Marspedia - User contributions [en]

Low gravity

2026-01-22T22:16:10Z

RichardWSmith: /* Martian gravity in fiction */

Mars' surface gravity is 3.711 meters / second^2, or about 38% of Earth's gravity.

There is no medical evidence for the effects on Mars' [[gravity]] on Earth life. Although we could simulate Mars' gravity on the International Space Station using a centrifuge, (with some mice in a cage for example), this experiment has never been done. Plants have been grown successfully in zero gee, so it is likely they would also be viable in 38% gee.

==Long term medical effects of 38% gravity==
As of 2026, no studies have researched this question. Once we have real data, please update this section. This should be a focus of [[Future research]].

==Short term medical effects of 38% gravity==
People with bad knee and hip joints may find Martian gravity to be a boon. In the far future, Mars might be seen as an attractive retirement location for that reason. (Tho Luna with 1/6 of Earth's gravity may be even more attractive.)

==Martian gravity in fiction==
Edgar Rice Burroughs wrote an 11 book series about John Carter who was teleported to Mars. On Mars the low gravity allowed him to leap 50 feet at a time (too far to be realistic), and compared to the natives, he had super human strength.

==Increasing Gravity Inside Long Term Mars Habitations==
There is a simple way to increase gravity within a major base on Mars... A centrifuge.

The formula for centripetal force is:

a = r(2 PI / T)^2. (a = acceleration, T = Time to rotate once, r = radius.)
(Note the acceleration only needs to be ~62% of Earth's gravity because we will add Mars' current gravity to this acceleration.)

So let's say we make an underground maglev railway on a circular track on an angle facing inwards (so when it is at speed, the gravity of the railcar plus Mars' gravity faces directly down to the floor of the car).

If this circular track was 500 meters in diameter, then the car would have to go around the loop every 57 seconds, (say one RPM to round off). The hyper loop railway would have to go 55 meters per second, or 198 km/hour.

The main reason to make it a magnetic levitation is to avoid wear on the wheel's bearings. This speed is doable with Earth trains using today's tech, which have to fight thru Earth's air pressure. (Japan's bullet trains go 320 km/h for example.)

Assuming the train is 100% as long as the track, and that it is two meters wide, then there is 0.684 square kilometres of area for people to exercise or sleep in. If we make the train 3 stories high, then this area triples.

If we find that there are no long term ill effects at, say, 0.8 gees of gravity, the speed of the train could be lowered. Alternately the radius could be increased (giving us more living area), without increasing the speed.

IF, and only if, we find that low gravity is a problem, then people could work 8 hours a day in the low gravity factories or farms. Then live and sleep in an underground habit at Earth gravity. Assuming it is underground, this would also reduce the radiation dose.

===Eureka Settlement Proposal:===
A [[Gravity|rotating settlement habitat]] is proposed [[Gravity|here]]. The Eureka <ref>https://macroinvent.com/wp-content/uploads/2019/03/Eureka-Mars-Settlement-Concept.pdf</ref>space Settlement was proposed for the [[Mars Colony Design Contest|2019 Mars society design contest.]]

==References==
<references />

Low gravity

2026-01-22T22:12:44Z

RichardWSmith: updated date.

Mars' surface gravity is 3.711 meters / second^2, or about 38% of Earth's gravity.

There is no medical evidence for the effects on Mars' [[gravity]] on Earth life. Although we could simulate Mars' gravity on the International Space Station using a centrifuge, (with some mice in a cage for example), this experiment has never been done. Plants have been grown successfully in zero gee, so it is likely they would also be viable in 38% gee.

==Long term medical effects of 38% gravity==
As of 2026, no studies have researched this question. Once we have real data, please update this section. This should be a focus of [[Future research]].

==Short term medical effects of 38% gravity==
People with bad knee and hip joints may find Martian gravity to be a boon. In the far future, Mars might be seen as an attractive retirement location for that reason. (Tho Luna with 1/6 of Earth's gravity may be even more attractive.)

==Martian gravity in fiction==

==Increasing Gravity Inside Long Term Mars Habitations==
There is a simple way to increase gravity within a major base on Mars... A centrifuge.

The formula for centripetal force is:

a = r(2 PI / T)^2. (a = acceleration, T = Time to rotate once, r = radius.)
(Note the acceleration only needs to be ~62% of Earth's gravity because we will add Mars' current gravity to this acceleration.)

So let's say we make an underground maglev railway on a circular track on an angle facing inwards (so when it is at speed, the gravity of the railcar plus Mars' gravity faces directly down to the floor of the car).

If this circular track was 500 meters in diameter, then the car would have to go around the loop every 57 seconds, (say one RPM to round off). The hyper loop railway would have to go 55 meters per second, or 198 km/hour.

The main reason to make it a magnetic levitation is to avoid wear on the wheel's bearings. This speed is doable with Earth trains using today's tech, which have to fight thru Earth's air pressure. (Japan's bullet trains go 320 km/h for example.)

Assuming the train is 100% as long as the track, and that it is two meters wide, then there is 0.684 square kilometres of area for people to exercise or sleep in. If we make the train 3 stories high, then this area triples.

If we find that there are no long term ill effects at, say, 0.8 gees of gravity, the speed of the train could be lowered. Alternately the radius could be increased (giving us more living area), without increasing the speed.

IF, and only if, we find that low gravity is a problem, then people could work 8 hours a day in the low gravity factories or farms. Then live and sleep in an underground habit at Earth gravity. Assuming it is underground, this would also reduce the radiation dose.

===Eureka Settlement Proposal:===
A [[Gravity|rotating settlement habitat]] is proposed [[Gravity|here]]. The Eureka <ref>https://macroinvent.com/wp-content/uploads/2019/03/Eureka-Mars-Settlement-Concept.pdf</ref>space Settlement was proposed for the [[Mars Colony Design Contest|2019 Mars society design contest.]]

==References==
<references />

Atmosphere

2025-11-18T19:39:42Z

RichardWSmith: /* Mesoscale Atmospheric Behaviour */ spelling

[[Image:sunset.jpg|thumb|right|300px|Sunset photographed by Mars Rover Spirit]]
The '''Atmosphere''' of [[Mars]] is not breathable. The pressure is too low, and there is too little [[oxygen]]. And yet, it gives Mars something that makes it the most habitable of all planets in our [[solar system]], except [[Earth]] of course. It provides valuable [[:category:chemistry|chemicals]], and it forms a visible sky, mostly from dispersed dust. Also note that it protects from lower energy radiation particles, stopping 1.58% of the radiation from space, even though it has only 0.6% of the Earth's air pressure.

Mars at one time had a much thicker atmosphere, see [[Ancient Atmosphere]] & [[Atmospheric loss]] for what happened to this thick atmosphere.

==Composition (gaseous parts)==
Composition of Mars atmosphere by volume<ref>[https://science.sciencemag.org/content/341/6143/263 Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover]</ref><ref>[http://www.daviddarling.info/encyclopedia/M/Marsatmos.html Water and trace gases based on table from David Darling Space Encyclopedia]</ref>
{|
! style="font-style: bold;text-align:left;" |Percentage
! style="font-style: bold;text-align:left;padding: 10px;" |Gas
|-
|96.0%
|[[Carbon dioxide]] (CO2)
|-
|1.93%
||[[Argon]] (Ar)
|-
|1.89%
|[[Nitrogen]] (N)
|-
|0.145%
|[[Oxygen]] (O2)
|-
|0.09%
|[[Carbon monoxide]] (CO)
|-
|0.03%
|[[water|Water vapor]] (H2O)
|-
| style="text-align:left;vertical-align:top;" |''Trace''

|[[Neon]] (Ne), [[Krypton]] (Kr), [[Xenon]] (Xe), [[Ozone]] (O3), [[Methane]] (CH4), C2H2, C2H4, C2H6, CH3OH
CH3Cl, N2O, NO2, NH3, PH3, SO2, OCS, H2S, H2CO, HCl, NCN.
|}

The total mass of the Martian atmosphere is 2.5x10^16 kg

==Air Pressure==
1-9 millibars (depending on altitude) or 600 Pa, average. This is 0.6% of Earth's air pressure at sea level (14,7 psi or 101,3 kPa)

Note that in the southern winter, approximately 30% of the atmosphere's carbon dioxide freezes out at the poles. (In the northern winter, about 12% freezes out.) Thus there is a strong seasonable component to the air pressure on Mars. The pressure ranges from about 1 bar to 0.7 bar over the course of the year (measured at the Viking lander sites). These values are lower at higher elevations.

If Mars had Earth's surface gravity, the atmosphere would be held more tightly, and compressed into a smaller volume. Since Mars' surface gravity is 38% of Earth's, the atmosphere is 'puffier' and extends farther into space.

The scale height of Mars (the relationship that says how quickly the atmosphere thins as you rise) is 10km. Thus every km you rise above the surface, the air pressure drops by about 10%. (This relationship works best near the ground, at very high altitudes, the atmosphere thins more slowly.)

==Vertical Organization==
Scientists have organized the Martian atmosphere into the following regions, starting from the ground and moving up:
*Troposphere (~0 to ~50 km). Tropopause is at ~40 to 50 km.
*Mesosphere (~50 to ~100 km). Mesopause is about ~120 km.
*Thermosphere (~100 to ~200 km). Homopause is at about 125 km.
*Exosphere. (Higher than ~200 km) Thins out to space.

These names are analogous to Earth, except with no ozone layer, Mars has no stratosphere. (The stratosphere of the Earth's atmosphere is strongly warmed by the ozone layer stopping UV light and warming.)

Other scientists prefer to use the more neutral names:
*Lower Atmosphere 0 to 50 km
*Middle Atmosphere 50 to 100 km
*Upper atmosphere more than 100 km.

When Mars is dusty, the [[Dust]] absorbs more solar heat and warms the air. This causes the upper edge of the Martian atmosphere to expand by ~25 km, and these divisions move higher.

Imbedded within the Thermosphere is the ionosphere, a region of thin plasma. Most of this plasma is formed by Extreme UltraViolet light (EUV) ionizing the air. EUV is the most energetic UV light, shading into soft X-rays.

Above the Homopause, the gas molecules take long paths with few collisions. They start to sort themselves by the types of gas (so carbon monoxide (CO) being lighter than carbon dioxide (CO2) becomes more common at higher attitudes. Thus, lighter gases (ones with low molecular mass) such as atomic oxygen sort themselves higher.

==Air Temperature==
In the Earth's atmosphere, the atmosphere is warm near the ground, cools as it rises, then gets warmer near the stratosphere. (Due to the ozone layer stopping incoming UV light.) It then cools again until you reach near space, where the atmosphere is warmed by impacting particles.

On Mars<ref>https://en.wikipedia.org/wiki/Climate_of_Mars#Temperature</ref> it is similar except that there is no warming in the stratosphere since Mars lacks an ozone layer. Generally, the higher you go, the colder it gets, until you reach the homopause at about 120 km above the ground. The temperature of the air near the ground is about 240°K while at 80 to 120 km the temperature is about 130°K (degrees Kelvin). It warms slightly above this level as the atmosphere fades into space.

At times of large dust storms, the atmosphere is warmer due to the dust intercepting and reradiating heat.

During the day, the temperature of near ground air lags the temperature of the ground. Thus in the morning, the ground heats faster than the boundary layer. The air immediately above the ground gradually warms from the ground. In the evenings, the air is warmer than the cooling rocks. Near the ground, the air temperature typically ranges from 190°K to 245°K in the lower latitudes (-163°C to -28°C).

In certain equatorial areas of Mars, the daytime temperature can reach 293°K 20°C in summer (or even up to 30°C), but will fall back to about -60°C at night.

==Color of the Sky==
The color of the sky is almost always reddish or salmon coloured due to the dust suspended in it. However, at times when the sky is very dust free, it can be blue, for the same reason that the Earth's sky is blue. (Red light is scattered more, so more blue light makes it down to the ground.) The sky may also be blue at sunset due to the fine dust scattering the other colors, leaving a blue color near the sun.

==Clouds (Carbon Dioxide)==
[[Carbon dioxide]] clouds require very cold conditions and are normally found high in the atmosphere (50 to 100 km). These form in the mesosphere, usually over the equator, and usually at night. They tend to be thin and hazy. However, at polar winters, the sun does not rise for months at a time; the air there gets very cold. Then carbon dioxide fogs appear, and very fine carbon dioxide snow falls on the polar cap. These clouds are more extensive and are low to the ground (from 0 to 25 km).

It is rare for the primary constitute of an atmosphere to form clouds. (This only happens on Mars, Triton and Pluto.) The freezing of the atmosphere during winter (especially the southern winter) causes the air pressure to vary significantly during each year.

CO2 clouds cool the planet (reflecting light away from the world), and warm it (blocking infrared radiation from leaving). On the whole, CO2 clouds seem to have a net warming effect. Early in Mars' history, the higher air pressure would have made CO2 clouds more common, and this warming may have been of more significance.

==Clouds (Water)==
Clouds on Mars are usually formed of ice particles, rather than water droplets. Thus most martian clouds are variations of cirrus clouds.

There are four main types of clouds found on Mars: aphelion cloud banks, lee waves behind tall volcanoes, polar hoods (which form polar spiral troughs), and ground fogs. Cumulus clouds are rare and usually require specific ground morphologies to support their formation.

--- Aphelion Cloud Banks (ACB) are clouds which form in the northern spring and summer from moisture coming from the warming South Pole. A Hadley cell forms, where rising air from the south move high into the atmosphere, and settles from south 10 degrees latitude, to 45 degrees north. A wide belt of hazy clouds can form, from about 25 to 40 km high. Late in the summer these clouds fade, except over the [[Tharsis]] Bulge. ACB cover vast areas and are visible with Earth telescopes.

--- Ground fogs are found in low laying regions such as the [[Hellas Basin]], [[Noctis Labyrinthus]], and [[Valles Marineris]]. They tend to form in the morning as frost and water rich ground warms, with the humidity at 100% over a wide area. They quickly disperse with wind and as the air warms.

--- Lee waves are clouds trailing behind large volcanoes. The Tharsis bulge sticks up so high into the atmosphere, that air loses heat as it rises up on to the bulge and gains potential energy (and thus heat) as it flows off the bulge. This has a strong effect on Martian circulation. It can lead to planet wide standing waves, ACB clouds over Tharsis (when they can no longer form elsewhere), and cloud formations before and after tall volcanos.

--- Polar hoods are clouds that cover the Martian poles. NPH is the north polar hood, where as SPH is the south polar hood.

A key difference between Earth and Martian clouds, is that Earth clouds have so much water mass, that the heat released by condensation of water vapour to water droplets, warms the air. This causes up drafts, and effects the circulation of the atmosphere. The amount of water in the Martian air is normally so low, that it has negligible effect on air movements.

The formation of clouds is largely independent of dust storms – both may occur at the same time.

==Radiative Process In Mar's Atmosphere==
Since Mars exists in a near vacuum, it was thought at first that the radiation hitting the ground would be radiated back into space with very little change by the atmosphere. This turned out to be wrong, the thin air, and the dust it carries, significantly effects the radiation hitting Mars. Of the incoming radiation, 1% of it is absorbed by clouds, 12% is absorbed by dust, and 1% is absorbed by the air itself. Of the infrared radiation emitted by the warm ground, 27% is absorbed by the atmosphere (including the air, dust, and clouds). The heating of the atmosphere is the major power source which causes the wide scale circulation (Hadley cell) of Mar's atmosphere.

==Martian Planetary Boundary Layer (PBL)==
The Planetary Boundary Layer (PBL) is the layer of the atmosphere which is strongly effected by the planet's surface. On Earth, it is 600 meters to 1 km high, usually. (Over deserts, it can occasionally reach 5 km on Earth.) On Mars, the thinner, 'puffier', atmosphere means that the PBL has greater height, normally between 5 and 10 km and occasionally going higher.

The greatest difference between the two, is on Mars, the thinner air results in very little heat being held by the atmosphere near the ground. In the morning, the air warms as the ground warms, and later cools as the ground cools at night. On Mars, temperature inversions are common at night, and low to the ground. (The air is cold touching the cold ground, warms as you go higher, and then starts to cool as you rise higher still.) On Earth, this is rarer, and happens higher up. While the Martian air has little effect on the ground below it, the opposite is not true. The ground modifies the thin air above it with moisture, dust, heat, momentum, and chemicals.

Mixing with the upper atmosphere is fast during the day, and slow at night. This is also true on Earth, but there is less difference between the day and night time rates. Generally, mixing of the lower air with the highest layers of the atmosphere happens more quickly on Mars than on Earth.

===Dust Devils:===
Dust devils are an important part of the PBL and normally rise to 60% of the boundary layer's current height. They are responsible for moving most of the dust into the lower atmosphere (tho they are not responsible for the continent sized massive [[Dust Storms]]). See the [[Dust devils]] entry for more information.

==Mesoscale Atmospheric Behaviour==
Weather systems can be microscale (purely local phenomenon which have no larger impact), global (or macro scale) which cover vast areas of the planet. Mesoscale phenomenon fall between these. Generally energy flows in the atmosphere start at the largest of scales, are transferred down into the mesoscale and then are dissipated at the microscale. These boundaries are not sharp, they are fuzzy, bleeding into each other.

The people most concerned with this are those scientists which are trying to make Global Circulation Models (GCM) – computer models which try to understand and predict Mars' atmosphere.

There are several differences between Mesoscale behaviour on Earth and Mars.

-- Dust devils on Earth are clearly micro-scale events. On Mars they are huge, sometimes 1 km in diameter and can rise ~10 km into the air. Further they move dust high in the air, which can have regional or global impacts.

-- On Mars, the ground can warm the air above it, on a slope. The warm air, flows up the slope until it reaches a significant ridge, breaks off from the ground and moves vertically into the air. On Earth these slope flows very short ranged, on Mars it can go on for many kilometres. (Over a thousand kilometres in some areas in the Tharsis region.) Several craters have been observed with cool air coming down in the centre, and warm air flowing along the floor and ridges, detaching at the crater rim.

-- Martian dust storms can be small and local, large, or planet wide. Mesoscale studies have shown that tho they may be global in extent the vast majority of the dust are lifted from fairly small lifting centres. The largest storms seem to occur when thermal tides (described below), interact with baroclinic wind fronts carrying plenty of dust. The turbulence caused when the 'tidal gate' is opened allows what would be a regional dust storm to go planet wide. This 'tidal gate' may only be open for as little as 11 hours.

-- Gravity Waves (described below) have been observed on Mars, and are thought to contribute to the equatorial CO2 clouds as well as other processes. They are a Mesoscale behaviour.

-- Carbon dioxide is the majority gas in Mars' atmosphere, and in polar winter it can condense out forming huge clouds, fogs, and carbon dioxide snow. This causes Mesoscale behaviour totally unlike anything seen on Earth. Gases such as Neon can be locally enriched (changing the atmospheric density), the energy released as the CO2 condenses and freezes causes localized updrafts in the area of generally settling air. More study of these complex processes would be welcome.

Local winds on Mars are very poorly studied, and the rarity of water clouds makes studying winds from orbit very difficult (weather fronts are generally invisible). Also there are no 'geosynchronous' satellites around Mars which can watch one area for hours at a time. Martian Global Circulation Models are gradually improving, but more data from balloons, ground weather stations, and specialized satellites is needed.

===Thermal Tide===
A thermal tide or atmospheric tide are global scale, daily oscillations of the atmosphere. On Mars these are caused by solar heating. (On Earth they are also effected by Lunar tides, and Rossby waves.) They are named after the rising and falling of the water caused by the tides caused by Luna and the Sun. (The underlying process is completely different, despite the name.)

The side of Mars warmed by the sun has a warmer atmosphere which rises. The mass of air, under the force of gravity then flows down slope causing strong winds in the Mesosphere moving to both the east and the west. On the surface, gentle plant wide winds sweep along every day. In places, the local geography can modify this flow strengthening or dispersing it.

Thermal tides are stronger on Mars than any other body in the Solar System. They are strongest when the air has a high load of dust, since that is when it is best able to absorb radiation from the Sun.

===Gravity Wave===
In fluid dynamics gravity waves are generated at the interface between two media with different densities; buoyancy (or equivalently gravity), tries to restore equilibrium with fluid flows. On Earth, gravity waves mediate the transfer of momentum between the troposphere and the higher portions of the atmosphere (stratosphere and mesosphere). An example is caused by air flow over mountains creating waves in the air. At first the air continues to move without a change of velocity, but as the waves reach thinner air higher up, the amplitude of the waves increases. Non linear effects can cause the waves to break, transferring the momentum to the flow of the air at that altitude. They can form regular patterns of clouds (long parallel clouds).

A strong updraft can also cause gravity waves as it breaks in the thinner, upper atmosphere.

On Mars gravity waves have been seen both around the large mountains, and they have been generated by the condensing atmosphere during polar winters.

==Global Circulation==
We lack the detailed weather observations standard on Earth, so many of the conclusions in this section are based on "Global Circulation Models" (computer simulations), bounded by the observations that we do have. More long term observations over more areas of the planet would be very welcome.

Global circulation is dominated by 3 facts on Mars. First the atmosphere is very thin, second, the southern highlands are about 4 km higher than the northern lowlands, third, the Martian orbit has an eccentricity of 0.093, which means that the Sun is significantly more distant during the Northern summer. The northern summer is ~3 times longer than the southern summer, but the light striking the top of the atmosphere when Mars is furthest from the sun is ~40% less than when Mars is closest to the sun. For these reasons, the South Pole gets about 5K colder than the North Pole during their respective winters.

There are no bodies of water, which adds to the daily temperatures swings, and no heat transfer between evaporating / condensing water. To some extent, the heat absorbed by dust mimics this.

The atmosphere is mostly CO2, which absorbs only 1% of the energy striking it from the sun. However, CO2 absorbs infrared radiation well, and as sunlight warms the planet, it reradiates the energy in heat wavelengths. About 10% of this energy is absorbed by the CO2 for typical dust loads. Thus the atmosphere is largely warmed from below.

Mars largely lacks an ozone layer, or other solar absorbing gases, so Mars lacks a tropopause.

At high altitudes, easterly winds dominate in the summer hemisphere, where as there are often westerly winds aloft in the winter hemisphere.

Southern summer is when Mars is closest to the sun, and it experiences strong ground heating. This combined with the evaporating CO2 ice cap, results in strong winds which move large amounts of dust into the upper atmosphere. The dust absorbs significant heat, and drives further large scale winds. During polar winters a significant amount of the atmosphere will condense as snow, or frost out. This results in a long term extremely low pressure zone over the winter pole, which has no analog on Earth.

One effect not seen on Earth is the condensing of CO2 at the poles during polar winter. This keeps the air temperature just above the CO2 frost point (the latent heat of freezing warms the air).

Unlike Earth, the air temperature is largely insensitive to the elevation, since most heating comes from the ground. The angle of the sun, the distance to the sun, the albedo (how much light the surface absorbs) of the ground determine the air temperature. The ground temperature varies little from the equator to the higher mid latitudes. The polar regions are much colder. For this reason, transient surface winds are less powerful in the late spring and summer seasons.

Because of the thin air, and lack of bodies of water, the air cools very quickly. A local heat wave on the planet's surface, will return to the average temperature in ~1 day on Mars, ~5 days on Earth, and ~1,000 days on Venus.

For much the same reason as Earth, Mars has 'Westerlies' (winds flowing from the southwest to the northeast), tho they are not as powerful.

Mars has more than a 20 km difference between the lowest points, and highest points on its surface. The large mountains impart significant momentum to the atmosphere which must be taken into account in Global Circulation Models. Winds flowing south from the northern basin along the west edge of highlands marks three major 'storm tracks'.

Note that Mars has high pressure systems and low pressure systems, but on Earth these are easy to track from cloud formation. On Mars, these are much harder to see since the dust loading is independent from these pressure systems.

===Thermal tides and the boundary layer===
This refers to large scale expanding of the atmosphere from solar heating on a daily basis, and have nothing to do with gravity. This expansion is much greater on Mars than on Earth, and drives daily wind patterns. (They are stronger on Mars than any other body in the solar system.) This effect is greater when there is a lot of dust in the air.

The boundary layer (where the surface winds stop effecting higher air) is much higher on Mars, reaching ~10 to 15 km. (On Earth this is usually less than 1 km deep.) The thermal tides and the thin air are thought to be responsible for this fact. At night the boundary layer is much lower, dropping to around 1 km.

The winds caused by thermal tides, can kick up fine dust, which further warms the air. This causes the boundary layer to rise, tho the exact details (of these and other factors) have been difficult to model. More data is wanted.

===Hadley Cells===
A Hadley cell is when warm air rises high into the atmosphere, spreads out and cools, then descends hundreds or thousands of kilometres away. For example, on Earth, warm equatorial air rises, cools, and descends in the horse latitudes, about 30 degrees north and south of the equator. (They are called 'horse latitudes', because sailing ships becalmed in these areas, sometimes had to eat their horses for food.) These tend to be desert zones on Earth since they are regions of dry, high pressure systems. Earth has 6 cells: 2 equatorial Hadley cells, 2 in the mid latitudes, and one over each pole. (In other words, there are 3 cells in the north, and these are reflected in the southern hemisphere.) The strongest, equatorial cells are the Hadley Cells, The middle latitude ones are called Ferrel cells, and the ones over the poles are called Polar cells. Mars only has Hadley cells.

Because of Mars' smaller size, the Hadley cell structure is very different. Half the year, there are two cells, and half the year, there is a single cell. The behaviour changes based on the season and follows the hottest area on the planet:

-- At the southern pole's summer, the hottest area is in the southern hemisphere, well away from the equator. Vast amounts of CO2 sublimate and warm. This air rises from around 60 degrees south, in a Hadley cell and descends from 45 degrees north to 10 degrees south. Large dust storms can form at this time. This is the strongest cell, moving the most material.

-- At the southern spring and fall, (the equinoxes), the heating is centred on the equator, and two cells form. Two weaker Hadley cells form, with air rising from near the equator and descending in the mid latitudes north and south.

-- At the southern winter (northern summer), the warmest area is north of the equator. a smaller amount of CO2 sublimes, then warm air rises from the northern mid latitudes and descends near the equator. This flow is much weaker than the first case covered.

Mars has the strongest Hadley Cell(s) observed in the solar system.

Note that air in the Earth's northern hemisphere takes a very long time to mix with air in the south since the Hadley Cells split the atmosphere at the equator. This is not the case on Mars, the atmosphere freely crosses the equator.

If Mars were to be terraformed, (and gain an ocean), it is unclear if this very simple system would become more complex. The low north, and 4 km higher southern highlands would cause a very different global circulation since the hydrological cycle would be dominated by the northern hemisphere. The ocean would also likely have surface ice, which would further minimize its effect.

Later modelling have suggested that Mars may have small, transitory Ferrel cells in the high latitudes. They are strongest in the northern hemisphere in the northern spring.

===Transient Eddies===
Transient eddies (or traveling weather systems), correspond to the high and low pressure systems that move around on Earth. They form an important part of Mars' atmospheric flow, are needed to form large dust storms, and are strongly effected by the high Martian terrain.

They transport a large amount of heat poleward. It turns out that the heat so moved is insufficient to prevent CO2 from frosting out at the pole, but this is due to the very thin air (and its limited heat carrying capacity) and not the strength of these weather systems. They are known as 'slantwise convection' as they have strong sideways and vertical motion.

In the northern hemisphere, these weather systems are most often found in (or west of) Acidalia, Utopia, and Arcadia. In northern winter, the storms are centered to the north of Alba Patera, and Tempe Terra. In the Southern Hemisphere, there is a single major storm zone confined to the Western Hemisphere between 200 and 300 degrees. The southern storms are less powerful than those in the north, due to the thinner air and the local terrain.

The northern weather systems can create local dust storms, but rarely (only once) cause global dust storms.

Water has been shown to have a significant effect on transient eddies. Should Mars regain an ocean (even one with a frozen surface) it is expected that these weather systems would grow more powerful.

Around the northern summer solstice these storm systems die out in what is called the 'solstical pause'. This pause typically lasts a month or so.

On Mars in the northern hemisphere, there are large transient eddies in the upper atmosphere. These upper level weather systems have no direct correlation on Earth, and are not well understood.

===Stationary Eddies===
These are much stronger on Mars than those on Earth. They are formed by the huge mountains rising out of much of the atmosphere, the planet-wide gradient (high southern pole to lower northern lowlands), and fixed regions where the ground heating is highest. They contribute to moving dust into the upper atmosphere, and do much to couple the lower and upper atmospheric momentum and heat flows.

There has been little science done on these, and most conclusions are based on climate models.

===Jet streams===
The air near Earth's equator is warm and rises. This air is moving at the same speed as the land and water below it – moving at a rate of 1670 km/hr to the east due to Earth's rotation. Now the atmosphere is higher at the equator (warm, less dense air) than at the pole (cool, dense air). So gravity causes the masses of air to slide down slope towards the poles. But this air high in the atmosphere, is moving eastward at 1670 km/hr, and the land below it is moving eastward at a slower rate (zero at the poles). Thus this huge mass of equatorial air, as it spills north and south, flows from the west to east. This powers the jet streams. (As Earth's poles warm, the air above the poles has expanded, meaning there is less tendency for air to flow down slope. The slope is simply not as steep. This weakens the jet stream on Earth, causing larger Rosby waves, which cause heat waves, heavy rains, and cold snaps to lock in for long periods of 'extreme weather'.)

Mars also has jet streams formed and powered in the same way. The southern one is weaker (~100 to 120 m/s), while the northern one is stronger (~160 m/s) and lower. They form at about 65 degree latitude.

==Dust cycle==
Dust very important in the Martian atmosphere. It effectively absorbs visible light and moves that energy (as heat) to the air around it. Further, it absorbs infrared light in the silica band, stopping heat from the planet from reaching space in these wave lengths. The atmosphere is so thin, it has a very low thermal mass, and dust has a major influence in heat flows and driving air circulation. Dust powers the massive Hadley cells found on Mars.

===Basics===
The most common sizes of dust particles seem to be around 1.4 to 1.7 micrometers (millionths of a meter). During dust storms, large particles have been detected: 1.8 to 2.5 micrometers.

Dust composition closely tracks what rocks are common on the surface. Silicates, plagioclase felspar, and zeolite are common. In addition, goethite and nano phase iron oxide, magnetite, olivine and ferric oxide are likely present.

The Martian surface is the primary source and sink for dust (micro-meteorites add an insignificant amount of dust to the top of the atmosphere). Dust is lifted into the atmosphere by convective winds, and settles due to gravity. Large particles of dust fall out near the equator, at higher latitudes finer dust lands. The time to erode surface material to the size of the dust on Mars is about equal to the orbital obliquity cycle. Thus there is no reason to assume that the dust is ancient. Large reservoirs of dust may be buried (tho if under sufficient pressure they will aggregate into rocks and no longer be dust).

Two mechanisms are responsible for lifting dust into the air. First [[Dust devils]] add haze in the lower atmosphere. This is then spread around to form local, regional, or global effects. They are most common when the dust in the air is low, and become less common when the upper air is warmer with more dust. Second, 'surface wind stress lifting' happens when local winds are strong enough to lift sand grains off the surface where they bounce back to the ground. (This is called 'saltation', and with Mars' lower gravity it is more important than on Earth. Once a strong wind gets sand particles bouncing, a lower speed wind can keep them bouncing for a long time.) The sand particles disturb smaller dust, which along with the wind lifts the dust into the air. Surface wind stress lifting (usually in southern summer) start almost all of the global dust storms. Dust devils contribute a minor, but not insignificant amount of dust to global dust storms. Some studies suggest, that dust devils are the major contributor of the ultra fine dust that is always seen at low altitudes.

Dust, carbon dioxide, and water are all interrelated. The seasonal evaporation of the southern CO2 ice cap creates strong winds which can make large dust storms. Dust can warm the air, causing strong winds which release more dust from the ground, and lift it higher into the air where it can spread vast distances. And water clouds are formed around dust particles. Conversely, water clouds already in existence can absorb dust, limiting how high it can rise in the atmosphere.

===Computer models of dust===
Martian Global Climate Models (GCM) are computer simulations of the Martian atmosphere. Considerable effort has been required to model dust, as it has a large effect on the Martian atmosphere. Early models which assumed a constant dust load, poorly reflected reality. Models which dynamically track dust of various sizes thru the air column work much better. Computer models do a good job of simulating the start of large dust storms, but do a poor job of 'turning off' the storm. Computer models of the storms tend to have them last longer than in real life. How the storms stop lifting dust off the surface is not well understood. (One guess is that all the dust has been removed from the high wind areas adding dust to the atmosphere.)

===Dust storms===
Dust storms are classified thus: Local (less than 1.6 million square km), Regional (greater than 1.6 million square km but which have boundaries), and Global. Local storms happen in both hemispheres at all times of the year, but are more common in the local hemisphere's summer months. The majority happen within 10 degrees of the subliming polar hood and trace local weather fronts. High terrain can concentrate winds which also can cause local dust storms. Low lying areas (with higher surface pressure) on storm tracks are also correlated with local dust storms. Regional storms are common, with 8 to 35 being observed in a typical Martian year. They appear in the areas which generate local dust storms, but most often near the subliming polar cap, and more often in the southern hemisphere. In the northern hemisphere, they are often observed in the summer in the 'storm tracks' (The Acidalia, Arcadia and Utopia storm tracks.) Global storms are simply large regional storms. Often a couple regional storms happening at the same time add to the total dust load, and form a global storm. Global storms are usually involved in the large scale Hadley Cells.

The Martian air is cleanest (least dust), during northern spring & summer, increases in northern fall, and is dirtiest in norther winter (southern summer). Southern summer almost always has local and regional dust storms. There is a global dust storm approximately one in 3 Martian years. With a single exception (Mars year 29), all global dust storms have occurred during souther summer. With a single exception, all global dust storms have started in the southern hemisphere. Global dust storms can block 99% of the light hitting the surface tho this is rare. 95% blockages can last for weeks. That said, most global dust storms reduce sunlight by 90% or less. 70% reductions which can last for months are typical. Dust deposition seems to happen at about the same rate from equatorial to near polar regions.

===Location of dust===
Several lines of evidence suggest that fine dust is well mixed in the lower layers of the atmosphere, including the boundary layer. The dust is well mixed below 4 km, above this water clouds create volumes of low dust and limit its rise. The Viking and other landers suggest that the dust scale height is between 10 and 13 km. (Thus the dust will be half as thick for each 10 to 13 km you rise in the air.) Above the dust scale height, dust concentration is seasonal. A region of very thin dust is persistent in the upper atmosphere in the tropical and sub-tropical regions. This is a relatively new result and newer climate models will have to take it into account. It is possible that these persistent regions remain as water clouds are rarer at those latitudes.

Dust rapidly loses heat after sunset. This suggests that there are three large regions where dust is at least a couple cm thick: Tharsis, Arabia, and Elysium. Dust may be several meters thick in these areas. Other observations suggest that Syrtis Major and much of Acidalia are largely devoid of surface dust. (Dust may well land in these regions, but surface winds soon remove it.)

===Effects of terraforming on dust===
Early terraforming will involve increasing the temperature, which will increase the air pressure and amount of moisture in Mars' atmosphere. Higher air pressure will result in much stronger winds, which will blow up more dust. This dust may cool the surface slightly, but it will further warm the air, and give the air more thermal inertia. This will result in Mars cooling slower at night, and reduce the thermal tides described above. Rain is in the far future, but it is likely that fine red snow will fall. Dust reserves that have been long buried will be exposed and contribute more dust, but the winds will stop picking up dust, once there are snow fields covering them.

It is likely that early terraforming will result in far more dust, until increasing water vapour starts quickly removing it with snow. At this point, it becomes difficult to predict what will happen. Eventually, (if we can start a robust hydrological cycle), dust will be washed into lake and sea bottoms where it may be cemented with carbonate minerals.

==Water Cycle in Atmosphere==
See [[Mars Water Cycle]] for more detail, but the high level view is that water sublimes from the north polar cap during northern spring & summer, and enters the air. There it circulates, moving planet wide, where some frosts out in southern winter. In northern fall, this process ends. With the coming of southern winter, this frost sublimes. But the southern ice cap is largely left alone because it is covered by 8 meters of solid carbon dioxide (CO2) which must sublime first. By the time the CO2 is gone, southern summer is close to over, and little of the southern polar cap sublimes. The water is transported to the north polar cap (and to frosts at lower northern latitudes). As we move into northern spring the process repeats.

The South Polar air is significantly dryer than in the northern hemisphere.

The water vapour is usually less than 10% of the maximum humidity that the air can hold. It does little climate forcing, but when it forms clouds it does effect the heat transfers from space to the planet and vice-versa. Water forming snow around dust particles is a significant way to remove dust from the air.

==Carbon Dioxide Atmospheric Effects==
Mars is the only world where a major part of its atmosphere freezes in winter, and one of only 3 worlds where the major gas of the atmosphere forms clouds. About 30% of the Martian atmosphere freezes out during the southern polar night, which has major consequences for the dynamics of the atmosphere. This can cause a six fold concentration of non-condensable gasses (primarily N2, Ar, & CO).

===Formation of Transient Ice Caps===
Carbon dioxide (CO2) ice is white, but when mixed with dust or water ice is darker. Also its albedo changes if it is in the form of frost or snow (brighter), or as a solid mass created by direct freezing of atmospheric gases (darker). This effects how quickly it sublimes in the local spring. Scientists have gradually discovered that a large amount of water ice is just under the surface in polar regions.

Water holds a great deal of latent heat, and this gives the land a fair bit of thermal inertia. In other words, the warmth of summer is stored in water ice in the ground, which slows the condensation of carbon dioxide in the fall and early winter.

The CO2 transient caps are brightest in late spring and summer then in the early spring, with the exception of the [[Cryptic region]] near the South Pole, which remains relatively dark. The reason for this is unclear.

When the atmosphere is dusty, it is easier for snow (both CO2 and water ice) to form. This results in the transient ice cap being brighter, and reflecting more light into space.

As the polar winter ends in spring, the furthest edges receive direct sunlight and sublime. However, the areas poleward remain very cold, and much of the newly freed CO2 gas will freeze out further poleward. Thus the pole's ice caps continue to get thicker in the early parts of spring.

The density of the seasonal ice caps is very hard to measure, but it is estimated that it is about 1,000 kg / m^2 when first deposited. It would likely increase in density from compression (of material falling on top) and by sintering.

===Climate Change===
Mars experiences large changes to its climate based on orbital dynamics (eccentricity, season of perihelion, and its obliquity) which have large effects on the isolation (amount of sunlight) in higher latitudes. When the poles are pointed more directly at the sun during local summer, ALL of the permanent ice cap (both water and CO2) will vanish each Martian year. This will put far more water and gas into play, bringing Mars into a 'warm wet' period for tens of thousands of years.

For more detail, see [[Periodic climate changes on Mars]].

===Polar Clouds & Haze===
Carbon dioxide can spontaneously nucleate ice particles in the atmosphere, forming hazes or clouds of CO2. (Water ice almost never spontaneously nucleates on Mars, it virtually always condenses around dust particles.) High clouds are visually always CO2, where as lower clouds could be CO2 or water ice.

When carbon dioxide freezes, it releases its latent heat of condensation, which warms the air around it. This causes chaotic air flows around the freezing ice cap, mixing the air, and causing updrafts. Compare this to the general slow flowing of atmosphere to the pole and the low pressure zone created by the atmosphere condensing out. This makes modelling the atmosphere and clouds in the polar night challenging. Observing this process going on is difficult as it takes place in unrelieved darkness, and is shrouded by clouds (both CO2 and water). We would like an orbiter with LIDAR scanners tuned to several IR frequencies to study this phenomena.

===Buried Carbon Dioxide Reserves at South Pole===
There is evidence that buried below the southern permanent ice cap is large reserves of solid CO2. The amount of this ice is speculative, but it is thought that if it were all to sublime, it would increase the pressure of the air to 180% of what it currently holds. There are also likely CO2 clathrates, tho they would only add a small amount of pressure (perhaps 0 to 1%).

===Fans and Spiders===
An unusual land form are the Martian [[Black spiders]] or 'araneiforms'. These are formed by the movement of dust under a slab of solid carbon dioxide, breaking free thru cracks in the ice. They are associated with fans of dust.

==Photo-Chemistry in the Martian Atmosphere==
See this link: [[Photochemistry]] for more information. But in short, high energy photons from the sun (mostly [[Ultraviolet]] light, break up atoms in the Martian atmosphere. The most important major gases are CO2 and H2O. These form a variety of short lived, unstable molecular species, including: OH, HO2, hydrogen peroxide (H2O2), atomic hydrogen, atomic oxygen, [[Ozone]] (O3), and others.

These react in a variety of ways, and produce [[Carbon monoxide]] (CO), [[Oxygen]] (O2), and hydroxide (HO), which last fairly long in the Martian atmosphere.

==Bibliography==
The following text book is strongly recommended for detailed information on the atmosphere of Mars:

"The Atmosphere and Climate of Mars", Edited by: Robert M. Haberle, R. Todd Clancy, Francois Forget, Michael D. Smith, & Richard W. Zurek, Published by Cambridge Planetary Science, ISBN: 9871-107-01618-7.

==References:==
<references />

[[Category:Atmospheric Sciences]]

Atmosphere

2025-11-18T19:32:50Z

RichardWSmith: /* Color of the Sky */ changed word choice.

[[Image:sunset.jpg|thumb|right|300px|Sunset photographed by Mars Rover Spirit]]
The '''Atmosphere''' of [[Mars]] is not breathable. The pressure is too low, and there is too little [[oxygen]]. And yet, it gives Mars something that makes it the most habitable of all planets in our [[solar system]], except [[Earth]] of course. It provides valuable [[:category:chemistry|chemicals]], and it forms a visible sky, mostly from dispersed dust. Also note that it protects from lower energy radiation particles, stopping 1.58% of the radiation from space, even though it has only 0.6% of the Earth's air pressure.

Mars at one time had a much thicker atmosphere, see [[Ancient Atmosphere]] & [[Atmospheric loss]] for what happened to this thick atmosphere.

==Composition (gaseous parts)==
Composition of Mars atmosphere by volume<ref>[https://science.sciencemag.org/content/341/6143/263 Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover]</ref><ref>[http://www.daviddarling.info/encyclopedia/M/Marsatmos.html Water and trace gases based on table from David Darling Space Encyclopedia]</ref>
{|
! style="font-style: bold;text-align:left;" |Percentage
! style="font-style: bold;text-align:left;padding: 10px;" |Gas
|-
|96.0%
|[[Carbon dioxide]] (CO2)
|-
|1.93%
||[[Argon]] (Ar)
|-
|1.89%
|[[Nitrogen]] (N)
|-
|0.145%
|[[Oxygen]] (O2)
|-
|0.09%
|[[Carbon monoxide]] (CO)
|-
|0.03%
|[[water|Water vapor]] (H2O)
|-
| style="text-align:left;vertical-align:top;" |''Trace''

|[[Neon]] (Ne), [[Krypton]] (Kr), [[Xenon]] (Xe), [[Ozone]] (O3), [[Methane]] (CH4), C2H2, C2H4, C2H6, CH3OH
CH3Cl, N2O, NO2, NH3, PH3, SO2, OCS, H2S, H2CO, HCl, NCN.
|}

The total mass of the Martian atmosphere is 2.5x10^16 kg

==Air Pressure==
1-9 millibars (depending on altitude) or 600 Pa, average. This is 0.6% of Earth's air pressure at sea level (14,7 psi or 101,3 kPa)

Note that in the southern winter, approximately 30% of the atmosphere's carbon dioxide freezes out at the poles. (In the northern winter, about 12% freezes out.) Thus there is a strong seasonable component to the air pressure on Mars. The pressure ranges from about 1 bar to 0.7 bar over the course of the year (measured at the Viking lander sites). These values are lower at higher elevations.

If Mars had Earth's surface gravity, the atmosphere would be held more tightly, and compressed into a smaller volume. Since Mars' surface gravity is 38% of Earth's, the atmosphere is 'puffier' and extends farther into space.

The scale height of Mars (the relationship that says how quickly the atmosphere thins as you rise) is 10km. Thus every km you rise above the surface, the air pressure drops by about 10%. (This relationship works best near the ground, at very high altitudes, the atmosphere thins more slowly.)

==Vertical Organization==
Scientists have organized the Martian atmosphere into the following regions, starting from the ground and moving up:
*Troposphere (~0 to ~50 km). Tropopause is at ~40 to 50 km.
*Mesosphere (~50 to ~100 km). Mesopause is about ~120 km.
*Thermosphere (~100 to ~200 km). Homopause is at about 125 km.
*Exosphere. (Higher than ~200 km) Thins out to space.

These names are analogous to Earth, except with no ozone layer, Mars has no stratosphere. (The stratosphere of the Earth's atmosphere is strongly warmed by the ozone layer stopping UV light and warming.)

Other scientists prefer to use the more neutral names:
*Lower Atmosphere 0 to 50 km
*Middle Atmosphere 50 to 100 km
*Upper atmosphere more than 100 km.

When Mars is dusty, the [[Dust]] absorbs more solar heat and warms the air. This causes the upper edge of the Martian atmosphere to expand by ~25 km, and these divisions move higher.

Imbedded within the Thermosphere is the ionosphere, a region of thin plasma. Most of this plasma is formed by Extreme UltraViolet light (EUV) ionizing the air. EUV is the most energetic UV light, shading into soft X-rays.

Above the Homopause, the gas molecules take long paths with few collisions. They start to sort themselves by the types of gas (so carbon monoxide (CO) being lighter than carbon dioxide (CO2) becomes more common at higher attitudes. Thus, lighter gases (ones with low molecular mass) such as atomic oxygen sort themselves higher.

==Air Temperature==
In the Earth's atmosphere, the atmosphere is warm near the ground, cools as it rises, then gets warmer near the stratosphere. (Due to the ozone layer stopping incoming UV light.) It then cools again until you reach near space, where the atmosphere is warmed by impacting particles.

On Mars<ref>https://en.wikipedia.org/wiki/Climate_of_Mars#Temperature</ref> it is similar except that there is no warming in the stratosphere since Mars lacks an ozone layer. Generally, the higher you go, the colder it gets, until you reach the homopause at about 120 km above the ground. The temperature of the air near the ground is about 240°K while at 80 to 120 km the temperature is about 130°K (degrees Kelvin). It warms slightly above this level as the atmosphere fades into space.

At times of large dust storms, the atmosphere is warmer due to the dust intercepting and reradiating heat.

During the day, the temperature of near ground air lags the temperature of the ground. Thus in the morning, the ground heats faster than the boundary layer. The air immediately above the ground gradually warms from the ground. In the evenings, the air is warmer than the cooling rocks. Near the ground, the air temperature typically ranges from 190°K to 245°K in the lower latitudes (-163°C to -28°C).

In certain equatorial areas of Mars, the daytime temperature can reach 293°K 20°C in summer (or even up to 30°C), but will fall back to about -60°C at night.

==Color of the Sky==
The color of the sky is almost always reddish or salmon coloured due to the dust suspended in it. However, at times when the sky is very dust free, it can be blue, for the same reason that the Earth's sky is blue. (Red light is scattered more, so more blue light makes it down to the ground.) The sky may also be blue at sunset due to the fine dust scattering the other colors, leaving a blue color near the sun.

==Clouds (Carbon Dioxide)==
[[Carbon dioxide]] clouds require very cold conditions and are normally found high in the atmosphere (50 to 100 km). These form in the mesosphere, usually over the equator, and usually at night. They tend to be thin and hazy. However, at polar winters, the sun does not rise for months at a time; the air there gets very cold. Then carbon dioxide fogs appear, and very fine carbon dioxide snow falls on the polar cap. These clouds are more extensive and are low to the ground (from 0 to 25 km).

It is rare for the primary constitute of an atmosphere to form clouds. (This only happens on Mars, Triton and Pluto.) The freezing of the atmosphere during winter (especially the southern winter) causes the air pressure to vary significantly during each year.

CO2 clouds cool the planet (reflecting light away from the world), and warm it (blocking infrared radiation from leaving). On the whole, CO2 clouds seem to have a net warming effect. Early in Mars' history, the higher air pressure would have made CO2 clouds more common, and this warming may have been of more significance.

==Clouds (Water)==
Clouds on Mars are usually formed of ice particles, rather than water droplets. Thus most martian clouds are variations of cirrus clouds.

There are four main types of clouds found on Mars: aphelion cloud banks, lee waves behind tall volcanoes, polar hoods (which form polar spiral troughs), and ground fogs. Cumulus clouds are rare and usually require specific ground morphologies to support their formation.

--- Aphelion Cloud Banks (ACB) are clouds which form in the northern spring and summer from moisture coming from the warming South Pole. A Hadley cell forms, where rising air from the south move high into the atmosphere, and settles from south 10 degrees latitude, to 45 degrees north. A wide belt of hazy clouds can form, from about 25 to 40 km high. Late in the summer these clouds fade, except over the [[Tharsis]] Bulge. ACB cover vast areas and are visible with Earth telescopes.

--- Ground fogs are found in low laying regions such as the [[Hellas Basin]], [[Noctis Labyrinthus]], and [[Valles Marineris]]. They tend to form in the morning as frost and water rich ground warms, with the humidity at 100% over a wide area. They quickly disperse with wind and as the air warms.

--- Lee waves are clouds trailing behind large volcanoes. The Tharsis bulge sticks up so high into the atmosphere, that air loses heat as it rises up on to the bulge and gains potential energy (and thus heat) as it flows off the bulge. This has a strong effect on Martian circulation. It can lead to planet wide standing waves, ACB clouds over Tharsis (when they can no longer form elsewhere), and cloud formations before and after tall volcanos.

--- Polar hoods are clouds that cover the Martian poles. NPH is the north polar hood, where as SPH is the south polar hood.

A key difference between Earth and Martian clouds, is that Earth clouds have so much water mass, that the heat released by condensation of water vapour to water droplets, warms the air. This causes up drafts, and effects the circulation of the atmosphere. The amount of water in the Martian air is normally so low, that it has negligible effect on air movements.

The formation of clouds is largely independent of dust storms – both may occur at the same time.

==Radiative Process In Mar's Atmosphere==
Since Mars exists in a near vacuum, it was thought at first that the radiation hitting the ground would be radiated back into space with very little change by the atmosphere. This turned out to be wrong, the thin air, and the dust it carries, significantly effects the radiation hitting Mars. Of the incoming radiation, 1% of it is absorbed by clouds, 12% is absorbed by dust, and 1% is absorbed by the air itself. Of the infrared radiation emitted by the warm ground, 27% is absorbed by the atmosphere (including the air, dust, and clouds). The heating of the atmosphere is the major power source which causes the wide scale circulation (Hadley cell) of Mar's atmosphere.

==Martian Planetary Boundary Layer (PBL)==
The Planetary Boundary Layer (PBL) is the layer of the atmosphere which is strongly effected by the planet's surface. On Earth, it is 600 meters to 1 km high, usually. (Over deserts, it can occasionally reach 5 km on Earth.) On Mars, the thinner, 'puffier', atmosphere means that the PBL has greater height, normally between 5 and 10 km and occasionally going higher.

The greatest difference between the two, is on Mars, the thinner air results in very little heat being held by the atmosphere near the ground. In the morning, the air warms as the ground warms, and later cools as the ground cools at night. On Mars, temperature inversions are common at night, and low to the ground. (The air is cold touching the cold ground, warms as you go higher, and then starts to cool as you rise higher still.) On Earth, this is rarer, and happens higher up. While the Martian air has little effect on the ground below it, the opposite is not true. The ground modifies the thin air above it with moisture, dust, heat, momentum, and chemicals.

Mixing with the upper atmosphere is fast during the day, and slow at night. This is also true on Earth, but there is less difference between the day and night time rates. Generally, mixing of the lower air with the highest layers of the atmosphere happens more quickly on Mars than on Earth.

===Dust Devils:===
Dust devils are an important part of the PBL and normally rise to 60% of the boundary layer's current height. They are responsible for moving most of the dust into the lower atmosphere (tho they are not responsible for the continent sized massive [[Dust Storms]]). See the [[Dust devils]] entry for more information.

==Mesoscale Atmospheric Behaviour==
Weather systems can be microscale (purely local phenomenon which have no larger impact), global (or macro scale) which cover vast areas of the planet. Mesoscale phenomenon fall between these. Generally energy flows in the atmosphere start at the largest of scales, are transferred down into the mesoscale and then are dissipated at the microscale. These boundaries are not sharp, they are fuzzy, bleeding into each other.

The people most concerned with this are those scientists which are trying to make Global Circulation Models (GCM) – computer models which try to understand and predict Mars' atmosphere.

There are several differences between Mesoscale behaviour on Earth and Mars.

-- Dust devils on Earth are clearly micro-scale events. On Mars they are huge, sometimes 1 km in diameter and can rise ~10 km into the air. Further they move dust high in the air, which can have regional or global impacts.

-- On Mars, the ground can warm the air above it, on a slope. The warm air, flows up the slope until it reaches a significant ridge, breaks off from the ground and moves vertically into the air. On Earth these slope flows very short ranged, on Mars it can go on for many kilometres. (Over a thousand kilometres in some areas in the Tharsis region.) Several craters have been observed with cool air coming down in the centre, and warm air flowing along the floor and ridges, detaching at the crater rim.

-- Martian dust storms can be small and local, large, or planet wide. Mesoscale studies have shown that tho they may be global in extent the vast majority of the dust are lifted from fairly small lifting centres. The largest storms seem to occur when thermal tides (described below), interact with baroclinic wind fronts carrying plenty of dust. The turbulence caused when the 'tidal gate' is opened allows what would be a regional dust storm to go planet wide. This 'tidal gate' may only be open for as little as 11 hours.

-- Gavity Waves (described below) have been observed on Mars, and are thought to contribute to the equatorial CO2 clouds as well as other processes. They are a Mesoscale behaviour.

-- Carbon dioxide is the majority gas in Mars' atmosphere, and in polar winter it can condense out forming huge clouds, fogs, and carbon dioxide snow. This causes Mesoscale behaviour totally unlike anything seen on Earth. Gases such as Neon can be locally enriched (changing the atmospheric density), the energy released as the CO2 condenses and freezes causes localized updrafts in the area of generally settling air. More study of these complex processes would be welcome.

Local winds on Mars are very poorly studied, and the rarity of water clouds makes studying winds from orbit very difficult (weather fronts are generally invisible). Also there are no 'geosynchronous' satellites around Mars which can watch one area for hours at a time. Martian Global Circulation Models are gradually improving, but more data from balloons, ground weather stations, and specialized satellites is needed.

===Thermal Tide===
A thermal tide or atmospheric tide are global scale, daily oscillations of the atmosphere. On Mars these are caused by solar heating. (On Earth they are also effected by Lunar tides, and Rossby waves.) They are named after the rising and falling of the water caused by the tides caused by Luna and the Sun. (The underlying process is completely different, despite the name.)

The side of Mars warmed by the sun has a warmer atmosphere which rises. The mass of air, under the force of gravity then flows down slope causing strong winds in the Mesosphere moving to both the east and the west. On the surface, gentle plant wide winds sweep along every day. In places, the local geography can modify this flow strengthening or dispersing it.

Thermal tides are stronger on Mars than any other body in the Solar System. They are strongest when the air has a high load of dust, since that is when it is best able to absorb radiation from the Sun.

===Gravity Wave===
In fluid dynamics gravity waves are generated at the interface between two media with different densities; buoyancy (or equivalently gravity), tries to restore equilibrium with fluid flows. On Earth, gravity waves mediate the transfer of momentum between the troposphere and the higher portions of the atmosphere (stratosphere and mesosphere). An example is caused by air flow over mountains creating waves in the air. At first the air continues to move without a change of velocity, but as the waves reach thinner air higher up, the amplitude of the waves increases. Non linear effects can cause the waves to break, transferring the momentum to the flow of the air at that altitude. They can form regular patterns of clouds (long parallel clouds).

A strong updraft can also cause gravity waves as it breaks in the thinner, upper atmosphere.

On Mars gravity waves have been seen both around the large mountains, and they have been generated by the condensing atmosphere during polar winters.

==Global Circulation==
We lack the detailed weather observations standard on Earth, so many of the conclusions in this section are based on "Global Circulation Models" (computer simulations), bounded by the observations that we do have. More long term observations over more areas of the planet would be very welcome.

Global circulation is dominated by 3 facts on Mars. First the atmosphere is very thin, second, the southern highlands are about 4 km higher than the northern lowlands, third, the Martian orbit has an eccentricity of 0.093, which means that the Sun is significantly more distant during the Northern summer. The northern summer is ~3 times longer than the southern summer, but the light striking the top of the atmosphere when Mars is furthest from the sun is ~40% less than when Mars is closest to the sun. For these reasons, the South Pole gets about 5K colder than the North Pole during their respective winters.

There are no bodies of water, which adds to the daily temperatures swings, and no heat transfer between evaporating / condensing water. To some extent, the heat absorbed by dust mimics this.

The atmosphere is mostly CO2, which absorbs only 1% of the energy striking it from the sun. However, CO2 absorbs infrared radiation well, and as sunlight warms the planet, it reradiates the energy in heat wavelengths. About 10% of this energy is absorbed by the CO2 for typical dust loads. Thus the atmosphere is largely warmed from below.

Mars largely lacks an ozone layer, or other solar absorbing gases, so Mars lacks a tropopause.

At high altitudes, easterly winds dominate in the summer hemisphere, where as there are often westerly winds aloft in the winter hemisphere.

Southern summer is when Mars is closest to the sun, and it experiences strong ground heating. This combined with the evaporating CO2 ice cap, results in strong winds which move large amounts of dust into the upper atmosphere. The dust absorbs significant heat, and drives further large scale winds. During polar winters a significant amount of the atmosphere will condense as snow, or frost out. This results in a long term extremely low pressure zone over the winter pole, which has no analog on Earth.

One effect not seen on Earth is the condensing of CO2 at the poles during polar winter. This keeps the air temperature just above the CO2 frost point (the latent heat of freezing warms the air).

Unlike Earth, the air temperature is largely insensitive to the elevation, since most heating comes from the ground. The angle of the sun, the distance to the sun, the albedo (how much light the surface absorbs) of the ground determine the air temperature. The ground temperature varies little from the equator to the higher mid latitudes. The polar regions are much colder. For this reason, transient surface winds are less powerful in the late spring and summer seasons.

Because of the thin air, and lack of bodies of water, the air cools very quickly. A local heat wave on the planet's surface, will return to the average temperature in ~1 day on Mars, ~5 days on Earth, and ~1,000 days on Venus.

For much the same reason as Earth, Mars has 'Westerlies' (winds flowing from the southwest to the northeast), tho they are not as powerful.

Mars has more than a 20 km difference between the lowest points, and highest points on its surface. The large mountains impart significant momentum to the atmosphere which must be taken into account in Global Circulation Models. Winds flowing south from the northern basin along the west edge of highlands marks three major 'storm tracks'.

Note that Mars has high pressure systems and low pressure systems, but on Earth these are easy to track from cloud formation. On Mars, these are much harder to see since the dust loading is independent from these pressure systems.

===Thermal tides and the boundary layer===
This refers to large scale expanding of the atmosphere from solar heating on a daily basis, and have nothing to do with gravity. This expansion is much greater on Mars than on Earth, and drives daily wind patterns. (They are stronger on Mars than any other body in the solar system.) This effect is greater when there is a lot of dust in the air.

The boundary layer (where the surface winds stop effecting higher air) is much higher on Mars, reaching ~10 to 15 km. (On Earth this is usually less than 1 km deep.) The thermal tides and the thin air are thought to be responsible for this fact. At night the boundary layer is much lower, dropping to around 1 km.

The winds caused by thermal tides, can kick up fine dust, which further warms the air. This causes the boundary layer to rise, tho the exact details (of these and other factors) have been difficult to model. More data is wanted.

===Hadley Cells===
A Hadley cell is when warm air rises high into the atmosphere, spreads out and cools, then descends hundreds or thousands of kilometres away. For example, on Earth, warm equatorial air rises, cools, and descends in the horse latitudes, about 30 degrees north and south of the equator. (They are called 'horse latitudes', because sailing ships becalmed in these areas, sometimes had to eat their horses for food.) These tend to be desert zones on Earth since they are regions of dry, high pressure systems. Earth has 6 cells: 2 equatorial Hadley cells, 2 in the mid latitudes, and one over each pole. (In other words, there are 3 cells in the north, and these are reflected in the southern hemisphere.) The strongest, equatorial cells are the Hadley Cells, The middle latitude ones are called Ferrel cells, and the ones over the poles are called Polar cells. Mars only has Hadley cells.

Because of Mars' smaller size, the Hadley cell structure is very different. Half the year, there are two cells, and half the year, there is a single cell. The behaviour changes based on the season and follows the hottest area on the planet:

-- At the southern pole's summer, the hottest area is in the southern hemisphere, well away from the equator. Vast amounts of CO2 sublimate and warm. This air rises from around 60 degrees south, in a Hadley cell and descends from 45 degrees north to 10 degrees south. Large dust storms can form at this time. This is the strongest cell, moving the most material.

-- At the southern spring and fall, (the equinoxes), the heating is centred on the equator, and two cells form. Two weaker Hadley cells form, with air rising from near the equator and descending in the mid latitudes north and south.

-- At the southern winter (northern summer), the warmest area is north of the equator. a smaller amount of CO2 sublimes, then warm air rises from the northern mid latitudes and descends near the equator. This flow is much weaker than the first case covered.

Mars has the strongest Hadley Cell(s) observed in the solar system.

Note that air in the Earth's northern hemisphere takes a very long time to mix with air in the south since the Hadley Cells split the atmosphere at the equator. This is not the case on Mars, the atmosphere freely crosses the equator.

If Mars were to be terraformed, (and gain an ocean), it is unclear if this very simple system would become more complex. The low north, and 4 km higher southern highlands would cause a very different global circulation since the hydrological cycle would be dominated by the northern hemisphere. The ocean would also likely have surface ice, which would further minimize its effect.

Later modelling have suggested that Mars may have small, transitory Ferrel cells in the high latitudes. They are strongest in the northern hemisphere in the northern spring.

===Transient Eddies===
Transient eddies (or traveling weather systems), correspond to the high and low pressure systems that move around on Earth. They form an important part of Mars' atmospheric flow, are needed to form large dust storms, and are strongly effected by the high Martian terrain.

They transport a large amount of heat poleward. It turns out that the heat so moved is insufficient to prevent CO2 from frosting out at the pole, but this is due to the very thin air (and its limited heat carrying capacity) and not the strength of these weather systems. They are known as 'slantwise convection' as they have strong sideways and vertical motion.

In the northern hemisphere, these weather systems are most often found in (or west of) Acidalia, Utopia, and Arcadia. In northern winter, the storms are centered to the north of Alba Patera, and Tempe Terra. In the Southern Hemisphere, there is a single major storm zone confined to the Western Hemisphere between 200 and 300 degrees. The southern storms are less powerful than those in the north, due to the thinner air and the local terrain.

The northern weather systems can create local dust storms, but rarely (only once) cause global dust storms.

Water has been shown to have a significant effect on transient eddies. Should Mars regain an ocean (even one with a frozen surface) it is expected that these weather systems would grow more powerful.

Around the northern summer solstice these storm systems die out in what is called the 'solstical pause'. This pause typically lasts a month or so.

On Mars in the northern hemisphere, there are large transient eddies in the upper atmosphere. These upper level weather systems have no direct correlation on Earth, and are not well understood.

===Stationary Eddies===
These are much stronger on Mars than those on Earth. They are formed by the huge mountains rising out of much of the atmosphere, the planet-wide gradient (high southern pole to lower northern lowlands), and fixed regions where the ground heating is highest. They contribute to moving dust into the upper atmosphere, and do much to couple the lower and upper atmospheric momentum and heat flows.

There has been little science done on these, and most conclusions are based on climate models.

===Jet streams===
The air near Earth's equator is warm and rises. This air is moving at the same speed as the land and water below it – moving at a rate of 1670 km/hr to the east due to Earth's rotation. Now the atmosphere is higher at the equator (warm, less dense air) than at the pole (cool, dense air). So gravity causes the masses of air to slide down slope towards the poles. But this air high in the atmosphere, is moving eastward at 1670 km/hr, and the land below it is moving eastward at a slower rate (zero at the poles). Thus this huge mass of equatorial air, as it spills north and south, flows from the west to east. This powers the jet streams. (As Earth's poles warm, the air above the poles has expanded, meaning there is less tendency for air to flow down slope. The slope is simply not as steep. This weakens the jet stream on Earth, causing larger Rosby waves, which cause heat waves, heavy rains, and cold snaps to lock in for long periods of 'extreme weather'.)

Mars also has jet streams formed and powered in the same way. The southern one is weaker (~100 to 120 m/s), while the northern one is stronger (~160 m/s) and lower. They form at about 65 degree latitude.

==Dust cycle==
Dust very important in the Martian atmosphere. It effectively absorbs visible light and moves that energy (as heat) to the air around it. Further, it absorbs infrared light in the silica band, stopping heat from the planet from reaching space in these wave lengths. The atmosphere is so thin, it has a very low thermal mass, and dust has a major influence in heat flows and driving air circulation. Dust powers the massive Hadley cells found on Mars.

===Basics===
The most common sizes of dust particles seem to be around 1.4 to 1.7 micrometers (millionths of a meter). During dust storms, large particles have been detected: 1.8 to 2.5 micrometers.

Dust composition closely tracks what rocks are common on the surface. Silicates, plagioclase felspar, and zeolite are common. In addition, goethite and nano phase iron oxide, magnetite, olivine and ferric oxide are likely present.

The Martian surface is the primary source and sink for dust (micro-meteorites add an insignificant amount of dust to the top of the atmosphere). Dust is lifted into the atmosphere by convective winds, and settles due to gravity. Large particles of dust fall out near the equator, at higher latitudes finer dust lands. The time to erode surface material to the size of the dust on Mars is about equal to the orbital obliquity cycle. Thus there is no reason to assume that the dust is ancient. Large reservoirs of dust may be buried (tho if under sufficient pressure they will aggregate into rocks and no longer be dust).

Two mechanisms are responsible for lifting dust into the air. First [[Dust devils]] add haze in the lower atmosphere. This is then spread around to form local, regional, or global effects. They are most common when the dust in the air is low, and become less common when the upper air is warmer with more dust. Second, 'surface wind stress lifting' happens when local winds are strong enough to lift sand grains off the surface where they bounce back to the ground. (This is called 'saltation', and with Mars' lower gravity it is more important than on Earth. Once a strong wind gets sand particles bouncing, a lower speed wind can keep them bouncing for a long time.) The sand particles disturb smaller dust, which along with the wind lifts the dust into the air. Surface wind stress lifting (usually in southern summer) start almost all of the global dust storms. Dust devils contribute a minor, but not insignificant amount of dust to global dust storms. Some studies suggest, that dust devils are the major contributor of the ultra fine dust that is always seen at low altitudes.

Dust, carbon dioxide, and water are all interrelated. The seasonal evaporation of the southern CO2 ice cap creates strong winds which can make large dust storms. Dust can warm the air, causing strong winds which release more dust from the ground, and lift it higher into the air where it can spread vast distances. And water clouds are formed around dust particles. Conversely, water clouds already in existence can absorb dust, limiting how high it can rise in the atmosphere.

===Computer models of dust===
Martian Global Climate Models (GCM) are computer simulations of the Martian atmosphere. Considerable effort has been required to model dust, as it has a large effect on the Martian atmosphere. Early models which assumed a constant dust load, poorly reflected reality. Models which dynamically track dust of various sizes thru the air column work much better. Computer models do a good job of simulating the start of large dust storms, but do a poor job of 'turning off' the storm. Computer models of the storms tend to have them last longer than in real life. How the storms stop lifting dust off the surface is not well understood. (One guess is that all the dust has been removed from the high wind areas adding dust to the atmosphere.)

===Dust storms===
Dust storms are classified thus: Local (less than 1.6 million square km), Regional (greater than 1.6 million square km but which have boundaries), and Global. Local storms happen in both hemispheres at all times of the year, but are more common in the local hemisphere's summer months. The majority happen within 10 degrees of the subliming polar hood and trace local weather fronts. High terrain can concentrate winds which also can cause local dust storms. Low lying areas (with higher surface pressure) on storm tracks are also correlated with local dust storms. Regional storms are common, with 8 to 35 being observed in a typical Martian year. They appear in the areas which generate local dust storms, but most often near the subliming polar cap, and more often in the southern hemisphere. In the northern hemisphere, they are often observed in the summer in the 'storm tracks' (The Acidalia, Arcadia and Utopia storm tracks.) Global storms are simply large regional storms. Often a couple regional storms happening at the same time add to the total dust load, and form a global storm. Global storms are usually involved in the large scale Hadley Cells.

The Martian air is cleanest (least dust), during northern spring & summer, increases in northern fall, and is dirtiest in norther winter (southern summer). Southern summer almost always has local and regional dust storms. There is a global dust storm approximately one in 3 Martian years. With a single exception (Mars year 29), all global dust storms have occurred during souther summer. With a single exception, all global dust storms have started in the southern hemisphere. Global dust storms can block 99% of the light hitting the surface tho this is rare. 95% blockages can last for weeks. That said, most global dust storms reduce sunlight by 90% or less. 70% reductions which can last for months are typical. Dust deposition seems to happen at about the same rate from equatorial to near polar regions.

===Location of dust===
Several lines of evidence suggest that fine dust is well mixed in the lower layers of the atmosphere, including the boundary layer. The dust is well mixed below 4 km, above this water clouds create volumes of low dust and limit its rise. The Viking and other landers suggest that the dust scale height is between 10 and 13 km. (Thus the dust will be half as thick for each 10 to 13 km you rise in the air.) Above the dust scale height, dust concentration is seasonal. A region of very thin dust is persistent in the upper atmosphere in the tropical and sub-tropical regions. This is a relatively new result and newer climate models will have to take it into account. It is possible that these persistent regions remain as water clouds are rarer at those latitudes.

Dust rapidly loses heat after sunset. This suggests that there are three large regions where dust is at least a couple cm thick: Tharsis, Arabia, and Elysium. Dust may be several meters thick in these areas. Other observations suggest that Syrtis Major and much of Acidalia are largely devoid of surface dust. (Dust may well land in these regions, but surface winds soon remove it.)

===Effects of terraforming on dust===
Early terraforming will involve increasing the temperature, which will increase the air pressure and amount of moisture in Mars' atmosphere. Higher air pressure will result in much stronger winds, which will blow up more dust. This dust may cool the surface slightly, but it will further warm the air, and give the air more thermal inertia. This will result in Mars cooling slower at night, and reduce the thermal tides described above. Rain is in the far future, but it is likely that fine red snow will fall. Dust reserves that have been long buried will be exposed and contribute more dust, but the winds will stop picking up dust, once there are snow fields covering them.

It is likely that early terraforming will result in far more dust, until increasing water vapour starts quickly removing it with snow. At this point, it becomes difficult to predict what will happen. Eventually, (if we can start a robust hydrological cycle), dust will be washed into lake and sea bottoms where it may be cemented with carbonate minerals.

==Water Cycle in Atmosphere==
See [[Mars Water Cycle]] for more detail, but the high level view is that water sublimes from the north polar cap during northern spring & summer, and enters the air. There it circulates, moving planet wide, where some frosts out in southern winter. In northern fall, this process ends. With the coming of southern winter, this frost sublimes. But the southern ice cap is largely left alone because it is covered by 8 meters of solid carbon dioxide (CO2) which must sublime first. By the time the CO2 is gone, southern summer is close to over, and little of the southern polar cap sublimes. The water is transported to the north polar cap (and to frosts at lower northern latitudes). As we move into northern spring the process repeats.

The South Polar air is significantly dryer than in the northern hemisphere.

The water vapour is usually less than 10% of the maximum humidity that the air can hold. It does little climate forcing, but when it forms clouds it does effect the heat transfers from space to the planet and vice-versa. Water forming snow around dust particles is a significant way to remove dust from the air.

==Carbon Dioxide Atmospheric Effects==
Mars is the only world where a major part of its atmosphere freezes in winter, and one of only 3 worlds where the major gas of the atmosphere forms clouds. About 30% of the Martian atmosphere freezes out during the southern polar night, which has major consequences for the dynamics of the atmosphere. This can cause a six fold concentration of non-condensable gasses (primarily N2, Ar, & CO).

===Formation of Transient Ice Caps===
Carbon dioxide (CO2) ice is white, but when mixed with dust or water ice is darker. Also its albedo changes if it is in the form of frost or snow (brighter), or as a solid mass created by direct freezing of atmospheric gases (darker). This effects how quickly it sublimes in the local spring. Scientists have gradually discovered that a large amount of water ice is just under the surface in polar regions.

Water holds a great deal of latent heat, and this gives the land a fair bit of thermal inertia. In other words, the warmth of summer is stored in water ice in the ground, which slows the condensation of carbon dioxide in the fall and early winter.

The CO2 transient caps are brightest in late spring and summer then in the early spring, with the exception of the [[Cryptic region]] near the South Pole, which remains relatively dark. The reason for this is unclear.

When the atmosphere is dusty, it is easier for snow (both CO2 and water ice) to form. This results in the transient ice cap being brighter, and reflecting more light into space.

As the polar winter ends in spring, the furthest edges receive direct sunlight and sublime. However, the areas poleward remain very cold, and much of the newly freed CO2 gas will freeze out further poleward. Thus the pole's ice caps continue to get thicker in the early parts of spring.

The density of the seasonal ice caps is very hard to measure, but it is estimated that it is about 1,000 kg / m^2 when first deposited. It would likely increase in density from compression (of material falling on top) and by sintering.

===Climate Change===
Mars experiences large changes to its climate based on orbital dynamics (eccentricity, season of perihelion, and its obliquity) which have large effects on the isolation (amount of sunlight) in higher latitudes. When the poles are pointed more directly at the sun during local summer, ALL of the permanent ice cap (both water and CO2) will vanish each Martian year. This will put far more water and gas into play, bringing Mars into a 'warm wet' period for tens of thousands of years.

For more detail, see [[Periodic climate changes on Mars]].

===Polar Clouds & Haze===
Carbon dioxide can spontaneously nucleate ice particles in the atmosphere, forming hazes or clouds of CO2. (Water ice almost never spontaneously nucleates on Mars, it virtually always condenses around dust particles.) High clouds are visually always CO2, where as lower clouds could be CO2 or water ice.

When carbon dioxide freezes, it releases its latent heat of condensation, which warms the air around it. This causes chaotic air flows around the freezing ice cap, mixing the air, and causing updrafts. Compare this to the general slow flowing of atmosphere to the pole and the low pressure zone created by the atmosphere condensing out. This makes modelling the atmosphere and clouds in the polar night challenging. Observing this process going on is difficult as it takes place in unrelieved darkness, and is shrouded by clouds (both CO2 and water). We would like an orbiter with LIDAR scanners tuned to several IR frequencies to study this phenomena.

===Buried Carbon Dioxide Reserves at South Pole===
There is evidence that buried below the southern permanent ice cap is large reserves of solid CO2. The amount of this ice is speculative, but it is thought that if it were all to sublime, it would increase the pressure of the air to 180% of what it currently holds. There are also likely CO2 clathrates, tho they would only add a small amount of pressure (perhaps 0 to 1%).

===Fans and Spiders===
An unusual land form are the Martian [[Black spiders]] or 'araneiforms'. These are formed by the movement of dust under a slab of solid carbon dioxide, breaking free thru cracks in the ice. They are associated with fans of dust.

==Photo-Chemistry in the Martian Atmosphere==
See this link: [[Photochemistry]] for more information. But in short, high energy photons from the sun (mostly [[Ultraviolet]] light, break up atoms in the Martian atmosphere. The most important major gases are CO2 and H2O. These form a variety of short lived, unstable molecular species, including: OH, HO2, hydrogen peroxide (H2O2), atomic hydrogen, atomic oxygen, [[Ozone]] (O3), and others.

These react in a variety of ways, and produce [[Carbon monoxide]] (CO), [[Oxygen]] (O2), and hydroxide (HO), which last fairly long in the Martian atmosphere.

==Bibliography==
The following text book is strongly recommended for detailed information on the atmosphere of Mars:

"The Atmosphere and Climate of Mars", Edited by: Robert M. Haberle, R. Todd Clancy, Francois Forget, Michael D. Smith, & Richard W. Zurek, Published by Cambridge Planetary Science, ISBN: 9871-107-01618-7.

==References:==
<references />

[[Category:Atmospheric Sciences]]

Radiation shielding

2025-11-18T02:14:39Z

RichardWSmith: /* General */ Made sentence more correct / clearer.

[[Image:WaterShieldGreenhouse.png|thumb|right|300px|Water-shield Greenhouse Concept]]

Early explorers will simply accept the radiation dose for the 2.5 year round trip (which should give approximately a 1% lifetime increase of a fatal cancer). See "The Case for Mars, chapter 5. However, radiation protection becomes much more of a concern for long durations habitats where people will live on Mars for years or decades. In such cases, thick shielding (of soil or ice), exotic modern materials, or electro-static or magnetic shields become more sensible. Radiation from the natural radioactive elements in the soil should be approximately equal to Earth doses. Radiation from Solar and Cosmic rays will be stronger, with the latter being much more difficult to shield against. See [[Cosmic Radiation]].

==General==
The average radiation on Earth is very low at 3 to 6.2 mSv / year depending on location (https://en.wikipedia.org/wiki/Background_radiation). However, in some areas of the city Ramsar, Iran, the radiation level is 260 mSv / year, with no measured increase in health problems. Thus the radiation safety levels in many countries on Earth may be too conservative. Mars starts at 200 to 250 mSv / year. The discussion below assumes we are aiming for 6.2 mSv, but there shouldn't be a disaster if this is missed, even by a significant amount.

Also note that sudden radiation doses are much more dangerous than a small steady dose over a long period of time. A solar storm is FAR more dangerous than the slow and steady trickle of cosmic rays.

Note that there is significant evidence that very low levels of radiation is healthy. See [[Radiation Hormesis]].

Long term settlers will need to shield against 3 types of radiation: [[Electromagnetic radiation]], [[Solar Cosmic Rays]], and more powerful [[Cosmic radiation | Cosmic Rays]]. [[Galactic Cosmic Rays]] are by far the most difficult to shield against (tho their total energy dosage is low).

===Electromagnetic Radiation===
The EM spectrum consists of radio waves, microwaves, heat, visible light, ultraviolet light, x-rays, and gamma rays. The low energy end of this spectrum are waves that pass thru us and are harmless. As the frequency becomes shorter, the energy of each particle of light increases. The high energy EM waves act progressively more like particles and can damage tissue. UV light can give sun burn and damage surface tissue, and X-rays and gamma rays can do deep tissue damage.

EM waves are stopped by atoms with many electrons. Thus heavy metals strongly mitigate them. Air, dust, soil, and water all reduce their effect. The only common dangerous EM radiation on Mars is Ultraviolet (UV) light. On Earth, the stronger UV rays are stopped by our ozone layer, on Mars, 95% of the UV light reaches the ground, sterilizing the top layers of soil. (Also note that Mars gets about half the UV light that Earth does, because Mars is further from the sun.)

UV light can make plastics brittle, which may not be a concern for an 18 month stay, but it will be a problem for long term settlers. Metals and certain composites are resistant to UV light. The space suits and habitat materials will be designed to shield against UV rays.

X-rays and gamma rays are much more penetrating. For example, compressed Martian soil will halve the amount of gamma rays for each 9.1 cm of soil. Water halves the gamma ray flux for every 18 cm. <ref>https://www.imagesco.com/geiger/lead-shielding-guide.html</ref>

That said, the sun produces few x-rays and almost no gamma rays. A tiny amount come from space. The amount is low and can be basically ignored.

If a Mars base has a local source of gamma rays, say a nuclear reactor, it must be properly shielded, far away, or both.

===Solar Cosmic Rays===
These are not the solar wind, but rather high energy particles (mostly protons) which are produced in [[Coronal Mass Ejections]]. Most are low energy, 50 kilo-electron volts (eV) or less. These are easy to shield against. However, fairly rarely, powerful CME release particles in the 200 mega-electron volt range are produced. Maybe once or twice a decade, particles reach the giga-electron volt ranges are reached. These very rare, super explosions will be discussed below.

Small CME happen all the time, about once a week at solar minimum. Not all of these burst of particles hit Mars, the vast majority will shoot off in some other direction. These small bursts are fairly easy to shield against, the Martian atmosphere giving significant protection, space suits a little protection, and the habitat (with thick shielding for Galactic Cosmic rays giving) excellent protection. Martian colonists will likely ignore these.

The more powerful CME (say from 10 to 200 m-eV) are the 'solar storms' which occasionally feature on the news. A big one happens a few times a year at [[Solar maximum]]. These are very dangerous on Mars. Lacking a magnetic field, the high energy protons can reach the surface, only slightly attenuated by the thin atmosphere. The fairly thick radiation protection on bases will not be enough. It will be deadly dangerous to be caught on the surface during a big solar storm.

With a few hours warning, Martians will go deep in the base, likely several meters underground (or under a few meters of water) to a [[Storm Shelter]] to hide for 2 to 4 hours that the solar storm is active. Since most Coronal Mass Ejections miss Mars (and if they happen at night time they are harmless) this would happen maybe 10 to 30 times in a human lifetime.

A super-CME in the giga-electron Volt range is like the above only worse. You are still safe at night. But if it happened during Martian day, people would go into the shelter and still take a significant dose of radiation. This might happen once in a lifetime, and the storm shelters should be designed to prevent lethal doses of radiation from reaching the people inside.

These only will give an hour or two warning, so people won't have a lot of time to beef up defences. The storm shelter you have is the one you get, so don't skimp in your preparations.

Note that if you are living in a habitat in a lava tube (likely dozens or scores of meters underground) you are completely safe.

===Galactic Cosmic Rays===
More powerful Cosmic rays come in 3 types: [[Galactic Cosmic Rays]], [[Extra Galactic Cosmic Rays]], and [[Extreme Energy Cosmic Rays]]. The latter two are so powerful that no amount of shielding will protect against them, we will accept the dose from these, just as we do on Earth. Fortunately they are so rare, that they don't add that much radiation in a lifetime.

But shielding against Galactic Cosmic Rays is a very difficult task. These range from millions to billions of electron volts, and come from every area of the sky, in a steady trickle all times. They are common enough to add significantly to a Martian's lifetime radiation exposure. Realistically, Martians will have to accept a higher radiation dose than on Earth, they are just too hard to shield against.

A proton or alpha particle cosmic ray of in the upper end of this energy range can pass through more than a meter of aluminium, not counting the effects of [[secondary radiation]]<ref name="Logan">''Operational medicine and health care delivery'' - J.S. Logan, in S.E. Churchill ed. ''Fundamentals of space life sciences, Volume 1'' - 1997, ISBN 0-89464-051-8 pp. 154-156.</ref>.

However, living on a planet halves the radiation dose compared to someone in space or in a space station. (The ground blocks half of the celestial sphere, providing thousands of km of shielding.) Living in a deep valley or near the bottom of a cliff, will also reduce this radiation load.

These particles are very hard to shield against. They are not deflected (enough) by magnetic fields; they punch right thru the Sun's and Earth's magnetic field and hit every part of Earth. A few cm of soil which reduces a gamma ray dose by 8 fold, would actually increase the damage from these particles since the impact will produce [[Secondary radiation]]. They can produce free neutrons (albedo neutrons) which are not directly dangerous, but will cause beta decays which are.

What you really want is thick masses of small atoms (such as hydrogen, helium, lithium, etc.) Water with 2 hydrogens is good. Plastics (with hydrogens clustered around carbon chains) are good. Lithium hydroxide (with both lithium and hydrogen) is good. As a bonus, these sorts of materials are very good at stopping albedo neutrons as well.

With this in mind, it is clear that any Martian colonists would have to take a holistic approach, reducing their radiation exposure at every possible opportunity through shielding and risk-mitigating behaviour.

==Passive shielding==
[[Image:Greenhouse_marsfoundation.jpg|thumb|left|300px|The [[Mars Foundation]] concept for a side-lit greenhouse.]]
Any matter placed between a person (or radiation-sensitive equipment) and a radiation source reduces the amount of radiation they absorb.

[[Mars One]]'s solution is a thick layer of [[regolith]] on top of the settlement modules. An effective shield will require at least several hundred grams of regolith per square centimeter, according to one study.<ref>Slaba, T. C., Mertens, C. J., & Blattnig, S. R. (2013). Radiation Shielding Optimization on Mars. ''NASA/TP–2013-217983.'' Retrieved from https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130012456.pdf</ref> Using a regolith density estimate of 1.4 g/cm3<ref name=":1">Kim, M. Y., Thibeault, S. A., Simonsen, L. C., & Wilson, J. W. Comparison of Martian Meteorites and Martian Regolith as Shield Materials for Galactic Cosmic Rays. ''NASA TP-1998-208724.'' Retrieved from https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980237030.pdf.</ref>, this means the regolith layer would need to be over 2 meters deep. For concrete with an average density of 2.4 g/cm3 the required thickness should be about 40% less.
 
 
For early explorers who will spend about 555 days on Mars, they will simply accept the radiation dose from the space flight. (It will be high enough to potentially permanently age them, but is unlikely to cause immediate [[Radiation sickness]].) However, a meaningful increase in radiation protection could be gained, by stacking sand bags with local soil above their habitat. (Assuming there is a hard shell that could support sandbags and not an inflatable structure.) However, all Extra Vehicular Activities (EVAs) carry risk, and hours spent filling sand bags and lifting them to the top of the habitat and stacking them is somewhat dangerous. There should be a scientific study to see if the risk of all these EVAs are worth the gain in radiation protection. See [[Future research]].
 
===Protection from Electromagnetic Radiation===
The attenuation of radiation follows the Beer Lamberth law.<ref>https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuationCoef.htm</ref>

Ix=Io*e-ux
{| class="wikitable"
|Where:
|Ix
|=
|the intensity of photons transmitted across some distance x
|-
|
|I0
|=
|the initial intensity of photons (or radiation in general)
|-
|
|s
|=
|a proportionality constant that reflects the total probability of a photon being scattered or absorbed ''(TBC)''
|-
|
|µ
|=
|the linear attenuation coefficient
|-
|
|x
|=
|distance traveled (thickness of material)
|}
{| class="wikitable"
|+Linear Attenuation Coefficients (in cm-1) for a range of materials at gamma-ray energies of 100, 200 and 500 keV.<ref name=":0">https://en.wikibooks.org/wiki/Basic_Physics_of_Nuclear_Medicine/Attenuation_of_Gamma-Rays</ref>
!Absorber
!100 keV
!200 keV
!500 keV
|-
|'''Air'''
|0.000195
|0.000159
|0.000112
|-
|'''Water'''
|0.167
|0.136
|0.097
|-
|'''Carbon'''
|0.335
|0.274
|0.196
|-
|'''Wood *'''
|0.60
|0.20
|0.08
|-
|'''[[Superwood]]*'''
|2.40
|0.80
|0.32
|-
|'''Aluminium'''
|0.435
|0.324
|0.227
|-
|'''Iron'''
|2.72
|1.09
|0.655
|-
|'''Copper'''
|3.8
|1.309
|0.73
|-
|'''Lead'''
|59.7
|10.15
|1.64
|}

'* Wood varies by species (hard woods are better) and water content (wet wood is better), but the values given are typical. [[Superwood]] is 5 times denser, but has less water in it, so the wood value was multiplied by 4 as a conservative estimate. Wood and superwood provide excellent alpha and beta protection, and the large number of light atoms will help stop secondary radiation and albedo neutrons.

The linear attenuation coefficient µ is not commonly found in the literature, the mass attenuation coefficient µm is usually used instead. The coefficient is also dependent on the type of radiation, so a complete solution for radiation protection requires multiple analysis of the type of radiation to be protected against.

Conversion is quite simple as:

µ=µm*density of the material

List of mass attenuation coefficients<ref>https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuationCoef.htm</ref> can be found at the NIST website. https://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html

Another common way of evaluating radiation shielding is to use the '''half value,''' that expresses the thickness of absorbing material which is needed to reduce the incident radiation intensity by a factor of two, or Ix=Io / 2.

The Half Value Layer for a range of absorbers is listed in the following table for three gamma-ray energies:
{| class="wikitable"
|+Half Value Layers (in cm) for a range of materials at gamma-ray energies of 100, 200 and 500 keV.<ref name=":0" />
!Absorber
!100 keV
!200 keV
!500 keV
|-
|'''Air'''
|3555
|4359
|6189
|-
|'''Water'''
|4.15
|5.1
|7.15
|-
|'''Carbon'''
|2.07
|2.53
|3.54
|-
|'''Wood*'''
|17.0
|20.5
|28.5
|-
|'''Aluminium'''
|1.59
|2.14
|3.05
|-
|'''Iron'''
|0.26
|0.64
|1.06
|-
|'''Copper'''
|0.18
|0.53
|0.95
|-
|'''Lead'''
|0.012
|0.068
|0.42
|}
'* The radiation protection of wood varies by species (hard woods are better) and water content (more water is good). These are typical values.<ref>https://www.sciencedirect.com/science/article/abs/pii/S0969806X21005776</ref> Superwood is 5 times the density, so the Wood values would be 1/4 to 1/5.

The first point to note is that the Half Value Layer decreases as the atomic number increases. For example, the value for air at 100 keV is about 35 meters and it decreases to just 0.12 mm for lead at this energy. In other words 35 m of air is needed to reduce the intensity of a 100 keV gamma-ray beam by a factor of two whereas just 0.12 mm of lead can do the same thing. The Half Value Layer increases with increasing gamma-ray energy. For example, from 0.18 cm for copper at 100 keV to about 1 cm at 500 keV.

Also note that this table is for gamma rays. Gamma rays in the kilo-electron volt ranger are not the big problem. Giga-electron volt cosmic rays are.

===Protection from Particulate Radiation===
On Earth, particulate radiation is often easily addressed because the particles have low enough energies that they can be stopped by a thin shield. In space and on the surface of Mars, shielding needs to account for high-energy particles. When it comes to particulate radiation, the effectiveness of shielding increases with the mass of the shielding and decreases with the atomic mass of the elements used for the shielding. The reason that low-atomic-mass elements are advantageous is that they generate less secondary radiation when impacted by particles.<ref>Wilson JW, Cucinotta FA, Thibeault SA, Kim M, Shinn JL, Badavi FF. Radiation Shielding Design Issues. In *Shielding Strategies for Human Space Exploration* (Chapter 7). <nowiki>http://hdl.handle.net/2060/19980137598</nowiki></ref>

For example, 1kg of [[hydrogen]] offers more protection then 1kg of [[aluminium]].

2kg of aluminium offers more protection than 1kg of aluminium.

Finally, 1kg of hydrogen offers more protection than 2kg of aluminium.<ref>''Radiation biology'' - J.R. Letaw, in S.E. Churchill ed. ''Fundamentals of space life sciences, Volume 1'' - 1997, ISBN 0-89464-051-8 pp. 16-17.</ref>

Also, particles interact with atomic nuclei, while electromagnetic radiation interacts with electrons. So while for electromagnetic radiations the effectiveness of shielding increases with the number of electrons, and therefore with heavier atoms that have more electrons, for particles the effectiveness of radiation protection increases with the number of nuclei per volume, and lighter materials such as hydrogen have more nuclei per volume.

This study looked at the cosmic ray protection of Martian regolith. <ref>https://www.sciencedirect.com/science/article/abs/pii/S0032063322001039 | Effectiveness of Martian Regolith as a Radiation Shield.</ref> It found that 1 meter of regolith would reduce cosmic radiation by 41%. So two meters would reduce the radiation load down to about 1/3.

===Possible Shielding Materials===
{| class="wikitable"
|+Comparison of Material Options
!Material
!Advantages
!Disadvantages
|-
|Metal
|Efficiency of using structural material for incidental shielding benefit; some metals block EM radiation very well
|Secondary radiation<ref name=":2">Parker LJ. (2016). Human radiation exposure tolerance and expected exposure during colonization of the Moon and Mars. <nowiki>http://www.marspapers.org/paper/Parker_2016_1.pdf</nowiki></ref>
|-
|Plastic
|High hydrogen content<ref name=":2" />
|Less structural utility than metal
|-
|Water
|High hydrogen content<ref name=":2" />
|Liquid
|-
|Liquid hydrogen
|Pure hydrogen
|Cryogenic liquid
|-
|Lithium Hydroxide (LiOH)
|Contains Li & H, both small molecules. Denser than water, best neutron absorber / kg.
|Needs to be kept dry
|-
|Regolith
|Obtainable through ISRU
|Large thickness required for thorough shielding<ref>James G, Chamitoff G, and Barker D. Resource Utilization and Site Selection for a Self-Sufficient Martian Outpost. NASA/TM-98-206538. <nowiki>http://hdl.handle.net/2060/19980147990</nowiki></ref>
|-
|Regolith plus epoxy
|Mostly obtainable through ISRU; greater hydrogen content than regolith alone; more durable and easier to shape than regolith alone<ref name=":1" />
|More complex to implement than regolith alone
|-
|Boron nitride nanotubes
|Low atomic numbers; boron absorbs secondary neutrons well compared to other elements; possible use as both shielding and structural material<ref>Tiano, Amanda L, et al. “Boron Nitride Nanotube: Synthesis and Applications.” NTRS Document ID 20140004051, 2014. <nowiki>http://hdl.handle.net/2060/20140004051</nowiki></ref>; hydrogen could be stored in or bonded to nanotubes to improve shielding<ref>Thibeault SA, Fay CC, Lowther SE, Earle KD, Sauti G, Kang JH, Park C, McMullen AM. (2012). ''Radiation Shielding Materials Containing Hydrogen, Boron, and Nitrogen: Systematic Computational and Experimental Study. Phase I''. <nowiki>https://ntrs.nasa.gov/search.jsp?R=20160010096</nowiki></ref>
|Difficult to manufacture
|}

==Active shielding==
For a Martian stay of 18 months, elaborate precautions are not needed. If people are going to spend many years on Mars, active shielding becomes more important.

Active shielding against radiation involves a man-made electromagnetic field which deflects ionized particles in a similar manner to the Earth's magnetic field. Several different types of active shield have been proposed, including electric field, magnetic field, and plasma shield designs. Such fields might require infeasible amounts of energy to generate (a Mars temperature superconductor would be useful here). Such a magnetic shield could also pose a risk to anyone approaching the craft or base, as it would create bands of trapped particles similar to the Van Allen belts.<ref name="Logan" /> However, the radiation exposure might be low, as traversing the magnetic shield should be a very brief event. (If these 'van Allen belts' cross into the ground, charged particles would be absorbed and the issue won't arise. For a fixed base, one pole could be in the air, and one underground.)

It might be possible to situate a base in such a location that one of the residual Martian magnetic fields offers a net benefit. Care should certainly be taken not to situate it where the fields concentrate radiation.

Also, it might be possible (assuming one could generate the required magnetic field in some way) to have the radiation belts of the habitat pass through some sort of physical barrier, which scrubs them of particles.

Note that magnetic shields are good against lower energy particles, they are of little help against high energy [[Cosmic Radiation |Cosmic Rays]]. However, every bit helps, so these ideas may help in a 'layered defence'.

===Design concepts===

====Protection during transit to Mars====
A straightforward approach to designing an active shield would be to surround the protected space with a spherical enclosure made of a conducting metal that is held at a positive electric charge. Solar radiation both consist of positively charged particles, which would be repelled by the positively charged shield. Positively charged cosmic rays have too much energy to be deflected by such a shield. To protect science experiments from cosmic rays on Earth, scientists go deep underground. They do not make a small metal charged dome.

One drawback to this concept is that the shield would attract free electrons and accelerate them, creating secondary radiation when they impact the shield. These electrons would also drain the shield's positive charge, increasing the power needed to maintain it.

This led to the idea of adding a negatively-charged outer shield to keep electrons away from the positively-charged inner shield. However, this introduces its own problems: the mass requirement increases substantially, and the two shields must be tightly fixed in precise concentric positions, because any asymmetry would result in a strong attractive force that would pull the two shields together.

More recent concepts have moved away from the idea of enclosing the protected space in a charged shell. A 2006 paper described a design in which trusses extended out from a spacecraft in the x, y, and z directions. The trusses are used to suspend individual sphere-shaped charge centers: positive charges are held 50 m from the spacecraft, and negative charges are 160 m out. These charges are meant to generate an electric field sufficient to deflect incoming ions approaching from any direction. Despite the elimination of the enclosing shells, the authors concluded that the mass requirement was still too high for this design to be practical.<ref>Smith JG, Smith T, Williams M, Youngquist R, and Mendell W. Potential Polymeric Sphere Construction Materials for a Spacecraft Electrostatic Shield. NASA/TM—2006–214302. <nowiki>https://ntrs.nasa.gov/search.jsp?R=20060013423</nowiki></ref>

In 2011, a concept using toroidal (doughnut-shaped) rings as charge centers was studied. Three conductive rings, each with a 45 m radius, are positioned with their axes of symmetry along the x, y, and z axes, and held at a positive voltage. In addition, six negatively-charged spheres are suspended from trusses, 160 m out. For comparison, the researchers ran simulations of this design and a similar design that used only spherical charge centers. Both designs offered good protection from SPE. The toroid design was more effective against GCR. Ring thicknesses of 1, 5, and 10 m were simulated, showing effectiveness against GCR increasing as a function of thickness. To minimize the mass of the shield, the use of an electrostatically inflated membrane structure was proposed for the charge centers: instead of using a rigid metal, the charge center surfaces would be made of a flexible membrane with a conductive coating. Unlike a balloon that is inflated by filling it with a gas, this type of structure would inflate to its intended shape when electrically charged because of repulsion between like charges distributed across the membrane.<ref>Tripathi RK. 2016. ''Meeting the Grand Challenge of Protecting Astronauts Health: Electrostatic Active Space Radiation Shielding for Deep Space Missions''. <nowiki>https://ntrs.nasa.gov/search.jsp?R=20160010094</nowiki></ref>

A 2012 paper by Westover et al. described a design that used a magnetic field. The field could be generated by multiple layers of superconducting material wrapped around a cylindrical spacecraft in double-helix-shaped coils. Their simulations of this design found that a large, weak field was easier to generate than a small, intense field of equivalent shielding effectiveness. They also examined another design, a cylindrical spaceship surrounded by six exterior solenoids, each centered on a separate central axis (unlike the double helix design, in which the magnetic coil and the spacecraft are co-axial). To reduce the magnetic field inside the spacecraft, a compensation coil, with a current moving in the opposite direction, would surround it. Overall this design was judged to be preferable to the double helix design. Shortcomings of the latter design include the fact that the multiple layers of coils make it difficult to design a shield that can be compacted for launch and later expanded in space. In addition, it would be more difficult to block the magnetic field intruding into the spacecraft because the field strength varies more at different points within the spacecraft; problems might also arise from forces created by the magnetic field acting on the shield or spacecraft.<ref>Westover SC, Meinke RB, BurgerWJ, Van Sciver S, Washburn S, et al. 2012. Magnet Architectures and Active Radiation Shielding Study. <nowiki>https://ntrs.nasa.gov/search.jsp?R=20190002579</nowiki></ref>

==== Minimagnetosphere ====
A minimagnetosphere<ref>https://www.researchgate.net/publication/325285302_Can_artificial_miniature_magnetospheres_be_used_to_protect_spacecraft</ref> may be a viable alternative to the strong local magnetic fields described above for spacecraft. Minimagnetospheres have been detected on the Moon<ref>https://www.universetoday.com/79781/moons-mini-magnetosphere/</ref>, and indications show that they have modified the interactions of the solar wind with the surface. The concept uses an electric field rather than the magnetic field. An plasma cloud is created around the vehicle by injecting charged particles and kept in place using a small superconducting magnet at the vehicle. The electrical charge can deflect particles due to the large size on the minimagnetosphere, that can reach a number of km in diameter. This solution might be particularly applicable to a cycler type vehicle.

The mass required to generate such a mini-magnetosphere must be low, or people will do without and accept the modest radiation exposure for such a trip.

==Types of radiation shielding and risk to tissue==
{| class="wikitable" style="margin:auto"
|+ Radiation Weighting Factors & Best Shielding Materials
|-
! Type of Radiation !! Tissue Weighting Factor !! Ideal Shielding Material
|-
| Photons (all energies) || 1 || Dense materials and metals
|-
| Electrons & muons (all energies) || 1 || Small atoms (e.g. H2, He, Li.)
|-
| Protons & charged pions || 2 || Small atoms
|-
| Alpha particles || 20 || Small atoms
|-
| Fission fragments & heavy ions || 20 || Thick or dense matter
|-
| Neutrons || ~10 but varies || Small atoms (e.g. H2, He, Li.)
|}

Note that small particles about the same mass of a proton or helium nucleus, are best slowed down by bouncing off of other small atoms. (As much energy as possible is removed by kinetic energy of the atom it bumps into.) Where as gamma rays, and heavy nucleus want dense matter, or metals to stop them. However, cosmic rays with obscene amounts of energy will break into [[Secondary radiation]] when hitting dense matter, and the secondary radiation is more dangerous than the cosmic rays themselves. Finally, cosmic rays can create free neutrons, which we want to slow with light atoms. See [[Radiation sickness]] for more information on these weighting factors.

So ideal radiation protection might have a couple meters (or more) or soil, then a metal wall a few cm thick, then some material with small atoms like water, plastic, or lithium hydroxide (LiOH). A storm shelter is similar, but might have 5 to 8 meters of soil.

==Risk-mitigating behaviour==
The possible sources of radiation on Mars are man-made sources, (such as nuclear reactors or medical equipment), [[solar radiation]], [[galactic cosmic radiation]] and naturally occurring [[radioactive elements]] on Mars.

Living in a deep [[Lava tube]] takes care of the problem. However many sites on Mars do not have these. For those areas...

Possible behavioral choices which minimize the risk from these include:

* Avoiding daytime [[EVA]] during solar storms.
* Working preferentially close to natural or man-made objects, such as habitats, rovers or cliffs which provide additional (if not omni-directional) shielding. For example, if a habitat was at the base of a tall cliff, then half of the cosmic rays, and half the high energy solar radiation would be shielded from.
* Entering a [[storm shelter]] when there is a high-radiation risk from [[Solar Cosmic Rays|solar particle events]].
* Placing blocks of ice around the outer walls of the base is an excellent radiation shield.
* Sandbags on the roof of the habitat provide excellent solar protection, and minor protection verses high energy cosmic rays.
* Sleeping in areas with higher radiation protection. e.g. under bladders at ceiling filled with local water.

==Example of using shielding and behavior to reduce radiation dosage==
We can combine passive shielding with risk mitigating behavior to achieve low radiation exposure but still allow for some views of the exterior through windows. For example:

*Martian background average radiation is 240-300 mSv per year<ref>NASA, Tony C. Slaba, Christopher J. Mertens, and Steve R. Blattnig ''Radiation Shielding Optimization on Mars'' , <nowiki>https://spaceradiation.larc.nasa.gov/nasapapers/NASA-TP-2013-217983.pdf</nowiki>, Apr 2013.</ref>.
*If you sleep in a radiation shielded space such as underground rooms with a compacted regolith cover several meters thick, 8/24 hours, then the dose would go down by 1/3, to 160-200 mSv per year.
*If you spend most of your living (work, study) time in a radiation shielded space, then your dose becomes another 1/3 less, or 80 to 100 mSv.
*With overhangs and a radiation proof roof, 70% of the incident radiation to a space close to windows can be stopped by geometries, then the dose is down to 20 to 25 mSv. this is about the 20 mSv per year for a 5 year period that is recommended for radiation workers. <ref>https://www.cnsc-ccsn.gc.ca/eng/resources/radiation/radiation-doses/</ref>
*Part of the surface dose on Mars is solar proton events (SPE). These are predictable and detectable, and a large settlement will mostly be built of shielded areas. So during Solar Proton Events you should stay away from the windows. This behavior might reduce the yearly radiation load another 25%, down to 15-18 mSv per year. (If people go into a storm shelter for the worst solar storms, the dose would be reduced further.)
*What is the portion of the dosage from SPE? I have a weak reference that puts this at 30%. If correct, then the radiation load from large windows under a radiation proof ceiling is acceptable.
*Mars should be low in radon because it seems to be low in [[Thorium]] and, by analogy, Uranium as well. However, the habitats are totally enclosed spaces and radon generated by radioactive decay of naturally occurring uranium in the soil might accumulate. As 1 to 3 mSv on Earth comes from atmospheric radon<ref>http://nuclearsafety.gc.ca/eng/resources/radiation/introduction-to-radiation/radiation-doses.cfm</ref>, this part of the yearly load might go away, just as it might need to be mitigated if radon accumulates in the enclosed habitats.
*Even just 1/2 to 1 inches of glass reduces radiation dosage significantly.

If the above is correct, then large windows are not really an issue. Geodesic glass domes over public spaces might be a poor choice, unless there is an understanding that you don't spend more than 2 to 4 hours per day under them.

==Bibliography==
"The Case For Mars", by Robert Zubrin, Simon and Schuster, ISBN 978-1-4516-0811-3. Chapter 5 has a very nice breakdown of radiation exposure from Conjunction and Opposition class missions.

==References==

<references />

[[Category:Radiation Protection]]

Radiation shielding

2025-11-18T02:12:45Z

RichardWSmith: /* General */ Adding a link.

[[Image:WaterShieldGreenhouse.png|thumb|right|300px|Water-shield Greenhouse Concept]]

Early explorers will simply accept the radiation dose for the 2.5 year round trip (which should give approximately a 1% lifetime increase of a fatal cancer). See "The Case for Mars, chapter 5. However, radiation protection becomes much more of a concern for long durations habitats where people will live on Mars for years or decades. In such cases, thick shielding (of soil or ice), exotic modern materials, or electro-static or magnetic shields become more sensible. Radiation from the natural radioactive elements in the soil should be approximately equal to Earth doses. Radiation from Solar and Cosmic rays will be stronger, with the latter being much more difficult to shield against. See [[Cosmic Radiation]].

==General==
The average radiation on Earth is very low at 3 to 6.2 mSv / year depending on location (https://en.wikipedia.org/wiki/Background_radiation). However, in some areas of the city Ramsar, Iran, the radiation level is 260 mSv / year, with no measured increase in health problems. Thus the radiation safety levels in many countries on Earth may be too conservative. Mars starts at 200 to 250 mSv / year. The discussion below assumes we are aiming for 6.2 mSv, but there shouldn't be a disaster if this is missed, even by a significant amount.

Also note that sudden radiation doses are much more dangerous than a small steady dose over a long period of time. A solar storm is FAR more dangerous than the slow and steady trickle of cosmic rays.

Note that there is significant evidence that very low levels of radiation is healthy. See [[Radiation Hormesis]].

Long term settlers will need to shield against 3 types of radiation: [[Electromagnetic radiation]], [[Solar Cosmic Rays]], and more powerful [[Cosmic radiation | Cosmic Rays]]. Of the last, [[Galactic Cosmic Rays]] are by far the most common fraction of these.

===Electromagnetic Radiation===
The EM spectrum consists of radio waves, microwaves, heat, visible light, ultraviolet light, x-rays, and gamma rays. The low energy end of this spectrum are waves that pass thru us and are harmless. As the frequency becomes shorter, the energy of each particle of light increases. The high energy EM waves act progressively more like particles and can damage tissue. UV light can give sun burn and damage surface tissue, and X-rays and gamma rays can do deep tissue damage.

EM waves are stopped by atoms with many electrons. Thus heavy metals strongly mitigate them. Air, dust, soil, and water all reduce their effect. The only common dangerous EM radiation on Mars is Ultraviolet (UV) light. On Earth, the stronger UV rays are stopped by our ozone layer, on Mars, 95% of the UV light reaches the ground, sterilizing the top layers of soil. (Also note that Mars gets about half the UV light that Earth does, because Mars is further from the sun.)

UV light can make plastics brittle, which may not be a concern for an 18 month stay, but it will be a problem for long term settlers. Metals and certain composites are resistant to UV light. The space suits and habitat materials will be designed to shield against UV rays.

X-rays and gamma rays are much more penetrating. For example, compressed Martian soil will halve the amount of gamma rays for each 9.1 cm of soil. Water halves the gamma ray flux for every 18 cm. <ref>https://www.imagesco.com/geiger/lead-shielding-guide.html</ref>

That said, the sun produces few x-rays and almost no gamma rays. A tiny amount come from space. The amount is low and can be basically ignored.

If a Mars base has a local source of gamma rays, say a nuclear reactor, it must be properly shielded, far away, or both.

===Solar Cosmic Rays===
These are not the solar wind, but rather high energy particles (mostly protons) which are produced in [[Coronal Mass Ejections]]. Most are low energy, 50 kilo-electron volts (eV) or less. These are easy to shield against. However, fairly rarely, powerful CME release particles in the 200 mega-electron volt range are produced. Maybe once or twice a decade, particles reach the giga-electron volt ranges are reached. These very rare, super explosions will be discussed below.

Small CME happen all the time, about once a week at solar minimum. Not all of these burst of particles hit Mars, the vast majority will shoot off in some other direction. These small bursts are fairly easy to shield against, the Martian atmosphere giving significant protection, space suits a little protection, and the habitat (with thick shielding for Galactic Cosmic rays giving) excellent protection. Martian colonists will likely ignore these.

The more powerful CME (say from 10 to 200 m-eV) are the 'solar storms' which occasionally feature on the news. A big one happens a few times a year at [[Solar maximum]]. These are very dangerous on Mars. Lacking a magnetic field, the high energy protons can reach the surface, only slightly attenuated by the thin atmosphere. The fairly thick radiation protection on bases will not be enough. It will be deadly dangerous to be caught on the surface during a big solar storm.

With a few hours warning, Martians will go deep in the base, likely several meters underground (or under a few meters of water) to a [[Storm Shelter]] to hide for 2 to 4 hours that the solar storm is active. Since most Coronal Mass Ejections miss Mars (and if they happen at night time they are harmless) this would happen maybe 10 to 30 times in a human lifetime.

A super-CME in the giga-electron Volt range is like the above only worse. You are still safe at night. But if it happened during Martian day, people would go into the shelter and still take a significant dose of radiation. This might happen once in a lifetime, and the storm shelters should be designed to prevent lethal doses of radiation from reaching the people inside.

These only will give an hour or two warning, so people won't have a lot of time to beef up defences. The storm shelter you have is the one you get, so don't skimp in your preparations.

Note that if you are living in a habitat in a lava tube (likely dozens or scores of meters underground) you are completely safe.

===Galactic Cosmic Rays===
More powerful Cosmic rays come in 3 types: [[Galactic Cosmic Rays]], [[Extra Galactic Cosmic Rays]], and [[Extreme Energy Cosmic Rays]]. The latter two are so powerful that no amount of shielding will protect against them, we will accept the dose from these, just as we do on Earth. Fortunately they are so rare, that they don't add that much radiation in a lifetime.

But shielding against Galactic Cosmic Rays is a very difficult task. These range from millions to billions of electron volts, and come from every area of the sky, in a steady trickle all times. They are common enough to add significantly to a Martian's lifetime radiation exposure. Realistically, Martians will have to accept a higher radiation dose than on Earth, they are just too hard to shield against.

A proton or alpha particle cosmic ray of in the upper end of this energy range can pass through more than a meter of aluminium, not counting the effects of [[secondary radiation]]<ref name="Logan">''Operational medicine and health care delivery'' - J.S. Logan, in S.E. Churchill ed. ''Fundamentals of space life sciences, Volume 1'' - 1997, ISBN 0-89464-051-8 pp. 154-156.</ref>.

However, living on a planet halves the radiation dose compared to someone in space or in a space station. (The ground blocks half of the celestial sphere, providing thousands of km of shielding.) Living in a deep valley or near the bottom of a cliff, will also reduce this radiation load.

These particles are very hard to shield against. They are not deflected (enough) by magnetic fields; they punch right thru the Sun's and Earth's magnetic field and hit every part of Earth. A few cm of soil which reduces a gamma ray dose by 8 fold, would actually increase the damage from these particles since the impact will produce [[Secondary radiation]]. They can produce free neutrons (albedo neutrons) which are not directly dangerous, but will cause beta decays which are.

What you really want is thick masses of small atoms (such as hydrogen, helium, lithium, etc.) Water with 2 hydrogens is good. Plastics (with hydrogens clustered around carbon chains) are good. Lithium hydroxide (with both lithium and hydrogen) is good. As a bonus, these sorts of materials are very good at stopping albedo neutrons as well.

With this in mind, it is clear that any Martian colonists would have to take a holistic approach, reducing their radiation exposure at every possible opportunity through shielding and risk-mitigating behaviour.

==Passive shielding==
[[Image:Greenhouse_marsfoundation.jpg|thumb|left|300px|The [[Mars Foundation]] concept for a side-lit greenhouse.]]
Any matter placed between a person (or radiation-sensitive equipment) and a radiation source reduces the amount of radiation they absorb.

[[Mars One]]'s solution is a thick layer of [[regolith]] on top of the settlement modules. An effective shield will require at least several hundred grams of regolith per square centimeter, according to one study.<ref>Slaba, T. C., Mertens, C. J., & Blattnig, S. R. (2013). Radiation Shielding Optimization on Mars. ''NASA/TP–2013-217983.'' Retrieved from https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130012456.pdf</ref> Using a regolith density estimate of 1.4 g/cm3<ref name=":1">Kim, M. Y., Thibeault, S. A., Simonsen, L. C., & Wilson, J. W. Comparison of Martian Meteorites and Martian Regolith as Shield Materials for Galactic Cosmic Rays. ''NASA TP-1998-208724.'' Retrieved from https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980237030.pdf.</ref>, this means the regolith layer would need to be over 2 meters deep. For concrete with an average density of 2.4 g/cm3 the required thickness should be about 40% less.
 
 
For early explorers who will spend about 555 days on Mars, they will simply accept the radiation dose from the space flight. (It will be high enough to potentially permanently age them, but is unlikely to cause immediate [[Radiation sickness]].) However, a meaningful increase in radiation protection could be gained, by stacking sand bags with local soil above their habitat. (Assuming there is a hard shell that could support sandbags and not an inflatable structure.) However, all Extra Vehicular Activities (EVAs) carry risk, and hours spent filling sand bags and lifting them to the top of the habitat and stacking them is somewhat dangerous. There should be a scientific study to see if the risk of all these EVAs are worth the gain in radiation protection. See [[Future research]].
 
===Protection from Electromagnetic Radiation===
The attenuation of radiation follows the Beer Lamberth law.<ref>https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuationCoef.htm</ref>

Ix=Io*e-ux
{| class="wikitable"
|Where:
|Ix
|=
|the intensity of photons transmitted across some distance x
|-
|
|I0
|=
|the initial intensity of photons (or radiation in general)
|-
|
|s
|=
|a proportionality constant that reflects the total probability of a photon being scattered or absorbed ''(TBC)''
|-
|
|µ
|=
|the linear attenuation coefficient
|-
|
|x
|=
|distance traveled (thickness of material)
|}
{| class="wikitable"
|+Linear Attenuation Coefficients (in cm-1) for a range of materials at gamma-ray energies of 100, 200 and 500 keV.<ref name=":0">https://en.wikibooks.org/wiki/Basic_Physics_of_Nuclear_Medicine/Attenuation_of_Gamma-Rays</ref>
!Absorber
!100 keV
!200 keV
!500 keV
|-
|'''Air'''
|0.000195
|0.000159
|0.000112
|-
|'''Water'''
|0.167
|0.136
|0.097
|-
|'''Carbon'''
|0.335
|0.274
|0.196
|-
|'''Wood *'''
|0.60
|0.20
|0.08
|-
|'''[[Superwood]]*'''
|2.40
|0.80
|0.32
|-
|'''Aluminium'''
|0.435
|0.324
|0.227
|-
|'''Iron'''
|2.72
|1.09
|0.655
|-
|'''Copper'''
|3.8
|1.309
|0.73
|-
|'''Lead'''
|59.7
|10.15
|1.64
|}

'* Wood varies by species (hard woods are better) and water content (wet wood is better), but the values given are typical. [[Superwood]] is 5 times denser, but has less water in it, so the wood value was multiplied by 4 as a conservative estimate. Wood and superwood provide excellent alpha and beta protection, and the large number of light atoms will help stop secondary radiation and albedo neutrons.

The linear attenuation coefficient µ is not commonly found in the literature, the mass attenuation coefficient µm is usually used instead. The coefficient is also dependent on the type of radiation, so a complete solution for radiation protection requires multiple analysis of the type of radiation to be protected against.

Conversion is quite simple as:

µ=µm*density of the material

List of mass attenuation coefficients<ref>https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuationCoef.htm</ref> can be found at the NIST website. https://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html

Another common way of evaluating radiation shielding is to use the '''half value,''' that expresses the thickness of absorbing material which is needed to reduce the incident radiation intensity by a factor of two, or Ix=Io / 2.

The Half Value Layer for a range of absorbers is listed in the following table for three gamma-ray energies:
{| class="wikitable"
|+Half Value Layers (in cm) for a range of materials at gamma-ray energies of 100, 200 and 500 keV.<ref name=":0" />
!Absorber
!100 keV
!200 keV
!500 keV
|-
|'''Air'''
|3555
|4359
|6189
|-
|'''Water'''
|4.15
|5.1
|7.15
|-
|'''Carbon'''
|2.07
|2.53
|3.54
|-
|'''Wood*'''
|17.0
|20.5
|28.5
|-
|'''Aluminium'''
|1.59
|2.14
|3.05
|-
|'''Iron'''
|0.26
|0.64
|1.06
|-
|'''Copper'''
|0.18
|0.53
|0.95
|-
|'''Lead'''
|0.012
|0.068
|0.42
|}
'* The radiation protection of wood varies by species (hard woods are better) and water content (more water is good). These are typical values.<ref>https://www.sciencedirect.com/science/article/abs/pii/S0969806X21005776</ref> Superwood is 5 times the density, so the Wood values would be 1/4 to 1/5.

The first point to note is that the Half Value Layer decreases as the atomic number increases. For example, the value for air at 100 keV is about 35 meters and it decreases to just 0.12 mm for lead at this energy. In other words 35 m of air is needed to reduce the intensity of a 100 keV gamma-ray beam by a factor of two whereas just 0.12 mm of lead can do the same thing. The Half Value Layer increases with increasing gamma-ray energy. For example, from 0.18 cm for copper at 100 keV to about 1 cm at 500 keV.

Also note that this table is for gamma rays. Gamma rays in the kilo-electron volt ranger are not the big problem. Giga-electron volt cosmic rays are.

===Protection from Particulate Radiation===
On Earth, particulate radiation is often easily addressed because the particles have low enough energies that they can be stopped by a thin shield. In space and on the surface of Mars, shielding needs to account for high-energy particles. When it comes to particulate radiation, the effectiveness of shielding increases with the mass of the shielding and decreases with the atomic mass of the elements used for the shielding. The reason that low-atomic-mass elements are advantageous is that they generate less secondary radiation when impacted by particles.<ref>Wilson JW, Cucinotta FA, Thibeault SA, Kim M, Shinn JL, Badavi FF. Radiation Shielding Design Issues. In *Shielding Strategies for Human Space Exploration* (Chapter 7). <nowiki>http://hdl.handle.net/2060/19980137598</nowiki></ref>

For example, 1kg of [[hydrogen]] offers more protection then 1kg of [[aluminium]].

2kg of aluminium offers more protection than 1kg of aluminium.

Finally, 1kg of hydrogen offers more protection than 2kg of aluminium.<ref>''Radiation biology'' - J.R. Letaw, in S.E. Churchill ed. ''Fundamentals of space life sciences, Volume 1'' - 1997, ISBN 0-89464-051-8 pp. 16-17.</ref>

Also, particles interact with atomic nuclei, while electromagnetic radiation interacts with electrons. So while for electromagnetic radiations the effectiveness of shielding increases with the number of electrons, and therefore with heavier atoms that have more electrons, for particles the effectiveness of radiation protection increases with the number of nuclei per volume, and lighter materials such as hydrogen have more nuclei per volume.

This study looked at the cosmic ray protection of Martian regolith. <ref>https://www.sciencedirect.com/science/article/abs/pii/S0032063322001039 | Effectiveness of Martian Regolith as a Radiation Shield.</ref> It found that 1 meter of regolith would reduce cosmic radiation by 41%. So two meters would reduce the radiation load down to about 1/3.

===Possible Shielding Materials===
{| class="wikitable"
|+Comparison of Material Options
!Material
!Advantages
!Disadvantages
|-
|Metal
|Efficiency of using structural material for incidental shielding benefit; some metals block EM radiation very well
|Secondary radiation<ref name=":2">Parker LJ. (2016). Human radiation exposure tolerance and expected exposure during colonization of the Moon and Mars. <nowiki>http://www.marspapers.org/paper/Parker_2016_1.pdf</nowiki></ref>
|-
|Plastic
|High hydrogen content<ref name=":2" />
|Less structural utility than metal
|-
|Water
|High hydrogen content<ref name=":2" />
|Liquid
|-
|Liquid hydrogen
|Pure hydrogen
|Cryogenic liquid
|-
|Lithium Hydroxide (LiOH)
|Contains Li & H, both small molecules. Denser than water, best neutron absorber / kg.
|Needs to be kept dry
|-
|Regolith
|Obtainable through ISRU
|Large thickness required for thorough shielding<ref>James G, Chamitoff G, and Barker D. Resource Utilization and Site Selection for a Self-Sufficient Martian Outpost. NASA/TM-98-206538. <nowiki>http://hdl.handle.net/2060/19980147990</nowiki></ref>
|-
|Regolith plus epoxy
|Mostly obtainable through ISRU; greater hydrogen content than regolith alone; more durable and easier to shape than regolith alone<ref name=":1" />
|More complex to implement than regolith alone
|-
|Boron nitride nanotubes
|Low atomic numbers; boron absorbs secondary neutrons well compared to other elements; possible use as both shielding and structural material<ref>Tiano, Amanda L, et al. “Boron Nitride Nanotube: Synthesis and Applications.” NTRS Document ID 20140004051, 2014. <nowiki>http://hdl.handle.net/2060/20140004051</nowiki></ref>; hydrogen could be stored in or bonded to nanotubes to improve shielding<ref>Thibeault SA, Fay CC, Lowther SE, Earle KD, Sauti G, Kang JH, Park C, McMullen AM. (2012). ''Radiation Shielding Materials Containing Hydrogen, Boron, and Nitrogen: Systematic Computational and Experimental Study. Phase I''. <nowiki>https://ntrs.nasa.gov/search.jsp?R=20160010096</nowiki></ref>
|Difficult to manufacture
|}

==Active shielding==
For a Martian stay of 18 months, elaborate precautions are not needed. If people are going to spend many years on Mars, active shielding becomes more important.

Active shielding against radiation involves a man-made electromagnetic field which deflects ionized particles in a similar manner to the Earth's magnetic field. Several different types of active shield have been proposed, including electric field, magnetic field, and plasma shield designs. Such fields might require infeasible amounts of energy to generate (a Mars temperature superconductor would be useful here). Such a magnetic shield could also pose a risk to anyone approaching the craft or base, as it would create bands of trapped particles similar to the Van Allen belts.<ref name="Logan" /> However, the radiation exposure might be low, as traversing the magnetic shield should be a very brief event. (If these 'van Allen belts' cross into the ground, charged particles would be absorbed and the issue won't arise. For a fixed base, one pole could be in the air, and one underground.)

It might be possible to situate a base in such a location that one of the residual Martian magnetic fields offers a net benefit. Care should certainly be taken not to situate it where the fields concentrate radiation.

Also, it might be possible (assuming one could generate the required magnetic field in some way) to have the radiation belts of the habitat pass through some sort of physical barrier, which scrubs them of particles.

Note that magnetic shields are good against lower energy particles, they are of little help against high energy [[Cosmic Radiation |Cosmic Rays]]. However, every bit helps, so these ideas may help in a 'layered defence'.

===Design concepts===

====Protection during transit to Mars====
A straightforward approach to designing an active shield would be to surround the protected space with a spherical enclosure made of a conducting metal that is held at a positive electric charge. Solar radiation both consist of positively charged particles, which would be repelled by the positively charged shield. Positively charged cosmic rays have too much energy to be deflected by such a shield. To protect science experiments from cosmic rays on Earth, scientists go deep underground. They do not make a small metal charged dome.

One drawback to this concept is that the shield would attract free electrons and accelerate them, creating secondary radiation when they impact the shield. These electrons would also drain the shield's positive charge, increasing the power needed to maintain it.

This led to the idea of adding a negatively-charged outer shield to keep electrons away from the positively-charged inner shield. However, this introduces its own problems: the mass requirement increases substantially, and the two shields must be tightly fixed in precise concentric positions, because any asymmetry would result in a strong attractive force that would pull the two shields together.

More recent concepts have moved away from the idea of enclosing the protected space in a charged shell. A 2006 paper described a design in which trusses extended out from a spacecraft in the x, y, and z directions. The trusses are used to suspend individual sphere-shaped charge centers: positive charges are held 50 m from the spacecraft, and negative charges are 160 m out. These charges are meant to generate an electric field sufficient to deflect incoming ions approaching from any direction. Despite the elimination of the enclosing shells, the authors concluded that the mass requirement was still too high for this design to be practical.<ref>Smith JG, Smith T, Williams M, Youngquist R, and Mendell W. Potential Polymeric Sphere Construction Materials for a Spacecraft Electrostatic Shield. NASA/TM—2006–214302. <nowiki>https://ntrs.nasa.gov/search.jsp?R=20060013423</nowiki></ref>

In 2011, a concept using toroidal (doughnut-shaped) rings as charge centers was studied. Three conductive rings, each with a 45 m radius, are positioned with their axes of symmetry along the x, y, and z axes, and held at a positive voltage. In addition, six negatively-charged spheres are suspended from trusses, 160 m out. For comparison, the researchers ran simulations of this design and a similar design that used only spherical charge centers. Both designs offered good protection from SPE. The toroid design was more effective against GCR. Ring thicknesses of 1, 5, and 10 m were simulated, showing effectiveness against GCR increasing as a function of thickness. To minimize the mass of the shield, the use of an electrostatically inflated membrane structure was proposed for the charge centers: instead of using a rigid metal, the charge center surfaces would be made of a flexible membrane with a conductive coating. Unlike a balloon that is inflated by filling it with a gas, this type of structure would inflate to its intended shape when electrically charged because of repulsion between like charges distributed across the membrane.<ref>Tripathi RK. 2016. ''Meeting the Grand Challenge of Protecting Astronauts Health: Electrostatic Active Space Radiation Shielding for Deep Space Missions''. <nowiki>https://ntrs.nasa.gov/search.jsp?R=20160010094</nowiki></ref>

A 2012 paper by Westover et al. described a design that used a magnetic field. The field could be generated by multiple layers of superconducting material wrapped around a cylindrical spacecraft in double-helix-shaped coils. Their simulations of this design found that a large, weak field was easier to generate than a small, intense field of equivalent shielding effectiveness. They also examined another design, a cylindrical spaceship surrounded by six exterior solenoids, each centered on a separate central axis (unlike the double helix design, in which the magnetic coil and the spacecraft are co-axial). To reduce the magnetic field inside the spacecraft, a compensation coil, with a current moving in the opposite direction, would surround it. Overall this design was judged to be preferable to the double helix design. Shortcomings of the latter design include the fact that the multiple layers of coils make it difficult to design a shield that can be compacted for launch and later expanded in space. In addition, it would be more difficult to block the magnetic field intruding into the spacecraft because the field strength varies more at different points within the spacecraft; problems might also arise from forces created by the magnetic field acting on the shield or spacecraft.<ref>Westover SC, Meinke RB, BurgerWJ, Van Sciver S, Washburn S, et al. 2012. Magnet Architectures and Active Radiation Shielding Study. <nowiki>https://ntrs.nasa.gov/search.jsp?R=20190002579</nowiki></ref>

==== Minimagnetosphere ====
A minimagnetosphere<ref>https://www.researchgate.net/publication/325285302_Can_artificial_miniature_magnetospheres_be_used_to_protect_spacecraft</ref> may be a viable alternative to the strong local magnetic fields described above for spacecraft. Minimagnetospheres have been detected on the Moon<ref>https://www.universetoday.com/79781/moons-mini-magnetosphere/</ref>, and indications show that they have modified the interactions of the solar wind with the surface. The concept uses an electric field rather than the magnetic field. An plasma cloud is created around the vehicle by injecting charged particles and kept in place using a small superconducting magnet at the vehicle. The electrical charge can deflect particles due to the large size on the minimagnetosphere, that can reach a number of km in diameter. This solution might be particularly applicable to a cycler type vehicle.

The mass required to generate such a mini-magnetosphere must be low, or people will do without and accept the modest radiation exposure for such a trip.

==Types of radiation shielding and risk to tissue==
{| class="wikitable" style="margin:auto"
|+ Radiation Weighting Factors & Best Shielding Materials
|-
! Type of Radiation !! Tissue Weighting Factor !! Ideal Shielding Material
|-
| Photons (all energies) || 1 || Dense materials and metals
|-
| Electrons & muons (all energies) || 1 || Small atoms (e.g. H2, He, Li.)
|-
| Protons & charged pions || 2 || Small atoms
|-
| Alpha particles || 20 || Small atoms
|-
| Fission fragments & heavy ions || 20 || Thick or dense matter
|-
| Neutrons || ~10 but varies || Small atoms (e.g. H2, He, Li.)
|}

Note that small particles about the same mass of a proton or helium nucleus, are best slowed down by bouncing off of other small atoms. (As much energy as possible is removed by kinetic energy of the atom it bumps into.) Where as gamma rays, and heavy nucleus want dense matter, or metals to stop them. However, cosmic rays with obscene amounts of energy will break into [[Secondary radiation]] when hitting dense matter, and the secondary radiation is more dangerous than the cosmic rays themselves. Finally, cosmic rays can create free neutrons, which we want to slow with light atoms. See [[Radiation sickness]] for more information on these weighting factors.

So ideal radiation protection might have a couple meters (or more) or soil, then a metal wall a few cm thick, then some material with small atoms like water, plastic, or lithium hydroxide (LiOH). A storm shelter is similar, but might have 5 to 8 meters of soil.

==Risk-mitigating behaviour==
The possible sources of radiation on Mars are man-made sources, (such as nuclear reactors or medical equipment), [[solar radiation]], [[galactic cosmic radiation]] and naturally occurring [[radioactive elements]] on Mars.

Living in a deep [[Lava tube]] takes care of the problem. However many sites on Mars do not have these. For those areas...

Possible behavioral choices which minimize the risk from these include:

* Avoiding daytime [[EVA]] during solar storms.
* Working preferentially close to natural or man-made objects, such as habitats, rovers or cliffs which provide additional (if not omni-directional) shielding. For example, if a habitat was at the base of a tall cliff, then half of the cosmic rays, and half the high energy solar radiation would be shielded from.
* Entering a [[storm shelter]] when there is a high-radiation risk from [[Solar Cosmic Rays|solar particle events]].
* Placing blocks of ice around the outer walls of the base is an excellent radiation shield.
* Sandbags on the roof of the habitat provide excellent solar protection, and minor protection verses high energy cosmic rays.
* Sleeping in areas with higher radiation protection. e.g. under bladders at ceiling filled with local water.

==Example of using shielding and behavior to reduce radiation dosage==
We can combine passive shielding with risk mitigating behavior to achieve low radiation exposure but still allow for some views of the exterior through windows. For example:

*Martian background average radiation is 240-300 mSv per year<ref>NASA, Tony C. Slaba, Christopher J. Mertens, and Steve R. Blattnig ''Radiation Shielding Optimization on Mars'' , <nowiki>https://spaceradiation.larc.nasa.gov/nasapapers/NASA-TP-2013-217983.pdf</nowiki>, Apr 2013.</ref>.
*If you sleep in a radiation shielded space such as underground rooms with a compacted regolith cover several meters thick, 8/24 hours, then the dose would go down by 1/3, to 160-200 mSv per year.
*If you spend most of your living (work, study) time in a radiation shielded space, then your dose becomes another 1/3 less, or 80 to 100 mSv.
*With overhangs and a radiation proof roof, 70% of the incident radiation to a space close to windows can be stopped by geometries, then the dose is down to 20 to 25 mSv. this is about the 20 mSv per year for a 5 year period that is recommended for radiation workers. <ref>https://www.cnsc-ccsn.gc.ca/eng/resources/radiation/radiation-doses/</ref>
*Part of the surface dose on Mars is solar proton events (SPE). These are predictable and detectable, and a large settlement will mostly be built of shielded areas. So during Solar Proton Events you should stay away from the windows. This behavior might reduce the yearly radiation load another 25%, down to 15-18 mSv per year. (If people go into a storm shelter for the worst solar storms, the dose would be reduced further.)
*What is the portion of the dosage from SPE? I have a weak reference that puts this at 30%. If correct, then the radiation load from large windows under a radiation proof ceiling is acceptable.
*Mars should be low in radon because it seems to be low in [[Thorium]] and, by analogy, Uranium as well. However, the habitats are totally enclosed spaces and radon generated by radioactive decay of naturally occurring uranium in the soil might accumulate. As 1 to 3 mSv on Earth comes from atmospheric radon<ref>http://nuclearsafety.gc.ca/eng/resources/radiation/introduction-to-radiation/radiation-doses.cfm</ref>, this part of the yearly load might go away, just as it might need to be mitigated if radon accumulates in the enclosed habitats.
*Even just 1/2 to 1 inches of glass reduces radiation dosage significantly.

If the above is correct, then large windows are not really an issue. Geodesic glass domes over public spaces might be a poor choice, unless there is an understanding that you don't spend more than 2 to 4 hours per day under them.

==Bibliography==
"The Case For Mars", by Robert Zubrin, Simon and Schuster, ISBN 978-1-4516-0811-3. Chapter 5 has a very nice breakdown of radiation exposure from Conjunction and Opposition class missions.

==References==

<references />

[[Category:Radiation Protection]]

Radiation Hormesis

2025-11-18T01:43:32Z

RichardWSmith: /* Discussion */ Added link.

Hormesis is a medical term that notes that many substances show positive effects to health at low dosages, but are dangerous at high dosages. Most drugs fall into this category. <ref>https://onlinelibrary.wiley.com/doi/full/10.1897/07-541.1</ref>. Hormesis was discovered by Hugo Schulz, a professor of pharmacology at the University of Greifsalf in the mid 1880's. He discovered that formic acid (which would kill yeast at high doses) was helpful in their growth at very low dosages. This discovery was so surprising, that he and his assistants checked, and rechecked their results for a couple of years before publishing.

For example, the drug aspirin (salicylate acid), is helpful at low doses, but at high doses is lethal. (In 2004, there were 20,000 cases of moderate aspirin poisoning, and 43 deaths.) <ref>https://en.wikipedia.org/wiki/Salicylate_poisoning</ref>

There is overwhelming evidence that very low levels of radiation are helpful to human health. <ref>https://pmc.ncbi.nlm.nih.gov/articles/PMC2477686/</ref>

==Discussion==
Studying the effects of massive doses of radiation in a very low time period (from survivors of the atomic bombs at Hiroshima, and Nagasaki) it was found that the chance of cancer increased linearly. The larger the dose, the greater the chance of cancer.

However, damage to cells occurs all the time, and they are constantly repairing themselves. Logically, radiation damage to cells, that is below the rate at which cells repair themselves would have little effect. There were three basic ideas on how to model radiation damage: 'Linear No Threshold', the 'Threshold Model', and 'Hormesis'.

[[File:LNT_Hormesis_TM.png|600 px|3 Ways to Model Radiation Damage]]

A threshold model was the most logical, but very early radiation safety standards assumed the wildly conservative Linear No Threshold model, assuming that the safety standards would be relaxed as more research came in.

Linear No Threshold (LNT) says that there is NO level of radiation which is safe. Given that we are bathed in radiation all the time, from [[Cosmic radiation|cosmic rays]], ground radiation, and radiation from inside our bodies, the LNT is very suspect. Surely if our radiation dose was equal to, or lower than, the normal background radiation, there would be no harm to our health?

Most scientists assumed that the Threshold Model would be correct. Below some dose the radiation damage was trivial or non-existent, above the threshold dose, it would do harm.

However, as time passed, the evidence accumulated that at very low levels of radiation, there were positive health effects, and some low amount of radiation was helpful to human life.

Sadly, the regulatory regime in most countries, did not update radiation safety standards in light of this new evidence. <ref>https://www.federalregister.gov/documents/2021/08/17/2021-17475/linear-no-threshold-model-and-standards-for-protection-against-radiation</ref>

Tho the preponderance of evidence suggests that very low levels of radiation are helpful, it is not easy to conduct these studies because the level at which radiation is beneficial is very close to the background radiation level. (So indications are hidden by the noise.) Since it is hard to lower the radiation level below normal background level, such studies are expensive and not performed on human subjects. Genetic testing has shown that while single strand breaks on DNA are easy to repair, double strand breaks are not. (Presumably, if the radiation is high enough to commonly cause double strand breaks, permanent cell damage is likely.)

==Samples of Research==
Science Direct's study on radiation hormesis (volume 902, Dec 2023). <ref>https://www.sciencedirect.com/science/article/pii/S0048969723046557</ref>

ResearchGate - A message to Fukushima: Nothing to Fear But Fear Itself.<ref>https://www.researchgate.net/publication/301313856_A_message_to_Fukushima_Nothing_to_fear_but_fear_itself</ref>

Radiation Hormesis: Historical & Current Perspectives. <ref>https://tech.snmjournals.org/content/43/4/242</ref>

==Speculation of Why Radiation is Helpful==
Life evolved (and is still evolving) in an environment filled with background radiation. (In Earth's early history, this background radiation dose was higher.) It is not surprising that some metabolic pathways would come to depend on a certain amount of radiation in the environment. That said, most speculation as to why low doses of radiation is helpful falls into two categories: 'Exercising the Immune System' and 'Stimulating Repair Mechanisms'.

===Exercising the Immune System===

===Stimulating Repair Mechanisms===
Stem cell therapy found that the stem cells injected into humans aged rapidly, but if irradiated, they lived longer. The radiation triggered improvements in proliferation, mobility, and chondorgenic differentiation capacity, (which improved cell longevity). This is thought to have been caused by stimulating repair mechanisms in these cells. <ref> https://www.cnl.ca/health-science-2/low-dose-radiation-research/#:~:text=There%20is%20growing%20evidence%20that,help%20protect%20people%20against%20diseases.</ref>

==Radiation Regulation and Mars Exploration==
In most countries, radiation safety standards are based on the Linear No Threshold model, which has been shown to be wildly incorrect.

In the city of Ramsar Iran, natural radiation is 50 times that of most areas on Earth, and they show no higher levels of cancer or other forms of radiation disease. Nor are the average lifespans lower than similar cities (with much lower levels of radiation) in other parts of Earth.<ref>https://pubmed.ncbi.nlm.nih.gov/11769138/ | Very high background radiation areas of Ramsar, Iran: preliminary biological studies.</ref><ref>https://www.ecolo.org/documents/documents_in_english/ramsar-natural-radioactivity/ramsar.html</ref><ref>https://aerb.gov.in/images/PDF/image/34086353.pdf | Ramsar suggests that current regulations for radiation are too strict.</ref> <ref>https://www.sciencedirect.com/science/article/abs/pii/S0531513104018369 | Natives of Ramsar had better response to high gamma ray doses</ref> <ref>https://www.researchgate.net/publication/228757260_Are_the_Inhabitants_of_High_Background_Radiation_Areas_of_Ramsar_More_Radioresistant_Scope_of_the_Problem_and_the_Need_for_Future_Studies</ref> <ref>https://www.ntanet.net/the-naturally-occurring-high-radiation-levels-of-ramsar-iran/</ref>

If the radiation dosage required for astronauts was at the level of Ramsar, far lighter and less expensive radiation protection would be needed.

This youTube video nicely summarizes the radiation levels of Mars exploration and discusses safety concerns.
https://www.youtube.com/watch?v=VluEllUrseE&list=LL&index=11

See also: https://www.youtube.com/watch?v=gzdLdNRaPKc&list=PL994EB042117A7F6D&index=66

==References==

Radiation Hormesis

2025-11-18T01:40:27Z

RichardWSmith: /* Radiation Regulation and Mars Exploration */ fixing formatting.

Hormesis is a medical term that notes that many substances show positive effects to health at low dosages, but are dangerous at high dosages. Most drugs fall into this category. <ref>https://onlinelibrary.wiley.com/doi/full/10.1897/07-541.1</ref>. Hormesis was discovered by Hugo Schulz, a professor of pharmacology at the University of Greifsalf in the mid 1880's. He discovered that formic acid (which would kill yeast at high doses) was helpful in their growth at very low dosages. This discovery was so surprising, that he and his assistants checked, and rechecked their results for a couple of years before publishing.

For example, the drug aspirin (salicylate acid), is helpful at low doses, but at high doses is lethal. (In 2004, there were 20,000 cases of moderate aspirin poisoning, and 43 deaths.) <ref>https://en.wikipedia.org/wiki/Salicylate_poisoning</ref>

There is overwhelming evidence that very low levels of radiation are helpful to human health. <ref>https://pmc.ncbi.nlm.nih.gov/articles/PMC2477686/</ref>

==Discussion==
Studying the effects of massive doses of radiation in a very low time period (from survivors of the atomic bombs at Hiroshima, and Nagasaki) it was found that the chance of cancer increased linearly. The larger the dose, the greater the chance of cancer.

However, damage to cells occurs all the time, and they are constantly repairing themselves. Logically, radiation damage to cells, that is below the rate at which cells repair themselves would have little effect. There were three basic ideas on how to model radiation damage: 'Linear No Threshold', the 'Threshold Model', and 'Hormesis'.

[[File:LNT_Hormesis_TM.png|600 px|3 Ways to Model Radiation Damage]]

A threshold model was the most logical, but very early radiation safety standards assumed the wildly conservative Linear No Threshold model, assuming that the safety standards would be relaxed as more research came in.

Linear No Threshold (LNT) says that there is NO level of radiation which is safe. Given that we are bathed in radiation all the time, from cosmic rays, ground radiation, and radiation from inside our bodies, the LNT is very suspect. Surely if our radiation dose was equal to, or lower than, the normal background radiation, there would be no harm to our health?

Most scientists assumed that the Threshold Model would be correct. Below some dose the radiation damage was trivial or non-existent, above the threshold dose, it would do harm.

However, as time passed, the evidence accumulated that at very low levels of radiation, there were positive health effects, and some low amount of radiation was helpful to human life.

Sadly, the regulatory regime in most countries, did not update radiation safety standards in light of this new evidence. <ref>https://www.federalregister.gov/documents/2021/08/17/2021-17475/linear-no-threshold-model-and-standards-for-protection-against-radiation</ref>

Tho the preponderance of evidence suggests that very low levels of radiation are helpful, it is not easy to conduct these studies because the level at which radiation is beneficial is very close to the background radiation level. (So indications are hidden by the noise.) Since it is hard to lower the radiation level below normal background level, such studies are expensive and not performed on human subjects. Genetic testing has shown that while single strand breaks on DNA are easy to repair, double strand breaks are not. (Presumably, if the radiation is high enough to commonly cause double strand breaks, permanent cell damage is likely.)

==Samples of Research==
Science Direct's study on radiation hormesis (volume 902, Dec 2023). <ref>https://www.sciencedirect.com/science/article/pii/S0048969723046557</ref>

ResearchGate - A message to Fukushima: Nothing to Fear But Fear Itself.<ref>https://www.researchgate.net/publication/301313856_A_message_to_Fukushima_Nothing_to_fear_but_fear_itself</ref>

Radiation Hormesis: Historical & Current Perspectives. <ref>https://tech.snmjournals.org/content/43/4/242</ref>

==Speculation of Why Radiation is Helpful==
Life evolved (and is still evolving) in an environment filled with background radiation. (In Earth's early history, this background radiation dose was higher.) It is not surprising that some metabolic pathways would come to depend on a certain amount of radiation in the environment. That said, most speculation as to why low doses of radiation is helpful falls into two categories: 'Exercising the Immune System' and 'Stimulating Repair Mechanisms'.

===Exercising the Immune System===

===Stimulating Repair Mechanisms===
Stem cell therapy found that the stem cells injected into humans aged rapidly, but if irradiated, they lived longer. The radiation triggered improvements in proliferation, mobility, and chondorgenic differentiation capacity, (which improved cell longevity). This is thought to have been caused by stimulating repair mechanisms in these cells. <ref> https://www.cnl.ca/health-science-2/low-dose-radiation-research/#:~:text=There%20is%20growing%20evidence%20that,help%20protect%20people%20against%20diseases.</ref>

==Radiation Regulation and Mars Exploration==
In most countries, radiation safety standards are based on the Linear No Threshold model, which has been shown to be wildly incorrect.

In the city of Ramsar Iran, natural radiation is 50 times that of most areas on Earth, and they show no higher levels of cancer or other forms of radiation disease. Nor are the average lifespans lower than similar cities (with much lower levels of radiation) in other parts of Earth.<ref>https://pubmed.ncbi.nlm.nih.gov/11769138/ | Very high background radiation areas of Ramsar, Iran: preliminary biological studies.</ref><ref>https://www.ecolo.org/documents/documents_in_english/ramsar-natural-radioactivity/ramsar.html</ref><ref>https://aerb.gov.in/images/PDF/image/34086353.pdf | Ramsar suggests that current regulations for radiation are too strict.</ref> <ref>https://www.sciencedirect.com/science/article/abs/pii/S0531513104018369 | Natives of Ramsar had better response to high gamma ray doses</ref> <ref>https://www.researchgate.net/publication/228757260_Are_the_Inhabitants_of_High_Background_Radiation_Areas_of_Ramsar_More_Radioresistant_Scope_of_the_Problem_and_the_Need_for_Future_Studies</ref> <ref>https://www.ntanet.net/the-naturally-occurring-high-radiation-levels-of-ramsar-iran/</ref>

If the radiation dosage required for astronauts was at the level of Ramsar, far lighter and less expensive radiation protection would be needed.

This youTube video nicely summarizes the radiation levels of Mars exploration and discusses safety concerns.
https://www.youtube.com/watch?v=VluEllUrseE&list=LL&index=11

See also: https://www.youtube.com/watch?v=gzdLdNRaPKc&list=PL994EB042117A7F6D&index=66

==References==

Radiation Hormesis

2025-11-18T01:39:04Z

RichardWSmith: /* Radiation Regulation and Mars Exploration */ Added references.

Hormesis is a medical term that notes that many substances show positive effects to health at low dosages, but are dangerous at high dosages. Most drugs fall into this category. <ref>https://onlinelibrary.wiley.com/doi/full/10.1897/07-541.1</ref>. Hormesis was discovered by Hugo Schulz, a professor of pharmacology at the University of Greifsalf in the mid 1880's. He discovered that formic acid (which would kill yeast at high doses) was helpful in their growth at very low dosages. This discovery was so surprising, that he and his assistants checked, and rechecked their results for a couple of years before publishing.

For example, the drug aspirin (salicylate acid), is helpful at low doses, but at high doses is lethal. (In 2004, there were 20,000 cases of moderate aspirin poisoning, and 43 deaths.) <ref>https://en.wikipedia.org/wiki/Salicylate_poisoning</ref>

There is overwhelming evidence that very low levels of radiation are helpful to human health. <ref>https://pmc.ncbi.nlm.nih.gov/articles/PMC2477686/</ref>

==Discussion==
Studying the effects of massive doses of radiation in a very low time period (from survivors of the atomic bombs at Hiroshima, and Nagasaki) it was found that the chance of cancer increased linearly. The larger the dose, the greater the chance of cancer.

However, damage to cells occurs all the time, and they are constantly repairing themselves. Logically, radiation damage to cells, that is below the rate at which cells repair themselves would have little effect. There were three basic ideas on how to model radiation damage: 'Linear No Threshold', the 'Threshold Model', and 'Hormesis'.

[[File:LNT_Hormesis_TM.png|600 px|3 Ways to Model Radiation Damage]]

A threshold model was the most logical, but very early radiation safety standards assumed the wildly conservative Linear No Threshold model, assuming that the safety standards would be relaxed as more research came in.

Linear No Threshold (LNT) says that there is NO level of radiation which is safe. Given that we are bathed in radiation all the time, from cosmic rays, ground radiation, and radiation from inside our bodies, the LNT is very suspect. Surely if our radiation dose was equal to, or lower than, the normal background radiation, there would be no harm to our health?

Most scientists assumed that the Threshold Model would be correct. Below some dose the radiation damage was trivial or non-existent, above the threshold dose, it would do harm.

However, as time passed, the evidence accumulated that at very low levels of radiation, there were positive health effects, and some low amount of radiation was helpful to human life.

Sadly, the regulatory regime in most countries, did not update radiation safety standards in light of this new evidence. <ref>https://www.federalregister.gov/documents/2021/08/17/2021-17475/linear-no-threshold-model-and-standards-for-protection-against-radiation</ref>

Tho the preponderance of evidence suggests that very low levels of radiation are helpful, it is not easy to conduct these studies because the level at which radiation is beneficial is very close to the background radiation level. (So indications are hidden by the noise.) Since it is hard to lower the radiation level below normal background level, such studies are expensive and not performed on human subjects. Genetic testing has shown that while single strand breaks on DNA are easy to repair, double strand breaks are not. (Presumably, if the radiation is high enough to commonly cause double strand breaks, permanent cell damage is likely.)

==Samples of Research==
Science Direct's study on radiation hormesis (volume 902, Dec 2023). <ref>https://www.sciencedirect.com/science/article/pii/S0048969723046557</ref>

ResearchGate - A message to Fukushima: Nothing to Fear But Fear Itself.<ref>https://www.researchgate.net/publication/301313856_A_message_to_Fukushima_Nothing_to_fear_but_fear_itself</ref>

Radiation Hormesis: Historical & Current Perspectives. <ref>https://tech.snmjournals.org/content/43/4/242</ref>

==Speculation of Why Radiation is Helpful==
Life evolved (and is still evolving) in an environment filled with background radiation. (In Earth's early history, this background radiation dose was higher.) It is not surprising that some metabolic pathways would come to depend on a certain amount of radiation in the environment. That said, most speculation as to why low doses of radiation is helpful falls into two categories: 'Exercising the Immune System' and 'Stimulating Repair Mechanisms'.

===Exercising the Immune System===

===Stimulating Repair Mechanisms===
Stem cell therapy found that the stem cells injected into humans aged rapidly, but if irradiated, they lived longer. The radiation triggered improvements in proliferation, mobility, and chondorgenic differentiation capacity, (which improved cell longevity). This is thought to have been caused by stimulating repair mechanisms in these cells. <ref> https://www.cnl.ca/health-science-2/low-dose-radiation-research/#:~:text=There%20is%20growing%20evidence%20that,help%20protect%20people%20against%20diseases.</ref>

==Radiation Regulation and Mars Exploration==
In most countries, radiation safety standards are based on the Linear No Threshold model, which has been shown to be wildly incorrect.

In the city of Ramsar Iran, natural radiation is 50 times that of most areas on Earth, and they show no higher levels of cancer or other forms of radiation disease. Nor are the average lifespans lower than similar cities (with much lower levels of radiation) in other parts of Earth.<ref>https://pubmed.ncbi.nlm.nih.gov/11769138/ | Very high background radiation areas of Ramsar, Iran: preliminary biological studies.</ref>
<ref>https://www.ecolo.org/documents/documents_in_english/ramsar-natural-radioactivity/ramsar.html</ref>
<ref>https://aerb.gov.in/images/PDF/image/34086353.pdf | Ramsar suggests that current regulations for radiation are too strict.</ref>
<ref>https://www.sciencedirect.com/science/article/abs/pii/S0531513104018369 | Natives of Ramsar had better response to high gamma ray doses</ref>
<ref>https://www.researchgate.net/publication/228757260_Are_the_Inhabitants_of_High_Background_Radiation_Areas_of_Ramsar_More_Radioresistant_Scope_of_the_Problem_and_the_Need_for_Future_Studies</ref>
<ref>https://www.ntanet.net/the-naturally-occurring-high-radiation-levels-of-ramsar-iran/</ref>

If the radiation dosage required for astronauts was at the level of Ramsar, far lighter and less expensive radiation protection would be needed.

This youTube video nicely summarizes the radiation levels of Mars exploration and discusses safety concerns.
https://www.youtube.com/watch?v=VluEllUrseE&list=LL&index=11

See also: https://www.youtube.com/watch?v=gzdLdNRaPKc&list=PL994EB042117A7F6D&index=66

==References==

Radiation

2025-11-18T01:35:10Z

RichardWSmith: /* Types of Radiation */ Added link.

[[Image:nuclear_warning_sign.png|right|Nuclear Danger Icon]]
Natural '''Radiation''' on [[Mars]] is much higher compared with [[Earth]]. The thin [[atmosphere]] provides only a small shielding effect against [[cosmic radiation]]. It provides moderate protection against [[solar radiation]]. Mars also lacks the [[magnetosphere]] that protects Earth.

The average natural radiation level on Mars is 24-30 rads or 240-300 mSv per year<ref name=":0" /><ref name=":5" />. This is about 40-50 times the average on Earth.

1 millisievert [mSv] = 0.1 rad [rd]

==Types of Radiation==
Radiation comes in a variety of forms:<ref>http://www.nas.nasa.gov/About/Education/SpaceSettlement/designer/needs.html#SHIELDING</ref>
{| border="1"
|-
!Name
!Relative Biological Effectiveness (RBE)
!Source
|-
|'''[[X-ray|X-Rays]] and [[gamma ray|Gamma Rays]]'''
|1
|[[Radiation belts]], [[solar radiation]], and bremsstrahlung electrons
|-
|'''[[electron|Electrons]]'''
1.0 MeV 
0.1 MeV
| 
1 
1.08
|Radiation belts
|-
|'''[[proton|Protons]]''' 
100 MeV 
1.5 MeV 
0.1 MeV
| 
1-2 
8.5 
10
|[[Cosmic radiation]], inner-radiation belts, and [[solar radiation]]
|-
|'''[[neutron|Neutrons]]''' 
0.05 ev (thermal) 
1.0 MeV 
10 MeV
| 
2.8 
10.5 
6.4
|Nuclear interactions in the [[sun]]; on Mars, produced when [[cosmic radiation]] interacts with [[regolith]]
|-
|'''[[alpha particles|Alpha Particles]]''' 
5.0 MeV 
1.0 MeV
| 
15 
20
|[[Cosmic radiation]], [[Cosmic rays]]
|-
|[[Heavy Ions|'''Heavy Ions''']]
|Varies widely
|[[Cosmic radiation]]
|-
| colspan="3" |Table 1: Types of radiation
|}
(RBE is a measure of the damage done to living tissue, relative to gamma rays)

[[Cosmic radiation]] comprises 85% protons, 14% alpha particles, and 1% heavy ions.<ref>Schimmerling W. (2011, Feb 5). The Space Radiation Environment: An Introduction. <nowiki>https://three.jsc.nasa.gov/concepts/SpaceRadiationEnviron.pdf</nowiki></ref> Solar radiation includes the same radiation types, but it a higher proportion of protons and its heavy primaries have lower energy levels. The high-energy heavy primaries in cosmic radiation can penetrate materials that effectively block lower-energy radiation<ref name=":1">Rapp D. (2006). Radiation Effects and Shielding Requirements in Human Missions to the Moon and Mars. Mars 2:46-71. <nowiki>https://doi.org/10.1555/mars.2006.0004</nowiki></ref>.

==Astronaut Exposures==
The following astronauts have experienced radiations doses equal to, or greater than, the radiation doses in a 2.5 year Mars mission. (Assuming a 520 mSv dose for this mission. Different missions have different amounts of radiation protection.) In three cases the astronauts took more than double the amount of radiation than what is expected this Mars mission.

They are: Carl Waltz, Colin Foale, Peggy Whitson, Jeffrey Williams, Oleg Kononenko, Sergei Krikalyov, Vladimir Solovyov, Valeri Polyakov, Fyodor Yurchikhin, Aleksandr Kaleri, Sergei Krikalyov, Yurki Malenchenko, Gennadi Padalka, & Sergei Avdeyev.

None have exhibited ANY radiation health effects.

Note that NASA has stricter radiation limits than Russia (or the Soviet Union), so as time passes the number of Russian names on this list will become more pronounced.

See [[Long duration space flight]] for more information.

==Exposure limits==

===Limits for humans===
Exposure to dangerous levels of radiation causes [[radiation sickness]] and cancer. The average exposure to radiation on Earth due to natural sources is 6.2 mSv per year<ref name=":3">http://www.ans.org/pi/resources/dosechart/msv.php</ref>. The highest natural exposure is recorded in Ramsar, Iran, where people are exposed up to 260 mSv/y for many generations, with no reported harmful effects<ref name="Ghiassi-Nejad et al 2002">Ghiassi-Nejad et al, Very high background radiation areas of Ramsar, Iran: Preliminary biological studies, Health Physics 82(1):87-93 (February 2002), DOI: 10.1097/00004032-200201000-00011 [https://www.researchgate.net/publication, /11588980_Very_high_background_radiation_areas_of_Ramsar_Iran_Preliminary_biological_studies abstract]</ref>. This is 13 times the maximum exposure allowed radiation workers each year. Importantly, this level of radiation is what Mars settlers (living inside shelters of reasonable cost) would expect. So the 'high' levels of the back ground radiation at Ramsar is good news for Mars settlement.<ref>https://pubmed.ncbi.nlm.nih.gov/17563407/</ref> A recent study showed that the people living in this city showed increased immunity to gamma ray exposure, tho if this is from evolutionary adaption over many generations, or from the immune system being 'exercised' regularly is not known. <ref>https://pubmed.ncbi.nlm.nih.gov/11769138/</ref>

It should be emphasized, that low level radiation doses spread over a long period of time (long enough that the bodies natural functions have time to repair the damage), are far less dangerous than large doses received in a short amount of time. (In fact numerous studies show health benefits from extremely low levels of radiation.) <ref>https://jnm.snmjournals.org/content/59/12/1786</ref> See [[Radiation Hormesis]]. <ref>https://pmc.ncbi.nlm.nih.gov/articles/PMC2477686/</ref>

See also: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6149023/. https://www.sciencedaily.com/releases/2017/09/170913104428.htm. https://www.ajronline.org/doi/full/10.2214/ajr.179.5.1791137. https://en.wikipedia.org/wiki/Radiation_hormesis

Some people believe that ANY exposure to radiation, no matter how slight, poses some risk. Small dose - small risk of cancer. High dose - high risk of cancer. This is the No Minimum Threshold theory of radiation dosage. This works quite well with fast radiation doses high enough to cause cancers later in life, but the evidence is much weaker for low level does over a long period of time. No Minimum Threshold is used by regulatory agencies when they wish to be extremely conservative about radiation risks.<ref>https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6043938/</ref> <ref>https://www.federalregister.gov/documents/2021/08/17/2021-17475/linear-no-threshold-model-and-standards-for-protection-against-radiation</ref> <ref>https://pubs.rsna.org/doi/10.1148/radiol.2511080671</ref>

Nevertheless, there are defined legal limits for exposure during work for several professional activities, such as for X-ray assistants, airplane personnel, etc. The International Commission on Radiation Protection recommends that occupational (work-related) radiation exposure be limited to 50 millisieverts (mSv) per year, and limited to 100 mSv over any 5-year period<ref>http://www.icrp.org/publication.asp?id=ICRP%20Publication%20103</ref>. NASA's radiation dose limits for astronauts are established in NASA-STD-3001<ref>NASA. (2015). NASA Space Flight Human-System Standard Volume 1, Revision A: Crew Health. Retrieved from https://standards.nasa.gov/standard/nasa/nasa-std-3001-vol-1</ref>.

There is scientific uncertainty surrounding the health hazard from cosmic and solar radiation, because most past research on the health effects of radiation studied only x-rays and gamma rays<ref name=":1" />.

===Limits for plants===
[[File:Plant radiation.PNG|thumb|Table 2: Need to find source for this table]]
"In general, plants are relatively radiation resistant when growing and extremely resistant as dormant seeds." Radiation should not interfere with raising plants as food sources, at least not on the time scales of exploration missions.<ref>National Research Council 1996. Radiation Hazards to Crews of Interplanetary Missions: Biological Issues and Research Strategies. Washington, DC: The National Academies Press. <nowiki>https://doi.org/10.17226/5540</nowiki>.</ref>

Table 2 shows the limits for plants(source of table required, in space or on mars?). It shows that in practically all cases plants can survive radiations events that are likely for Mars without any kind of protection.

However, a 2021 study in the Netherlands, conducted on two types of plants, has shown that radiation at the Mars surface may reduce yields substantially<ref>TACK, Nynke, WAMELINK, G. W. W., DENKOVA, A. G., ''et al.'' Influence of Martian Radiation-like Conditions on the Growth of Secale cereale and Lepidium sativum. ''Frontiers in Astronomy and Space Sciences'', 2021, p. 127.</ref>. Germination of plants in a protected environment before setting them out in greenhouses might be a potential mitigation measure but further research is needed.

==Martian Environment==

===Effects of the Martian atmosphere===
Most Solar Proton Events (SPE) particles are low energy & will be stopped by the atmosphere before they reach the surface. However, interactions with atmospheric particles can produce neutrons; those neutrons can reach the surface, so the health hazard is not eliminated. However, the rare, very powerful SPE have energies of medium energy [[Cosmic radiation| cosmic rays]] and are a dangerous hazard.

Cosmic radiation protons are likely to penetrate the atmosphere. Cosmic ray heavy ions may fragment in the atmosphere, producing lower-mass ions that can still harm astronauts on the surface.

Mars' thin atmosphere allows more [[Ultraviolet]] light to reach the surface, compared to Earth. However, habitat structural materials and standard space suits should be sufficient to protect humans from UV radiation.<ref name=":4">Beaty DW, Snook K, Carlton A, Eppler D, Farrell B, Heldmann J,...Zeitlin C, on behalf of the Mars Human Precursor Science Steering Group. (2005). An Analysis of the Precursor Measurements of Mars Needed to Reduce the Risk of the First Human Mission to Mars. Available at <nowiki>https://mepag.jpl.nasa.gov/reports/MHP_SSG_(06-02-05).pdf</nowiki></ref>

Mars atmosphere effect depends on the considered inclination, as the incoming radiations will cross more matter if it's coming from the horizon rather than from the zenith. For inclination angles greater than ~45°, the atmospheric thickness is in the range from 20-30 g/cm2, and for lower inclination angles, the atmospherie thickness can exceed 100 g/cm2<ref name=":2" />. In other words, solar radiation is less dangerous when the sun in near the horizon.

===Low gravity effects on atmospheric thickness===
Note that the radiation protection given by the Martian atmosphere is higher than would be expected considering the air pressure. Pressure can be thought of the weight of the air above you in the atmosphere. Mars' gravity is 38% of Earths. So the weight of that air is less than it would be on Earth. On other words, more air must be above you on Mars to give the same pressure, compared to Earth. For example, you might think that since Mars' air pressure is 0.6% of Earth's, the radiation protection would also be 0.6%. However, the mass of air above you on Mars is 1/38% or 2.6 times thicker than that pressure on Earth. So even tho Mars' air pressure is 0.6% of Earth's, it gives 1.56% of Earth's atmospheric protection. Likewise, if we were to terraform Mars to have 10% Earth's pressure, the radiation protection by that atmosphere would be 26%. If we were to give Mars an atmosphere of 50% Earth's air pressure, then the Martian atmosphere's radiation protection would be 132% that of Earth.

===Effects of cosmic rays striking the regolith===
When [[cosmic rays]] strikes regolith, it can cause the impacted atoms to emit their own radiation. Surrounding regolith particles absorb much of this radiation, with the exception of neutrons. Neutrons generated in this way are called albedo neutrons. These neutrons have the potential to add substantially to the radiation dose for astronauts on the surface.<ref name=":4" />

Neutrons are well absorbed by water, so blocks of ice or water around habitats would be useful radiation protection. Lithium hydride is thought to be the most effective neutron absorber ever discovered, and it might be built into the floor of long term habitats. <ref>https://www.tandfonline.com/doi/abs/10.1179/1743284715Y.0000000105?journalCode=ymst20
</ref> (Lithium hydroxide reacts with water, so it must be kept away from the humid interior of the settlement.) If a long duration habitat is has a space under it, blocks of local ice could protect against secondary neutrons.

===Dose received by an unprotected human on Mars===

====Cosmic radiation====
The equivalent dose rate from cosmic radiation on Earth's surface at sea level is .26 mSv per year<ref name=":3" /> <ref>http://nuclearsafety.gc.ca/eng/resources/radiation/introduction-to-radiation/radiation-doses.cfm</ref> (this makes up ~10% of the annual natural radiation dose, and it increase with altitude). Based on measurements made by the Curiosity rover, the corresponding figure for the surface of Mars is approximately 230 mSv/year<ref name=":0">Hassler DM, Zeitlin C, Wimmer-Schweingruber RF, Ehresmann B, Rafkin S, Eigenbrode JL,...MSL Science Team. (2014). Mars' Surface Radiation Environment Measured with the Mars Science Laboratory's Curiosity Rover. Science 343(6169). <nowiki>https://doi.org/10.1126/science.1244797</nowiki></ref>. More generally, one model estimated that the dose equivalent rate on the surface of Mars ranges from 156.4 mSv/year (at solar maximum) to 273.8 mSv/year (at solar minimum)<ref name=":2" /><ref name=":5">McKenna-Lawlor S, Goncalves P, Keating A, Reitz G, Matthia D. (2012). Overview of Energetic Particle Hazards During Prospective Manned Missions to Mars. Planetary and Space Science. 63: 123-132.</ref>. A 2005 report by the Mars Human Precursor Science Steering Group estimated that (at solar minimum) the dose from cosmic radiation would be 1.2 +/- 0.5 mSv/day; this includes 0.4 +/- 0.4 mSv/day from albedo neutrons.<ref name=":4" />

====Solar Proton Events====
These are also known a [[Coronal Mass Ejections]]. Curiosity also measured the temporary increase in radiation during a single SPE. The results indicate an increase in equivalent dose rate of approximately 25% over a 1-day interval<ref name=":0" />. This figure will vary depending on the intensity of a particular SPE. See [[Solar Cosmic Rays]] for further discussion.

==Effect on material==
Radiation can change the properties of [[plastics]] and metals, making them brittle after a period of time.

As semi-conductor computer chips have become smaller and smaller, they become more likely to be confused or damaged by radiation. Electronic equipment sent to Mars, or built on Mars should be radiation hardened.

==Protection==
Long term [[Habitat|habitats]] should be equipped with a [[radiation shielding]], thick enough to reduce the radiation to a level equal to Earth, that is, almost zero. Best protection may be achieved with houses built in natural [[caves]] or set into cliffs or hillsides. (If, however, it is decided that radiation levels equal to the city of Ramsar, Iran, are safe enough, then the thickness of the radiation shielding suggested below can be reduced up to 13 fold.)

Early exploration habitats could have water tanks, or sand bags above where people live. When radiation goes thru water, every 18 cm reduces the radiation by half. So a water tank 108 cm thick (6 halvings) will reduce the radiation level by 64 times. (As a bonus, water is a good neutron absorber.) Packed soil has a halving-distance of 9.1 cm, so 55 cm of hard soil would provide a similar level of protection. In general, it is far better to use local materials for radiation protection, rather than hauling them from Earth. See: https://www.imagesco.com/geiger/lead-shielding-guide.html.

[[Space suit]]s must be designed with radiation in mind. The suit should provide adequate shielding for the occupant. It may be necessary to design suits with several grades of protection. Suits designed for short-term use can carry lighter shielding which would reduce weight and improve maneuverability. That said, radiation protection from suits will be much less than inside habitats, so minimizing time on the surface will be the largest protection. Going outside during solar storms would likely be banned. Jobs that require regular EVA's (such as cleaning solar cells of dust) should be avoided.

During severe radiation events, such as [[solar flare|solar flares]], surface [[settlement|settlements]] may use [[storm shelter|storm shelters]] with heavier than normal shielding.

"In this work, it is shown that on the Martian surface, almost any amount of aluminum shielding increases exposure levels for humans. The increased exposure levels are attributed to neutron production in the shield and Martian regolith as well as the electromagnetic cascade induced in the Martian atmosphere. This result is significant for optimization of vehicle and shield designs intended for the surface of Mars." <ref name=":2">NASA, Tony C. Slaba, Christopher J. Mertens, and Steve R. Blattnig '' Radiation Shielding Optimization on Mars '', https://spaceradiation.larc.nasa.gov/nasapapers/NASA-TP-2013-217983.pdf, Apr 2013.</ref>

"An in-situ shielding strategy will also be of little help unless several hundred g/cm2 of regolith is utilized. Such a strategy would probably require large scale excavation making it an unlikely candidate. Instead, the shielding strategy would rely primarily on material optimization. Options, such as replacing aluminum structures with high hydrogen content carbon composites, could be pursued." <ref name=":2" /> This opinion is open to argument as in-situ resources utilization for any type of settlement should make large amounts of regolith available for construction. It mainly is true for the very first level of habitats.

The use of g/cm2 can be translated into an equivalent thickness that depends on the material density. For martian regolith at 2000 kg/m3, a thickness of 1m of regolith is 200 g/cm2. Water (or ice) is 100 g/cm2. So the minimum covering for a long term settlement would be 5m or more. For water, although the radiation absorption is better the density is lower, so about the same thickness would be required for protection. (to be discussed)

For people living on Mars for many years, they may prefer to have habitats with more than the minimum radiation protection. For example, if people decide that 2 meters of packed soil is sufficient as minimum protection, they may wish to have 4 or 5 meters for most of the base. They may spend a significant amount of time outside the habitat, so to balance the low radiation protection when outside they have with higher radiation protection inside.

==References==
<references />

==External links==

*[http://www.ips.gov.au/ IPS:] [http://www.ips.gov.au/Category/Educational/Space%20Weather/Space%20Weather%20Effects/guide-to-space-radiation.pdf A Guide to Space Radiation]
*[http://www.niauk.org/radiation-and-safety.html Nuclear Industry Association: Radiation, health and nuclear safety]
*[https://hesperia.gsfc.nasa.gov/sspvse/posters/DF_Smart/poster.pdf The frequency distribution of solar proton events: 5 solar cycles and 45 solar cycles]
*[[w:Radiation_hormesis|Low levels of radiation can be stimulatory: Radiation Hormesis]]

[[Category:Radiation Protection]]

Radiation Hormesis

2025-11-18T01:31:31Z

RichardWSmith: /* Discussion */

Hormesis is a medical term that notes that many substances show positive effects to health at low dosages, but are dangerous at high dosages. Most drugs fall into this category. <ref>https://onlinelibrary.wiley.com/doi/full/10.1897/07-541.1</ref>. Hormesis was discovered by Hugo Schulz, a professor of pharmacology at the University of Greifsalf in the mid 1880's. He discovered that formic acid (which would kill yeast at high doses) was helpful in their growth at very low dosages. This discovery was so surprising, that he and his assistants checked, and rechecked their results for a couple of years before publishing.

For example, the drug aspirin (salicylate acid), is helpful at low doses, but at high doses is lethal. (In 2004, there were 20,000 cases of moderate aspirin poisoning, and 43 deaths.) <ref>https://en.wikipedia.org/wiki/Salicylate_poisoning</ref>

There is overwhelming evidence that very low levels of radiation are helpful to human health. <ref>https://pmc.ncbi.nlm.nih.gov/articles/PMC2477686/</ref>

==Discussion==
Studying the effects of massive doses of radiation in a very low time period (from survivors of the atomic bombs at Hiroshima, and Nagasaki) it was found that the chance of cancer increased linearly. The larger the dose, the greater the chance of cancer.

However, damage to cells occurs all the time, and they are constantly repairing themselves. Logically, radiation damage to cells, that is below the rate at which cells repair themselves would have little effect. There were three basic ideas on how to model radiation damage: 'Linear No Threshold', the 'Threshold Model', and 'Hormesis'.

[[File:LNT_Hormesis_TM.png|600 px|3 Ways to Model Radiation Damage]]

A threshold model was the most logical, but very early radiation safety standards assumed the wildly conservative Linear No Threshold model, assuming that the safety standards would be relaxed as more research came in.

Linear No Threshold (LNT) says that there is NO level of radiation which is safe. Given that we are bathed in radiation all the time, from cosmic rays, ground radiation, and radiation from inside our bodies, the LNT is very suspect. Surely if our radiation dose was equal to, or lower than, the normal background radiation, there would be no harm to our health?

Most scientists assumed that the Threshold Model would be correct. Below some dose the radiation damage was trivial or non-existent, above the threshold dose, it would do harm.

However, as time passed, the evidence accumulated that at very low levels of radiation, there were positive health effects, and some low amount of radiation was helpful to human life.

Sadly, the regulatory regime in most countries, did not update radiation safety standards in light of this new evidence. <ref>https://www.federalregister.gov/documents/2021/08/17/2021-17475/linear-no-threshold-model-and-standards-for-protection-against-radiation</ref>

Tho the preponderance of evidence suggests that very low levels of radiation are helpful, it is not easy to conduct these studies because the level at which radiation is beneficial is very close to the background radiation level. (So indications are hidden by the noise.) Since it is hard to lower the radiation level below normal background level, such studies are expensive and not performed on human subjects. Genetic testing has shown that while single strand breaks on DNA are easy to repair, double strand breaks are not. (Presumably, if the radiation is high enough to commonly cause double strand breaks, permanent cell damage is likely.)

==Samples of Research==
Science Direct's study on radiation hormesis (volume 902, Dec 2023). <ref>https://www.sciencedirect.com/science/article/pii/S0048969723046557</ref>

ResearchGate - A message to Fukushima: Nothing to Fear But Fear Itself.<ref>https://www.researchgate.net/publication/301313856_A_message_to_Fukushima_Nothing_to_fear_but_fear_itself</ref>

Radiation Hormesis: Historical & Current Perspectives. <ref>https://tech.snmjournals.org/content/43/4/242</ref>

==Speculation of Why Radiation is Helpful==
Life evolved (and is still evolving) in an environment filled with background radiation. (In Earth's early history, this background radiation dose was higher.) It is not surprising that some metabolic pathways would come to depend on a certain amount of radiation in the environment. That said, most speculation as to why low doses of radiation is helpful falls into two categories: 'Exercising the Immune System' and 'Stimulating Repair Mechanisms'.

===Exercising the Immune System===

===Stimulating Repair Mechanisms===
Stem cell therapy found that the stem cells injected into humans aged rapidly, but if irradiated, they lived longer. The radiation triggered improvements in proliferation, mobility, and chondorgenic differentiation capacity, (which improved cell longevity). This is thought to have been caused by stimulating repair mechanisms in these cells. <ref> https://www.cnl.ca/health-science-2/low-dose-radiation-research/#:~:text=There%20is%20growing%20evidence%20that,help%20protect%20people%20against%20diseases.</ref>

==Radiation Regulation and Mars Exploration==
In most countries, radiation safety standards are based on the Linear No Threshold model, which has been shown to be wildly incorrect.

In the city of Ramsar Iran, natural radiation is 50 times that of most areas on Earth, and they show no higher levels of cancer or other forms of radiation disease. Nor are the average lifespans lower than similar cities (with much lower levels of radiation) in other parts of Earth.

If the radiation dosage required for astronauts was at the level of Ramsar, far lighter and less expensive radiation protection would be needed.

This youTube video nicely summarizes the radiation levels of Mars exploration and discusses safety concerns.
https://www.youtube.com/watch?v=VluEllUrseE&list=LL&index=11

See also: https://www.youtube.com/watch?v=gzdLdNRaPKc&list=PL994EB042117A7F6D&index=66

==References==

Radiation Hormesis

2025-11-18T01:18:35Z

RichardWSmith: /* Samples of Research */ Added reference.

Hormesis is a medical term that notes that many substances show positive effects to health at low dosages, but are dangerous at high dosages. Most drugs fall into this category. <ref>https://onlinelibrary.wiley.com/doi/full/10.1897/07-541.1</ref>. Hormesis was discovered by Hugo Schulz, a professor of pharmacology at the University of Greifsalf in the mid 1880's. He discovered that formic acid (which would kill yeast at high doses) was helpful in their growth at very low dosages. This discovery was so surprising, that he and his assistants checked, and rechecked their results for a couple of years before publishing.

For example, the drug aspirin (salicylate acid), is helpful at low doses, but at high doses is lethal. (In 2004, there were 20,000 cases of moderate aspirin poisoning, and 43 deaths.) <ref>https://en.wikipedia.org/wiki/Salicylate_poisoning</ref>

There is overwhelming evidence that very low levels of radiation are helpful to human health. <ref>https://pmc.ncbi.nlm.nih.gov/articles/PMC2477686/</ref>

==Discussion==
Studying the effects of massive doses of radiation in a very low time period (from survivors of the atomic bombs at Hiroshima, and Nagasaki) it was found that the chance of cancer increased linearly. The larger the dose, the greater the chance of cancer.

However, damage to cells occurs all the time, and they are constantly repairing themselves. Logically, radiation damage to cells, that is below the rate at which cells repair themselves would have little effect. There were three basic ideas on how to model radiation damage: 'Linear No Threshold', the 'Threshold Model', and 'Hormesis'.

[[File:LNT_Hormesis_TM.png|600 px|3 Ways to Model Radiation Damage]]

A threshold model was the most logical, but very early radiation safety standards assumed the wildly conservative Linear No Threshold model, assuming that the safety standards would be relaxed as more research came in.

Linear No Threshold (LNT) says that there is NO level of radiation which is safe. Given that we are bathed in radiation all the time, from cosmic rays, ground radiation, and radiation from inside our bodies, the LNT is very suspect. Surely if our radiation dose was equal to, or lower than, the normal background radiation, there would be no harm to our health?

Most scientists assumed that the Threshold Model would be correct. Below some dose the radiation damage was trivial or non-existent, above the threshold dose, it would do harm.

However, as time passed, the evidence accumulated that at very low levels of radiation, there were positive health effects, and some low amount of radiation was helpful to human life.

Sadly, the regulatory regime in most countries, did not update radiation safety standards in light of this new evidence. <ref>https://www.federalregister.gov/documents/2021/08/17/2021-17475/linear-no-threshold-model-and-standards-for-protection-against-radiation</ref>

==Samples of Research==
Science Direct's study on radiation hormesis (volume 902, Dec 2023). <ref>https://www.sciencedirect.com/science/article/pii/S0048969723046557</ref>

ResearchGate - A message to Fukushima: Nothing to Fear But Fear Itself.<ref>https://www.researchgate.net/publication/301313856_A_message_to_Fukushima_Nothing_to_fear_but_fear_itself</ref>

Radiation Hormesis: Historical & Current Perspectives. <ref>https://tech.snmjournals.org/content/43/4/242</ref>

==Speculation of Why Radiation is Helpful==
Life evolved (and is still evolving) in an environment filled with background radiation. (In Earth's early history, this background radiation dose was higher.) It is not surprising that some metabolic pathways would come to depend on a certain amount of radiation in the environment. That said, most speculation as to why low doses of radiation is helpful falls into two categories: 'Exercising the Immune System' and 'Stimulating Repair Mechanisms'.

===Exercising the Immune System===

===Stimulating Repair Mechanisms===
Stem cell therapy found that the stem cells injected into humans aged rapidly, but if irradiated, they lived longer. The radiation triggered improvements in proliferation, mobility, and chondorgenic differentiation capacity, (which improved cell longevity). This is thought to have been caused by stimulating repair mechanisms in these cells. <ref> https://www.cnl.ca/health-science-2/low-dose-radiation-research/#:~:text=There%20is%20growing%20evidence%20that,help%20protect%20people%20against%20diseases.</ref>

==Radiation Regulation and Mars Exploration==
In most countries, radiation safety standards are based on the Linear No Threshold model, which has been shown to be wildly incorrect.

In the city of Ramsar Iran, natural radiation is 50 times that of most areas on Earth, and they show no higher levels of cancer or other forms of radiation disease. Nor are the average lifespans lower than similar cities (with much lower levels of radiation) in other parts of Earth.

If the radiation dosage required for astronauts was at the level of Ramsar, far lighter and less expensive radiation protection would be needed.

This youTube video nicely summarizes the radiation levels of Mars exploration and discusses safety concerns.
https://www.youtube.com/watch?v=VluEllUrseE&list=LL&index=11

See also: https://www.youtube.com/watch?v=gzdLdNRaPKc&list=PL994EB042117A7F6D&index=66

==References==

Radiation Hormesis

2025-11-18T01:13:21Z

RichardWSmith: /* Discussion */ Added reference.

Hormesis is a medical term that notes that many substances show positive effects to health at low dosages, but are dangerous at high dosages. Most drugs fall into this category. <ref>https://onlinelibrary.wiley.com/doi/full/10.1897/07-541.1</ref>. Hormesis was discovered by Hugo Schulz, a professor of pharmacology at the University of Greifsalf in the mid 1880's. He discovered that formic acid (which would kill yeast at high doses) was helpful in their growth at very low dosages. This discovery was so surprising, that he and his assistants checked, and rechecked their results for a couple of years before publishing.

For example, the drug aspirin (salicylate acid), is helpful at low doses, but at high doses is lethal. (In 2004, there were 20,000 cases of moderate aspirin poisoning, and 43 deaths.) <ref>https://en.wikipedia.org/wiki/Salicylate_poisoning</ref>

There is overwhelming evidence that very low levels of radiation are helpful to human health. <ref>https://pmc.ncbi.nlm.nih.gov/articles/PMC2477686/</ref>

==Discussion==
Studying the effects of massive doses of radiation in a very low time period (from survivors of the atomic bombs at Hiroshima, and Nagasaki) it was found that the chance of cancer increased linearly. The larger the dose, the greater the chance of cancer.

However, damage to cells occurs all the time, and they are constantly repairing themselves. Logically, radiation damage to cells, that is below the rate at which cells repair themselves would have little effect. There were three basic ideas on how to model radiation damage: 'Linear No Threshold', the 'Threshold Model', and 'Hormesis'.

[[File:LNT_Hormesis_TM.png|600 px|3 Ways to Model Radiation Damage]]

A threshold model was the most logical, but very early radiation safety standards assumed the wildly conservative Linear No Threshold model, assuming that the safety standards would be relaxed as more research came in.

Linear No Threshold (LNT) says that there is NO level of radiation which is safe. Given that we are bathed in radiation all the time, from cosmic rays, ground radiation, and radiation from inside our bodies, the LNT is very suspect. Surely if our radiation dose was equal to, or lower than, the normal background radiation, there would be no harm to our health?

Most scientists assumed that the Threshold Model would be correct. Below some dose the radiation damage was trivial or non-existent, above the threshold dose, it would do harm.

However, as time passed, the evidence accumulated that at very low levels of radiation, there were positive health effects, and some low amount of radiation was helpful to human life.

Sadly, the regulatory regime in most countries, did not update radiation safety standards in light of this new evidence. <ref>https://www.federalregister.gov/documents/2021/08/17/2021-17475/linear-no-threshold-model-and-standards-for-protection-against-radiation</ref>

==Samples of Research==
Science Direct's study on radiation hormesis (volume 902, Dec 2023). <ref>https://www.sciencedirect.com/science/article/pii/S0048969723046557</ref>

ResearchGate - A message to Fukushima: Nothing to Fear But Fear Itself.<ref>https://www.researchgate.net/publication/301313856_A_message_to_Fukushima_Nothing_to_fear_but_fear_itself</ref>

==Speculation of Why Radiation is Helpful==
Life evolved (and is still evolving) in an environment filled with background radiation. (In Earth's early history, this background radiation dose was higher.) It is not surprising that some metabolic pathways would come to depend on a certain amount of radiation in the environment. That said, most speculation as to why low doses of radiation is helpful falls into two categories: 'Exercising the Immune System' and 'Stimulating Repair Mechanisms'.

===Exercising the Immune System===

===Stimulating Repair Mechanisms===
Stem cell therapy found that the stem cells injected into humans aged rapidly, but if irradiated, they lived longer. The radiation triggered improvements in proliferation, mobility, and chondorgenic differentiation capacity, (which improved cell longevity). This is thought to have been caused by stimulating repair mechanisms in these cells. <ref> https://www.cnl.ca/health-science-2/low-dose-radiation-research/#:~:text=There%20is%20growing%20evidence%20that,help%20protect%20people%20against%20diseases.</ref>

==Radiation Regulation and Mars Exploration==
In most countries, radiation safety standards are based on the Linear No Threshold model, which has been shown to be wildly incorrect.

In the city of Ramsar Iran, natural radiation is 50 times that of most areas on Earth, and they show no higher levels of cancer or other forms of radiation disease. Nor are the average lifespans lower than similar cities (with much lower levels of radiation) in other parts of Earth.

If the radiation dosage required for astronauts was at the level of Ramsar, far lighter and less expensive radiation protection would be needed.

This youTube video nicely summarizes the radiation levels of Mars exploration and discusses safety concerns.
https://www.youtube.com/watch?v=VluEllUrseE&list=LL&index=11

See also: https://www.youtube.com/watch?v=gzdLdNRaPKc&list=PL994EB042117A7F6D&index=66

==References==

Radiation

2025-11-18T01:10:01Z

RichardWSmith: /* Limits for humans */ Formatting

[[Image:nuclear_warning_sign.png|right|Nuclear Danger Icon]]
Natural '''Radiation''' on [[Mars]] is much higher compared with [[Earth]]. The thin [[atmosphere]] provides only a small shielding effect against [[cosmic radiation]]. It provides moderate protection against [[solar radiation]]. Mars also lacks the [[magnetosphere]] that protects Earth.

The average natural radiation level on Mars is 24-30 rads or 240-300 mSv per year<ref name=":0" /><ref name=":5" />. This is about 40-50 times the average on Earth.

1 millisievert [mSv] = 0.1 rad [rd]

==Types of Radiation==
Radiation comes in a variety of forms:<ref>http://www.nas.nasa.gov/About/Education/SpaceSettlement/designer/needs.html#SHIELDING</ref>
{| border="1"
|-
!Name
!Relative Biological Effectiveness (RBE)
!Source
|-
|'''[[X-ray|X-Rays]] and [[gamma ray|Gamma Rays]]'''
|1
|[[Radiation belts]], [[solar radiation]], and bremsstrahlung electrons
|-
|'''[[electron|Electrons]]'''
1.0 MeV 
0.1 MeV
| 
1 
1.08
|Radiation belts
|-
|'''[[proton|Protons]]''' 
100 MeV 
1.5 MeV 
0.1 MeV
| 
1-2 
8.5 
10
|[[Cosmic radiation]], inner-radiation belts, and [[solar radiation]]
|-
|'''[[neutron|Neutrons]]''' 
0.05 ev (thermal) 
1.0 MeV 
10 MeV
| 
2.8 
10.5 
6.4
|Nuclear interactions in the [[sun]]; on Mars, produced when [[cosmic radiation]] interacts with [[regolith]]
|-
|'''[[alpha particles|Alpha Particles]]''' 
5.0 MeV 
1.0 MeV
| 
15 
20
|[[Cosmic radiation]], [[Cosmic rays]]
|-
|[[Heavy Ions|'''Heavy Ions''']]
|Varies widely
|[[Cosmic radiation]]
|-
| colspan="3" |Table 1: Types of radiation
|}
(RBE is a measure of the damage done to living tissue, relative to gamma rays)

Cosmic radiation comprises 85% protons, 14% alpha particles, and 1% heavy ions.<ref>Schimmerling W. (2011, Feb 5). The Space Radiation Environment: An Introduction. <nowiki>https://three.jsc.nasa.gov/concepts/SpaceRadiationEnviron.pdf</nowiki></ref> Solar radiation includes the same radiation types, but it a higher proportion of protons and its heavy primaries have lower energy levels. The high-energy heavy primaries in cosmic radiation can penetrate materials that effectively block lower-energy radiation<ref name=":1">Rapp D. (2006). Radiation Effects and Shielding Requirements in Human Missions to the Moon and Mars. Mars 2:46-71. <nowiki>https://doi.org/10.1555/mars.2006.0004</nowiki></ref>.

==Astronaut Exposures==
The following astronauts have experienced radiations doses equal to, or greater than, the radiation doses in a 2.5 year Mars mission. (Assuming a 520 mSv dose for this mission. Different missions have different amounts of radiation protection.) In three cases the astronauts took more than double the amount of radiation than what is expected this Mars mission.

They are: Carl Waltz, Colin Foale, Peggy Whitson, Jeffrey Williams, Oleg Kononenko, Sergei Krikalyov, Vladimir Solovyov, Valeri Polyakov, Fyodor Yurchikhin, Aleksandr Kaleri, Sergei Krikalyov, Yurki Malenchenko, Gennadi Padalka, & Sergei Avdeyev.

None have exhibited ANY radiation health effects.

Note that NASA has stricter radiation limits than Russia (or the Soviet Union), so as time passes the number of Russian names on this list will become more pronounced.

See [[Long duration space flight]] for more information.

==Exposure limits==

===Limits for humans===
Exposure to dangerous levels of radiation causes [[radiation sickness]] and cancer. The average exposure to radiation on Earth due to natural sources is 6.2 mSv per year<ref name=":3">http://www.ans.org/pi/resources/dosechart/msv.php</ref>. The highest natural exposure is recorded in Ramsar, Iran, where people are exposed up to 260 mSv/y for many generations, with no reported harmful effects<ref name="Ghiassi-Nejad et al 2002">Ghiassi-Nejad et al, Very high background radiation areas of Ramsar, Iran: Preliminary biological studies, Health Physics 82(1):87-93 (February 2002), DOI: 10.1097/00004032-200201000-00011 [https://www.researchgate.net/publication, /11588980_Very_high_background_radiation_areas_of_Ramsar_Iran_Preliminary_biological_studies abstract]</ref>. This is 13 times the maximum exposure allowed radiation workers each year. Importantly, this level of radiation is what Mars settlers (living inside shelters of reasonable cost) would expect. So the 'high' levels of the back ground radiation at Ramsar is good news for Mars settlement.<ref>https://pubmed.ncbi.nlm.nih.gov/17563407/</ref> A recent study showed that the people living in this city showed increased immunity to gamma ray exposure, tho if this is from evolutionary adaption over many generations, or from the immune system being 'exercised' regularly is not known. <ref>https://pubmed.ncbi.nlm.nih.gov/11769138/</ref>

It should be emphasized, that low level radiation doses spread over a long period of time (long enough that the bodies natural functions have time to repair the damage), are far less dangerous than large doses received in a short amount of time. (In fact numerous studies show health benefits from extremely low levels of radiation.) <ref>https://jnm.snmjournals.org/content/59/12/1786</ref> See [[Radiation Hormesis]]. <ref>https://pmc.ncbi.nlm.nih.gov/articles/PMC2477686/</ref>

See also: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6149023/. https://www.sciencedaily.com/releases/2017/09/170913104428.htm. https://www.ajronline.org/doi/full/10.2214/ajr.179.5.1791137. https://en.wikipedia.org/wiki/Radiation_hormesis

Some people believe that ANY exposure to radiation, no matter how slight, poses some risk. Small dose - small risk of cancer. High dose - high risk of cancer. This is the No Minimum Threshold theory of radiation dosage. This works quite well with fast radiation doses high enough to cause cancers later in life, but the evidence is much weaker for low level does over a long period of time. No Minimum Threshold is used by regulatory agencies when they wish to be extremely conservative about radiation risks.<ref>https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6043938/</ref> <ref>https://www.federalregister.gov/documents/2021/08/17/2021-17475/linear-no-threshold-model-and-standards-for-protection-against-radiation</ref> <ref>https://pubs.rsna.org/doi/10.1148/radiol.2511080671</ref>

Nevertheless, there are defined legal limits for exposure during work for several professional activities, such as for X-ray assistants, airplane personnel, etc. The International Commission on Radiation Protection recommends that occupational (work-related) radiation exposure be limited to 50 millisieverts (mSv) per year, and limited to 100 mSv over any 5-year period<ref>http://www.icrp.org/publication.asp?id=ICRP%20Publication%20103</ref>. NASA's radiation dose limits for astronauts are established in NASA-STD-3001<ref>NASA. (2015). NASA Space Flight Human-System Standard Volume 1, Revision A: Crew Health. Retrieved from https://standards.nasa.gov/standard/nasa/nasa-std-3001-vol-1</ref>.

There is scientific uncertainty surrounding the health hazard from cosmic and solar radiation, because most past research on the health effects of radiation studied only x-rays and gamma rays<ref name=":1" />.

===Limits for plants===
[[File:Plant radiation.PNG|thumb|Table 2: Need to find source for this table]]
"In general, plants are relatively radiation resistant when growing and extremely resistant as dormant seeds." Radiation should not interfere with raising plants as food sources, at least not on the time scales of exploration missions.<ref>National Research Council 1996. Radiation Hazards to Crews of Interplanetary Missions: Biological Issues and Research Strategies. Washington, DC: The National Academies Press. <nowiki>https://doi.org/10.17226/5540</nowiki>.</ref>

Table 2 shows the limits for plants(source of table required, in space or on mars?). It shows that in practically all cases plants can survive radiations events that are likely for Mars without any kind of protection.

However, a 2021 study in the Netherlands, conducted on two types of plants, has shown that radiation at the Mars surface may reduce yields substantially<ref>TACK, Nynke, WAMELINK, G. W. W., DENKOVA, A. G., ''et al.'' Influence of Martian Radiation-like Conditions on the Growth of Secale cereale and Lepidium sativum. ''Frontiers in Astronomy and Space Sciences'', 2021, p. 127.</ref>. Germination of plants in a protected environment before setting them out in greenhouses might be a potential mitigation measure but further research is needed.

==Martian Environment==

===Effects of the Martian atmosphere===
Most Solar Proton Events (SPE) particles are low energy & will be stopped by the atmosphere before they reach the surface. However, interactions with atmospheric particles can produce neutrons; those neutrons can reach the surface, so the health hazard is not eliminated. However, the rare, very powerful SPE have energies of medium energy [[Cosmic radiation| cosmic rays]] and are a dangerous hazard.

Cosmic radiation protons are likely to penetrate the atmosphere. Cosmic ray heavy ions may fragment in the atmosphere, producing lower-mass ions that can still harm astronauts on the surface.

Mars' thin atmosphere allows more [[Ultraviolet]] light to reach the surface, compared to Earth. However, habitat structural materials and standard space suits should be sufficient to protect humans from UV radiation.<ref name=":4">Beaty DW, Snook K, Carlton A, Eppler D, Farrell B, Heldmann J,...Zeitlin C, on behalf of the Mars Human Precursor Science Steering Group. (2005). An Analysis of the Precursor Measurements of Mars Needed to Reduce the Risk of the First Human Mission to Mars. Available at <nowiki>https://mepag.jpl.nasa.gov/reports/MHP_SSG_(06-02-05).pdf</nowiki></ref>

Mars atmosphere effect depends on the considered inclination, as the incoming radiations will cross more matter if it's coming from the horizon rather than from the zenith. For inclination angles greater than ~45°, the atmospheric thickness is in the range from 20-30 g/cm2, and for lower inclination angles, the atmospherie thickness can exceed 100 g/cm2<ref name=":2" />. In other words, solar radiation is less dangerous when the sun in near the horizon.

===Low gravity effects on atmospheric thickness===
Note that the radiation protection given by the Martian atmosphere is higher than would be expected considering the air pressure. Pressure can be thought of the weight of the air above you in the atmosphere. Mars' gravity is 38% of Earths. So the weight of that air is less than it would be on Earth. On other words, more air must be above you on Mars to give the same pressure, compared to Earth. For example, you might think that since Mars' air pressure is 0.6% of Earth's, the radiation protection would also be 0.6%. However, the mass of air above you on Mars is 1/38% or 2.6 times thicker than that pressure on Earth. So even tho Mars' air pressure is 0.6% of Earth's, it gives 1.56% of Earth's atmospheric protection. Likewise, if we were to terraform Mars to have 10% Earth's pressure, the radiation protection by that atmosphere would be 26%. If we were to give Mars an atmosphere of 50% Earth's air pressure, then the Martian atmosphere's radiation protection would be 132% that of Earth.

===Effects of cosmic rays striking the regolith===
When [[cosmic rays]] strikes regolith, it can cause the impacted atoms to emit their own radiation. Surrounding regolith particles absorb much of this radiation, with the exception of neutrons. Neutrons generated in this way are called albedo neutrons. These neutrons have the potential to add substantially to the radiation dose for astronauts on the surface.<ref name=":4" />

Neutrons are well absorbed by water, so blocks of ice or water around habitats would be useful radiation protection. Lithium hydride is thought to be the most effective neutron absorber ever discovered, and it might be built into the floor of long term habitats. <ref>https://www.tandfonline.com/doi/abs/10.1179/1743284715Y.0000000105?journalCode=ymst20
</ref> (Lithium hydroxide reacts with water, so it must be kept away from the humid interior of the settlement.) If a long duration habitat is has a space under it, blocks of local ice could protect against secondary neutrons.

===Dose received by an unprotected human on Mars===

====Cosmic radiation====
The equivalent dose rate from cosmic radiation on Earth's surface at sea level is .26 mSv per year<ref name=":3" /> <ref>http://nuclearsafety.gc.ca/eng/resources/radiation/introduction-to-radiation/radiation-doses.cfm</ref> (this makes up ~10% of the annual natural radiation dose, and it increase with altitude). Based on measurements made by the Curiosity rover, the corresponding figure for the surface of Mars is approximately 230 mSv/year<ref name=":0">Hassler DM, Zeitlin C, Wimmer-Schweingruber RF, Ehresmann B, Rafkin S, Eigenbrode JL,...MSL Science Team. (2014). Mars' Surface Radiation Environment Measured with the Mars Science Laboratory's Curiosity Rover. Science 343(6169). <nowiki>https://doi.org/10.1126/science.1244797</nowiki></ref>. More generally, one model estimated that the dose equivalent rate on the surface of Mars ranges from 156.4 mSv/year (at solar maximum) to 273.8 mSv/year (at solar minimum)<ref name=":2" /><ref name=":5">McKenna-Lawlor S, Goncalves P, Keating A, Reitz G, Matthia D. (2012). Overview of Energetic Particle Hazards During Prospective Manned Missions to Mars. Planetary and Space Science. 63: 123-132.</ref>. A 2005 report by the Mars Human Precursor Science Steering Group estimated that (at solar minimum) the dose from cosmic radiation would be 1.2 +/- 0.5 mSv/day; this includes 0.4 +/- 0.4 mSv/day from albedo neutrons.<ref name=":4" />

====Solar Proton Events====
These are also known a [[Coronal Mass Ejections]]. Curiosity also measured the temporary increase in radiation during a single SPE. The results indicate an increase in equivalent dose rate of approximately 25% over a 1-day interval<ref name=":0" />. This figure will vary depending on the intensity of a particular SPE. See [[Solar Cosmic Rays]] for further discussion.

==Effect on material==
Radiation can change the properties of [[plastics]] and metals, making them brittle after a period of time.

As semi-conductor computer chips have become smaller and smaller, they become more likely to be confused or damaged by radiation. Electronic equipment sent to Mars, or built on Mars should be radiation hardened.

==Protection==
Long term [[Habitat|habitats]] should be equipped with a [[radiation shielding]], thick enough to reduce the radiation to a level equal to Earth, that is, almost zero. Best protection may be achieved with houses built in natural [[caves]] or set into cliffs or hillsides. (If, however, it is decided that radiation levels equal to the city of Ramsar, Iran, are safe enough, then the thickness of the radiation shielding suggested below can be reduced up to 13 fold.)

Early exploration habitats could have water tanks, or sand bags above where people live. When radiation goes thru water, every 18 cm reduces the radiation by half. So a water tank 108 cm thick (6 halvings) will reduce the radiation level by 64 times. (As a bonus, water is a good neutron absorber.) Packed soil has a halving-distance of 9.1 cm, so 55 cm of hard soil would provide a similar level of protection. In general, it is far better to use local materials for radiation protection, rather than hauling them from Earth. See: https://www.imagesco.com/geiger/lead-shielding-guide.html.

[[Space suit]]s must be designed with radiation in mind. The suit should provide adequate shielding for the occupant. It may be necessary to design suits with several grades of protection. Suits designed for short-term use can carry lighter shielding which would reduce weight and improve maneuverability. That said, radiation protection from suits will be much less than inside habitats, so minimizing time on the surface will be the largest protection. Going outside during solar storms would likely be banned. Jobs that require regular EVA's (such as cleaning solar cells of dust) should be avoided.

During severe radiation events, such as [[solar flare|solar flares]], surface [[settlement|settlements]] may use [[storm shelter|storm shelters]] with heavier than normal shielding.

"In this work, it is shown that on the Martian surface, almost any amount of aluminum shielding increases exposure levels for humans. The increased exposure levels are attributed to neutron production in the shield and Martian regolith as well as the electromagnetic cascade induced in the Martian atmosphere. This result is significant for optimization of vehicle and shield designs intended for the surface of Mars." <ref name=":2">NASA, Tony C. Slaba, Christopher J. Mertens, and Steve R. Blattnig '' Radiation Shielding Optimization on Mars '', https://spaceradiation.larc.nasa.gov/nasapapers/NASA-TP-2013-217983.pdf, Apr 2013.</ref>

"An in-situ shielding strategy will also be of little help unless several hundred g/cm2 of regolith is utilized. Such a strategy would probably require large scale excavation making it an unlikely candidate. Instead, the shielding strategy would rely primarily on material optimization. Options, such as replacing aluminum structures with high hydrogen content carbon composites, could be pursued." <ref name=":2" /> This opinion is open to argument as in-situ resources utilization for any type of settlement should make large amounts of regolith available for construction. It mainly is true for the very first level of habitats.

The use of g/cm2 can be translated into an equivalent thickness that depends on the material density. For martian regolith at 2000 kg/m3, a thickness of 1m of regolith is 200 g/cm2. Water (or ice) is 100 g/cm2. So the minimum covering for a long term settlement would be 5m or more. For water, although the radiation absorption is better the density is lower, so about the same thickness would be required for protection. (to be discussed)

For people living on Mars for many years, they may prefer to have habitats with more than the minimum radiation protection. For example, if people decide that 2 meters of packed soil is sufficient as minimum protection, they may wish to have 4 or 5 meters for most of the base. They may spend a significant amount of time outside the habitat, so to balance the low radiation protection when outside they have with higher radiation protection inside.

==References==
<references />

==External links==

*[http://www.ips.gov.au/ IPS:] [http://www.ips.gov.au/Category/Educational/Space%20Weather/Space%20Weather%20Effects/guide-to-space-radiation.pdf A Guide to Space Radiation]
*[http://www.niauk.org/radiation-and-safety.html Nuclear Industry Association: Radiation, health and nuclear safety]
*[https://hesperia.gsfc.nasa.gov/sspvse/posters/DF_Smart/poster.pdf The frequency distribution of solar proton events: 5 solar cycles and 45 solar cycles]
*[[w:Radiation_hormesis|Low levels of radiation can be stimulatory: Radiation Hormesis]]

[[Category:Radiation Protection]]

Radiation Hormesis

2025-11-18T01:07:00Z

RichardWSmith: Added link.

Hormesis is a medical term that notes that many substances show positive effects to health at low dosages, but are dangerous at high dosages. Most drugs fall into this category. <ref>https://onlinelibrary.wiley.com/doi/full/10.1897/07-541.1</ref>. Hormesis was discovered by Hugo Schulz, a professor of pharmacology at the University of Greifsalf in the mid 1880's. He discovered that formic acid (which would kill yeast at high doses) was helpful in their growth at very low dosages. This discovery was so surprising, that he and his assistants checked, and rechecked their results for a couple of years before publishing.

For example, the drug aspirin (salicylate acid), is helpful at low doses, but at high doses is lethal. (In 2004, there were 20,000 cases of moderate aspirin poisoning, and 43 deaths.) <ref>https://en.wikipedia.org/wiki/Salicylate_poisoning</ref>

There is overwhelming evidence that very low levels of radiation are helpful to human health. <ref>https://pmc.ncbi.nlm.nih.gov/articles/PMC2477686/</ref>

==Discussion==
Studying the effects of massive doses of radiation in a very low time period (from survivors of the atomic bombs at Hiroshima, and Nagasaki) it was found that the chance of cancer increased linearly. The larger the dose, the greater the chance of cancer.

However, damage to cells occurs all the time, and they are constantly repairing themselves. Logically, radiation damage to cells, that is below the rate at which cells repair themselves would have little effect. There were three basic ideas on how to model radiation damage: 'Linear No Threshold', the 'Threshold Model', and 'Hormesis'.

[[File:LNT_Hormesis_TM.png|600 px|3 Ways to Model Radiation Damage]]

A threshold model was the most logical, but very early radiation safety standards assumed the wildly conservative Linear No Threshold model, assuming that the safety standards would be relaxed as more research came in.

Linear No Threshold (LNT) says that there is NO level of radiation which is safe. Given that we are bathed in radiation all the time, from cosmic rays, ground radiation, and radiation from inside our bodies, the LNT is very suspect. Surely if our radiation dose was equal to, or lower than, the normal background radiation, there would be no harm to our health?

Most scientists assumed that the Threshold Model would be correct. Below some dose the radiation damage was trivial or non-existent, above the threshold dose, it would do harm.

However, as time passed, the evidence accumulated that at very low levels of radiation, there were positive health effects, and some low amount of radiation was helpful to human life.

Sadly, the regulatory regime in most countries, did not update radiation safety standards in light of this new evidence.

==Samples of Research==
Science Direct's study on radiation hormesis (volume 902, Dec 2023). <ref>https://www.sciencedirect.com/science/article/pii/S0048969723046557</ref>

ResearchGate - A message to Fukushima: Nothing to Fear But Fear Itself.<ref>https://www.researchgate.net/publication/301313856_A_message_to_Fukushima_Nothing_to_fear_but_fear_itself</ref>

==Speculation of Why Radiation is Helpful==
Life evolved (and is still evolving) in an environment filled with background radiation. (In Earth's early history, this background radiation dose was higher.) It is not surprising that some metabolic pathways would come to depend on a certain amount of radiation in the environment. That said, most speculation as to why low doses of radiation is helpful falls into two categories: 'Exercising the Immune System' and 'Stimulating Repair Mechanisms'.

===Exercising the Immune System===

===Stimulating Repair Mechanisms===
Stem cell therapy found that the stem cells injected into humans aged rapidly, but if irradiated, they lived longer. The radiation triggered improvements in proliferation, mobility, and chondorgenic differentiation capacity, (which improved cell longevity). This is thought to have been caused by stimulating repair mechanisms in these cells. <ref> https://www.cnl.ca/health-science-2/low-dose-radiation-research/#:~:text=There%20is%20growing%20evidence%20that,help%20protect%20people%20against%20diseases.</ref>

==Radiation Regulation and Mars Exploration==
In most countries, radiation safety standards are based on the Linear No Threshold model, which has been shown to be wildly incorrect.

In the city of Ramsar Iran, natural radiation is 50 times that of most areas on Earth, and they show no higher levels of cancer or other forms of radiation disease. Nor are the average lifespans lower than similar cities (with much lower levels of radiation) in other parts of Earth.

If the radiation dosage required for astronauts was at the level of Ramsar, far lighter and less expensive radiation protection would be needed.

This youTube video nicely summarizes the radiation levels of Mars exploration and discusses safety concerns.
https://www.youtube.com/watch?v=VluEllUrseE&list=LL&index=11

See also: https://www.youtube.com/watch?v=gzdLdNRaPKc&list=PL994EB042117A7F6D&index=66

==References==

Radiation Hormesis

2025-11-18T01:02:50Z

RichardWSmith: /* Radiation Regulation and Mars Exploration */

Hormesis is a medical term that notes that many substances show positive effects to health at low dosages, but are dangerous at high dosages. Most drugs fall into this category. <ref>https://onlinelibrary.wiley.com/doi/full/10.1897/07-541.1</ref>. Hormesis was discovered by Hugo Schulz, a professor of pharmacology at the University of Greifsalf in the mid 1880's. He discovered that formic acid (which would kill yeast at high doses) was helpful in their growth at very low dosages. This discovery was so surprising, that he and his assistants checked, and rechecked their results for a couple of years before publishing.

For example, the drug aspirin (salicylate acid), is helpful at low doses, but at high doses is lethal. (In 2004, there were 20,000 cases of moderate aspirin poisoning, and 43 deaths.) <ref>https://en.wikipedia.org/wiki/Salicylate_poisoning</ref>

There is overwhelming evidence that very low levels of radiation are helpful to human health. <ref>https://pmc.ncbi.nlm.nih.gov/articles/PMC2477686/</ref>

==Discussion==
Studying the effects of massive doses of radiation in a very low time period (from survivors of the atomic bombs at Hiroshima, and Nagasaki) it was found that the chance of cancer increased linearly. The larger the dose, the greater the chance of cancer.

However, damage to cells occurs all the time, and they are constantly repairing themselves. Logically, radiation damage to cells, that is below the rate at which cells repair themselves would have little effect. There were three basic ideas on how to model radiation damage: 'Linear No Threshold', the 'Threshold Model', and 'Hormesis'.

[[File:LNT_Hormesis_TM.png|600 px|3 Ways to Model Radiation Damage]]

A threshold model was the most logical, but very early radiation safety standards assumed the wildly conservative Linear No Threshold model, assuming that the safety standards would be relaxed as more research came in.

Linear No Threshold (LNT) says that there is NO level of radiation which is safe. Given that we are bathed in radiation all the time, from cosmic rays, ground radiation, and radiation from inside our bodies, the LNT is very suspect. Surely if our radiation dose was equal to, or lower than, the normal background radiation, there would be no harm to our health?

Most scientists assumed that the Threshold Model would be correct. Below some dose the radiation damage was trivial or non-existent, above the threshold dose, it would do harm.

However, as time passed, the evidence accumulated that at very low levels of radiation, there were positive health effects, and some low amount of radiation was helpful to human life.

Sadly, the regulatory regime in most countries, did not update radiation safety standards in light of this new evidence.

==Samples of Research==
Science Direct's study on radiation hormesis (volume 902, Dec 2023). <ref>https://www.sciencedirect.com/science/article/pii/S0048969723046557</ref>

ResearchGate - A message to Fukushima: Nothing to Fear But Fear Itself.<ref>https://www.researchgate.net/publication/301313856_A_message_to_Fukushima_Nothing_to_fear_but_fear_itself</ref>

==Speculation of Why Radiation is Helpful==
Life evolved (and is still evolving) in an environment filled with background radiation. (In Earth's early history, this background radiation dose was higher.) It is not surprising that some metabolic pathways would come to depend on a certain amount of radiation in the environment. That said, most speculation as to why low doses of radiation is helpful falls into two categories: 'Exercising the Immune System' and 'Stimulating Repair Mechanisms'.

===Exercising the Immune System===

===Stimulating Repair Mechanisms===
Stem cell therapy found that the stem cells injected into humans aged rapidly, but if irradiated, they lived longer. The radiation triggered improvements in proliferation, mobility, and chondorgenic differentiation capacity, (which improved cell longevity). This is thought to have been caused by stimulating repair mechanisms in these cells. <ref> https://www.cnl.ca/health-science-2/low-dose-radiation-research/#:~:text=There%20is%20growing%20evidence%20that,help%20protect%20people%20against%20diseases.</ref>

==Radiation Regulation and Mars Exploration==
In most countries, radiation safety standards are based on the Linear No Threshold model, which has been shown to be wildly incorrect.

In the city of Ramsar Iran, natural radiation is 50 times that of most areas on Earth, and they show no higher levels of cancer or other forms of radiation disease. Nor are the average lifespans lower than similar cities (with much lower levels of radiation) in other parts of Earth.

If the radiation dosage required for astronauts was at the level of Ramsar, far lighter and less expensive radiation protection would be needed.

This youTube video nicely summarizes the radiation levels of Mars exploration and discusses safety concerns.

See also: https://www.youtube.com/watch?v=gzdLdNRaPKc&list=PL994EB042117A7F6D&index=66

==References==

Radiation Hormesis

2025-11-18T01:02:06Z

RichardWSmith: /* Stimulating Repair Mechanisms */

Hormesis is a medical term that notes that many substances show positive effects to health at low dosages, but are dangerous at high dosages. Most drugs fall into this category. <ref>https://onlinelibrary.wiley.com/doi/full/10.1897/07-541.1</ref>. Hormesis was discovered by Hugo Schulz, a professor of pharmacology at the University of Greifsalf in the mid 1880's. He discovered that formic acid (which would kill yeast at high doses) was helpful in their growth at very low dosages. This discovery was so surprising, that he and his assistants checked, and rechecked their results for a couple of years before publishing.

For example, the drug aspirin (salicylate acid), is helpful at low doses, but at high doses is lethal. (In 2004, there were 20,000 cases of moderate aspirin poisoning, and 43 deaths.) <ref>https://en.wikipedia.org/wiki/Salicylate_poisoning</ref>

There is overwhelming evidence that very low levels of radiation are helpful to human health. <ref>https://pmc.ncbi.nlm.nih.gov/articles/PMC2477686/</ref>

==Discussion==
Studying the effects of massive doses of radiation in a very low time period (from survivors of the atomic bombs at Hiroshima, and Nagasaki) it was found that the chance of cancer increased linearly. The larger the dose, the greater the chance of cancer.

However, damage to cells occurs all the time, and they are constantly repairing themselves. Logically, radiation damage to cells, that is below the rate at which cells repair themselves would have little effect. There were three basic ideas on how to model radiation damage: 'Linear No Threshold', the 'Threshold Model', and 'Hormesis'.

[[File:LNT_Hormesis_TM.png|600 px|3 Ways to Model Radiation Damage]]

A threshold model was the most logical, but very early radiation safety standards assumed the wildly conservative Linear No Threshold model, assuming that the safety standards would be relaxed as more research came in.

Linear No Threshold (LNT) says that there is NO level of radiation which is safe. Given that we are bathed in radiation all the time, from cosmic rays, ground radiation, and radiation from inside our bodies, the LNT is very suspect. Surely if our radiation dose was equal to, or lower than, the normal background radiation, there would be no harm to our health?

Most scientists assumed that the Threshold Model would be correct. Below some dose the radiation damage was trivial or non-existent, above the threshold dose, it would do harm.

However, as time passed, the evidence accumulated that at very low levels of radiation, there were positive health effects, and some low amount of radiation was helpful to human life.

Sadly, the regulatory regime in most countries, did not update radiation safety standards in light of this new evidence.

==Samples of Research==
Science Direct's study on radiation hormesis (volume 902, Dec 2023). <ref>https://www.sciencedirect.com/science/article/pii/S0048969723046557</ref>

ResearchGate - A message to Fukushima: Nothing to Fear But Fear Itself.<ref>https://www.researchgate.net/publication/301313856_A_message_to_Fukushima_Nothing_to_fear_but_fear_itself</ref>

==Speculation of Why Radiation is Helpful==
Life evolved (and is still evolving) in an environment filled with background radiation. (In Earth's early history, this background radiation dose was higher.) It is not surprising that some metabolic pathways would come to depend on a certain amount of radiation in the environment. That said, most speculation as to why low doses of radiation is helpful falls into two categories: 'Exercising the Immune System' and 'Stimulating Repair Mechanisms'.

===Exercising the Immune System===

===Stimulating Repair Mechanisms===
Stem cell therapy found that the stem cells injected into humans aged rapidly, but if irradiated, they lived longer. The radiation triggered improvements in proliferation, mobility, and chondorgenic differentiation capacity, (which improved cell longevity). This is thought to have been caused by stimulating repair mechanisms in these cells. <ref> https://www.cnl.ca/health-science-2/low-dose-radiation-research/#:~:text=There%20is%20growing%20evidence%20that,help%20protect%20people%20against%20diseases.</ref>

==Radiation Regulation and Mars Exploration==
In most countries, radiation safety standards are based on the Linear No Threshold model, which has been shown to be wildly incorrect.

In the city of Ramsar Iran, natural radiation is 50 times that of most areas on Earth, and they show no higher levels of cancer or other forms of radiation disease. Nor are the average lifespans lower than similar cities in other parts of Earth.

If the radiation dosage required for astronauts was at the level of Ramsar, far lighter and less expensive radiation protection would be needed.

This youTube video nicely summarizes the radiation levels of Mars exploration and discusses safety concerns.

See also: https://www.youtube.com/watch?v=gzdLdNRaPKc&list=PL994EB042117A7F6D&index=66

==References==

Radiation Hormesis

2025-11-18T00:35:07Z

RichardWSmith: Discussion of how overreaction to radiation danger causes real harm.

Radiation Hormesis

2025-11-18T00:29:53Z

RichardWSmith: /* Samples of Research */

Radiation Hormesis

2025-11-18T00:28:49Z

RichardWSmith: /* Samples of Research */ Added reference.

Radiation Hormesis

2025-11-18T00:26:43Z

RichardWSmith: /* Discussion */

Radiation Hormesis

2025-11-18T00:14:47Z

RichardWSmith: /* Discussion */

Radiation Hormesis

2025-11-18T00:13:00Z

RichardWSmith: /* Discussion */ Fiddling with image formatting.

Radiation Hormesis

2025-11-18T00:09:33Z

RichardWSmith: /* Discussion */ Adding Image

File:LNT Hormesis TM.png

2025-11-18T00:05:12Z

RichardWSmith: Three basic ways of modelling harm from Radiation: LNT, Hormesis, Threshold Model

== Summary ==
Three basic ways of modelling harm from Radiation: LNT, Hormesis, Threshold Model
== Licensing ==
{{PD}}

Radiation Hormesis

2025-11-18T00:01:07Z

RichardWSmith: Started discussion section. 3 major models.

Radiation Hormesis

2025-11-17T23:43:06Z

RichardWSmith: Reference to Stimulating Repair Mechanisms.

Radiation Hormesis

2025-11-17T23:26:21Z

RichardWSmith:

Radiation Hormesis

2025-11-17T23:24:28Z

RichardWSmith: Formatting.

Radiation Hormesis

2025-11-17T23:09:33Z

RichardWSmith: New page.

Radiation

2025-11-17T22:48:44Z

RichardWSmith: /* Exposure limits */ Fixed link.

[[Image:nuclear_warning_sign.png|right|Nuclear Danger Icon]]
Natural '''Radiation''' on [[Mars]] is much higher compared with [[Earth]]. The thin [[atmosphere]] provides only a small shielding effect against [[cosmic radiation]]. It provides moderate protection against [[solar radiation]]. Mars also lacks the [[magnetosphere]] that protects Earth.

The average natural radiation level on Mars is 24-30 rads or 240-300 mSv per year<ref name=":0" /><ref name=":5" />. This is about 40-50 times the average on Earth.

1 millisievert [mSv] = 0.1 rad [rd]

==Types of Radiation==
Radiation comes in a variety of forms:<ref>http://www.nas.nasa.gov/About/Education/SpaceSettlement/designer/needs.html#SHIELDING</ref>
{| border="1"
|-
!Name
!Relative Biological Effectiveness (RBE)
!Source
|-
|'''[[X-ray|X-Rays]] and [[gamma ray|Gamma Rays]]'''
|1
|[[Radiation belts]], [[solar radiation]], and bremsstrahlung electrons
|-
|'''[[electron|Electrons]]'''
1.0 MeV 
0.1 MeV
| 
1 
1.08
|Radiation belts
|-
|'''[[proton|Protons]]''' 
100 MeV 
1.5 MeV 
0.1 MeV
| 
1-2 
8.5 
10
|[[Cosmic radiation]], inner-radiation belts, and [[solar radiation]]
|-
|'''[[neutron|Neutrons]]''' 
0.05 ev (thermal) 
1.0 MeV 
10 MeV
| 
2.8 
10.5 
6.4
|Nuclear interactions in the [[sun]]; on Mars, produced when [[cosmic radiation]] interacts with [[regolith]]
|-
|'''[[alpha particles|Alpha Particles]]''' 
5.0 MeV 
1.0 MeV
| 
15 
20
|[[Cosmic radiation]], [[Cosmic rays]]
|-
|[[Heavy Ions|'''Heavy Ions''']]
|Varies widely
|[[Cosmic radiation]]
|-
| colspan="3" |Table 1: Types of radiation
|}
(RBE is a measure of the damage done to living tissue, relative to gamma rays)

Cosmic radiation comprises 85% protons, 14% alpha particles, and 1% heavy ions.<ref>Schimmerling W. (2011, Feb 5). The Space Radiation Environment: An Introduction. <nowiki>https://three.jsc.nasa.gov/concepts/SpaceRadiationEnviron.pdf</nowiki></ref> Solar radiation includes the same radiation types, but it a higher proportion of protons and its heavy primaries have lower energy levels. The high-energy heavy primaries in cosmic radiation can penetrate materials that effectively block lower-energy radiation<ref name=":1">Rapp D. (2006). Radiation Effects and Shielding Requirements in Human Missions to the Moon and Mars. Mars 2:46-71. <nowiki>https://doi.org/10.1555/mars.2006.0004</nowiki></ref>.

==Astronaut Exposures==
The following astronauts have experienced radiations doses equal to, or greater than, the radiation doses in a 2.5 year Mars mission. (Assuming a 520 mSv dose for this mission. Different missions have different amounts of radiation protection.) In three cases the astronauts took more than double the amount of radiation than what is expected this Mars mission.

They are: Carl Waltz, Colin Foale, Peggy Whitson, Jeffrey Williams, Oleg Kononenko, Sergei Krikalyov, Vladimir Solovyov, Valeri Polyakov, Fyodor Yurchikhin, Aleksandr Kaleri, Sergei Krikalyov, Yurki Malenchenko, Gennadi Padalka, & Sergei Avdeyev.

None have exhibited ANY radiation health effects.

Note that NASA has stricter radiation limits than Russia (or the Soviet Union), so as time passes the number of Russian names on this list will become more pronounced.

See [[Long duration space flight]] for more information.

==Exposure limits==

===Limits for humans===
Exposure to dangerous levels of radiation causes [[radiation sickness]] and cancer. The average exposure to radiation on Earth due to natural sources is 6.2 mSv per year<ref name=":3">http://www.ans.org/pi/resources/dosechart/msv.php</ref>. The highest natural exposure is recorded in Ramsar, Iran, where people are exposed up to 260 mSv/y for many generations, with no reported harmful effects<ref name="Ghiassi-Nejad et al 2002">Ghiassi-Nejad et al, Very high background radiation areas of Ramsar, Iran: Preliminary biological studies, Health Physics 82(1):87-93 (February 2002), DOI: 10.1097/00004032-200201000-00011 [https://www.researchgate.net/publication, /11588980_Very_high_background_radiation_areas_of_Ramsar_Iran_Preliminary_biological_studies abstract]</ref>. This is 13 times the maximum exposure allowed radiation workers each year. Importantly, this level of radiation is what Mars settlers (living inside shelters of reasonable cost) would expect. So the 'high' levels of the back ground radiation at Ramsar is good news for Mars settlement.<ref>https://pubmed.ncbi.nlm.nih.gov/17563407/</ref> A recent study showed that the people living in this city showed increased immunity to gamma ray exposure, tho if this is from evolutionary adaption over many generations, or from the immune system being 'exercised' regularly is not known. <ref>https://pubmed.ncbi.nlm.nih.gov/11769138/</ref>

It should be emphasized, that low level radiation doses spread over a long period of time (long enough that the bodies natural functions have time to repair the damage), are far less dangerous than large doses received in a short amount of time. (In fact numerous studies show health benefits from extremely low levels of radiation.) <ref>https://jnm.snmjournals.org/content/59/12/1786</ref> See [[Radiation Hormesis]]. <ref>https://pmc.ncbi.nlm.nih.gov/articles/PMC2477686/</ref>
See also: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6149023/. https://www.sciencedaily.com/releases/2017/09/170913104428.htm. https://www.ajronline.org/doi/full/10.2214/ajr.179.5.1791137. https://en.wikipedia.org/wiki/Radiation_hormesis

Some people believe that ANY exposure to radiation, no matter how slight, poses some risk. Small dose - small risk of cancer. High dose - high risk of cancer. This is the No Minimum Threshold theory of radiation dosage. This works quite well with fast radiation doses high enough to cause cancers later in life, but the evidence is much weaker for low level does over a long period of time. No Minimum Threshold is used by regulatory agencies when they wish to be extremely conservative about radiation risks.<ref>https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6043938/</ref> <ref>https://www.federalregister.gov/documents/2021/08/17/2021-17475/linear-no-threshold-model-and-standards-for-protection-against-radiation</ref> <ref>https://pubs.rsna.org/doi/10.1148/radiol.2511080671</ref>

Nevertheless, there are defined legal limits for exposure during work for several professional activities, such as for X-ray assistants, airplane personnel, etc. The International Commission on Radiation Protection recommends that occupational (work-related) radiation exposure be limited to 50 millisieverts (mSv) per year, and limited to 100 mSv over any 5-year period<ref>http://www.icrp.org/publication.asp?id=ICRP%20Publication%20103</ref>. NASA's radiation dose limits for astronauts are established in NASA-STD-3001<ref>NASA. (2015). NASA Space Flight Human-System Standard Volume 1, Revision A: Crew Health. Retrieved from https://standards.nasa.gov/standard/nasa/nasa-std-3001-vol-1</ref>.

There is scientific uncertainty surrounding the health hazard from cosmic and solar radiation, because most past research on the health effects of radiation studied only x-rays and gamma rays<ref name=":1" />.

===Limits for plants===
[[File:Plant radiation.PNG|thumb|Table 2: Need to find source for this table]]
"In general, plants are relatively radiation resistant when growing and extremely resistant as dormant seeds." Radiation should not interfere with raising plants as food sources, at least not on the time scales of exploration missions.<ref>National Research Council 1996. Radiation Hazards to Crews of Interplanetary Missions: Biological Issues and Research Strategies. Washington, DC: The National Academies Press. <nowiki>https://doi.org/10.17226/5540</nowiki>.</ref>

Table 2 shows the limits for plants(source of table required, in space or on mars?). It shows that in practically all cases plants can survive radiations events that are likely for Mars without any kind of protection.

However, a 2021 study in the Netherlands, conducted on two types of plants, has shown that radiation at the Mars surface may reduce yields substantially<ref>TACK, Nynke, WAMELINK, G. W. W., DENKOVA, A. G., ''et al.'' Influence of Martian Radiation-like Conditions on the Growth of Secale cereale and Lepidium sativum. ''Frontiers in Astronomy and Space Sciences'', 2021, p. 127.</ref>. Germination of plants in a protected environment before setting them out in greenhouses might be a potential mitigation measure but further research is needed.

==Martian Environment==

===Effects of the Martian atmosphere===
Most Solar Proton Events (SPE) particles are low energy & will be stopped by the atmosphere before they reach the surface. However, interactions with atmospheric particles can produce neutrons; those neutrons can reach the surface, so the health hazard is not eliminated. However, the rare, very powerful SPE have energies of medium energy [[Cosmic radiation| cosmic rays]] and are a dangerous hazard.

Cosmic radiation protons are likely to penetrate the atmosphere. Cosmic ray heavy ions may fragment in the atmosphere, producing lower-mass ions that can still harm astronauts on the surface.

Mars' thin atmosphere allows more [[Ultraviolet]] light to reach the surface, compared to Earth. However, habitat structural materials and standard space suits should be sufficient to protect humans from UV radiation.<ref name=":4">Beaty DW, Snook K, Carlton A, Eppler D, Farrell B, Heldmann J,...Zeitlin C, on behalf of the Mars Human Precursor Science Steering Group. (2005). An Analysis of the Precursor Measurements of Mars Needed to Reduce the Risk of the First Human Mission to Mars. Available at <nowiki>https://mepag.jpl.nasa.gov/reports/MHP_SSG_(06-02-05).pdf</nowiki></ref>

Mars atmosphere effect depends on the considered inclination, as the incoming radiations will cross more matter if it's coming from the horizon rather than from the zenith. For inclination angles greater than ~45°, the atmospheric thickness is in the range from 20-30 g/cm2, and for lower inclination angles, the atmospherie thickness can exceed 100 g/cm2<ref name=":2" />. In other words, solar radiation is less dangerous when the sun in near the horizon.

===Low gravity effects on atmospheric thickness===
Note that the radiation protection given by the Martian atmosphere is higher than would be expected considering the air pressure. Pressure can be thought of the weight of the air above you in the atmosphere. Mars' gravity is 38% of Earths. So the weight of that air is less than it would be on Earth. On other words, more air must be above you on Mars to give the same pressure, compared to Earth. For example, you might think that since Mars' air pressure is 0.6% of Earth's, the radiation protection would also be 0.6%. However, the mass of air above you on Mars is 1/38% or 2.6 times thicker than that pressure on Earth. So even tho Mars' air pressure is 0.6% of Earth's, it gives 1.56% of Earth's atmospheric protection. Likewise, if we were to terraform Mars to have 10% Earth's pressure, the radiation protection by that atmosphere would be 26%. If we were to give Mars an atmosphere of 50% Earth's air pressure, then the Martian atmosphere's radiation protection would be 132% that of Earth.

===Effects of cosmic rays striking the regolith===
When [[cosmic rays]] strikes regolith, it can cause the impacted atoms to emit their own radiation. Surrounding regolith particles absorb much of this radiation, with the exception of neutrons. Neutrons generated in this way are called albedo neutrons. These neutrons have the potential to add substantially to the radiation dose for astronauts on the surface.<ref name=":4" />

Neutrons are well absorbed by water, so blocks of ice or water around habitats would be useful radiation protection. Lithium hydride is thought to be the most effective neutron absorber ever discovered, and it might be built into the floor of long term habitats. <ref>https://www.tandfonline.com/doi/abs/10.1179/1743284715Y.0000000105?journalCode=ymst20
</ref> (Lithium hydroxide reacts with water, so it must be kept away from the humid interior of the settlement.) If a long duration habitat is has a space under it, blocks of local ice could protect against secondary neutrons.

===Dose received by an unprotected human on Mars===

====Cosmic radiation====
The equivalent dose rate from cosmic radiation on Earth's surface at sea level is .26 mSv per year<ref name=":3" /> <ref>http://nuclearsafety.gc.ca/eng/resources/radiation/introduction-to-radiation/radiation-doses.cfm</ref> (this makes up ~10% of the annual natural radiation dose, and it increase with altitude). Based on measurements made by the Curiosity rover, the corresponding figure for the surface of Mars is approximately 230 mSv/year<ref name=":0">Hassler DM, Zeitlin C, Wimmer-Schweingruber RF, Ehresmann B, Rafkin S, Eigenbrode JL,...MSL Science Team. (2014). Mars' Surface Radiation Environment Measured with the Mars Science Laboratory's Curiosity Rover. Science 343(6169). <nowiki>https://doi.org/10.1126/science.1244797</nowiki></ref>. More generally, one model estimated that the dose equivalent rate on the surface of Mars ranges from 156.4 mSv/year (at solar maximum) to 273.8 mSv/year (at solar minimum)<ref name=":2" /><ref name=":5">McKenna-Lawlor S, Goncalves P, Keating A, Reitz G, Matthia D. (2012). Overview of Energetic Particle Hazards During Prospective Manned Missions to Mars. Planetary and Space Science. 63: 123-132.</ref>. A 2005 report by the Mars Human Precursor Science Steering Group estimated that (at solar minimum) the dose from cosmic radiation would be 1.2 +/- 0.5 mSv/day; this includes 0.4 +/- 0.4 mSv/day from albedo neutrons.<ref name=":4" />

====Solar Proton Events====
These are also known a [[Coronal Mass Ejections]]. Curiosity also measured the temporary increase in radiation during a single SPE. The results indicate an increase in equivalent dose rate of approximately 25% over a 1-day interval<ref name=":0" />. This figure will vary depending on the intensity of a particular SPE. See [[Solar Cosmic Rays]] for further discussion.

==Effect on material==
Radiation can change the properties of [[plastics]] and metals, making them brittle after a period of time.

As semi-conductor computer chips have become smaller and smaller, they become more likely to be confused or damaged by radiation. Electronic equipment sent to Mars, or built on Mars should be radiation hardened.

==Protection==
Long term [[Habitat|habitats]] should be equipped with a [[radiation shielding]], thick enough to reduce the radiation to a level equal to Earth, that is, almost zero. Best protection may be achieved with houses built in natural [[caves]] or set into cliffs or hillsides. (If, however, it is decided that radiation levels equal to the city of Ramsar, Iran, are safe enough, then the thickness of the radiation shielding suggested below can be reduced up to 13 fold.)

Early exploration habitats could have water tanks, or sand bags above where people live. When radiation goes thru water, every 18 cm reduces the radiation by half. So a water tank 108 cm thick (6 halvings) will reduce the radiation level by 64 times. (As a bonus, water is a good neutron absorber.) Packed soil has a halving-distance of 9.1 cm, so 55 cm of hard soil would provide a similar level of protection. In general, it is far better to use local materials for radiation protection, rather than hauling them from Earth. See: https://www.imagesco.com/geiger/lead-shielding-guide.html.

[[Space suit]]s must be designed with radiation in mind. The suit should provide adequate shielding for the occupant. It may be necessary to design suits with several grades of protection. Suits designed for short-term use can carry lighter shielding which would reduce weight and improve maneuverability. That said, radiation protection from suits will be much less than inside habitats, so minimizing time on the surface will be the largest protection. Going outside during solar storms would likely be banned. Jobs that require regular EVA's (such as cleaning solar cells of dust) should be avoided.

During severe radiation events, such as [[solar flare|solar flares]], surface [[settlement|settlements]] may use [[storm shelter|storm shelters]] with heavier than normal shielding.

"In this work, it is shown that on the Martian surface, almost any amount of aluminum shielding increases exposure levels for humans. The increased exposure levels are attributed to neutron production in the shield and Martian regolith as well as the electromagnetic cascade induced in the Martian atmosphere. This result is significant for optimization of vehicle and shield designs intended for the surface of Mars." <ref name=":2">NASA, Tony C. Slaba, Christopher J. Mertens, and Steve R. Blattnig '' Radiation Shielding Optimization on Mars '', https://spaceradiation.larc.nasa.gov/nasapapers/NASA-TP-2013-217983.pdf, Apr 2013.</ref>

"An in-situ shielding strategy will also be of little help unless several hundred g/cm2 of regolith is utilized. Such a strategy would probably require large scale excavation making it an unlikely candidate. Instead, the shielding strategy would rely primarily on material optimization. Options, such as replacing aluminum structures with high hydrogen content carbon composites, could be pursued." <ref name=":2" /> This opinion is open to argument as in-situ resources utilization for any type of settlement should make large amounts of regolith available for construction. It mainly is true for the very first level of habitats.

The use of g/cm2 can be translated into an equivalent thickness that depends on the material density. For martian regolith at 2000 kg/m3, a thickness of 1m of regolith is 200 g/cm2. Water (or ice) is 100 g/cm2. So the minimum covering for a long term settlement would be 5m or more. For water, although the radiation absorption is better the density is lower, so about the same thickness would be required for protection. (to be discussed)

For people living on Mars for many years, they may prefer to have habitats with more than the minimum radiation protection. For example, if people decide that 2 meters of packed soil is sufficient as minimum protection, they may wish to have 4 or 5 meters for most of the base. They may spend a significant amount of time outside the habitat, so to balance the low radiation protection when outside they have with higher radiation protection inside.

==References==
<references />

==External links==

*[http://www.ips.gov.au/ IPS:] [http://www.ips.gov.au/Category/Educational/Space%20Weather/Space%20Weather%20Effects/guide-to-space-radiation.pdf A Guide to Space Radiation]
*[http://www.niauk.org/radiation-and-safety.html Nuclear Industry Association: Radiation, health and nuclear safety]
*[https://hesperia.gsfc.nasa.gov/sspvse/posters/DF_Smart/poster.pdf The frequency distribution of solar proton events: 5 solar cycles and 45 solar cycles]
*[[w:Radiation_hormesis|Low levels of radiation can be stimulatory: Radiation Hormesis]]

[[Category:Radiation Protection]]

Radiation

2025-11-17T22:48:08Z

RichardWSmith: /* Exposure limits */ Added link to National Institute of Health's page on Radiation Hormesis.

[[Image:nuclear_warning_sign.png|right|Nuclear Danger Icon]]
Natural '''Radiation''' on [[Mars]] is much higher compared with [[Earth]]. The thin [[atmosphere]] provides only a small shielding effect against [[cosmic radiation]]. It provides moderate protection against [[solar radiation]]. Mars also lacks the [[magnetosphere]] that protects Earth.

The average natural radiation level on Mars is 24-30 rads or 240-300 mSv per year<ref name=":0" /><ref name=":5" />. This is about 40-50 times the average on Earth.

1 millisievert [mSv] = 0.1 rad [rd]

==Types of Radiation==
Radiation comes in a variety of forms:<ref>http://www.nas.nasa.gov/About/Education/SpaceSettlement/designer/needs.html#SHIELDING</ref>
{| border="1"
|-
!Name
!Relative Biological Effectiveness (RBE)
!Source
|-
|'''[[X-ray|X-Rays]] and [[gamma ray|Gamma Rays]]'''
|1
|[[Radiation belts]], [[solar radiation]], and bremsstrahlung electrons
|-
|'''[[electron|Electrons]]'''
1.0 MeV 
0.1 MeV
| 
1 
1.08
|Radiation belts
|-
|'''[[proton|Protons]]''' 
100 MeV 
1.5 MeV 
0.1 MeV
| 
1-2 
8.5 
10
|[[Cosmic radiation]], inner-radiation belts, and [[solar radiation]]
|-
|'''[[neutron|Neutrons]]''' 
0.05 ev (thermal) 
1.0 MeV 
10 MeV
| 
2.8 
10.5 
6.4
|Nuclear interactions in the [[sun]]; on Mars, produced when [[cosmic radiation]] interacts with [[regolith]]
|-
|'''[[alpha particles|Alpha Particles]]''' 
5.0 MeV 
1.0 MeV
| 
15 
20
|[[Cosmic radiation]], [[Cosmic rays]]
|-
|[[Heavy Ions|'''Heavy Ions''']]
|Varies widely
|[[Cosmic radiation]]
|-
| colspan="3" |Table 1: Types of radiation
|}
(RBE is a measure of the damage done to living tissue, relative to gamma rays)

Cosmic radiation comprises 85% protons, 14% alpha particles, and 1% heavy ions.<ref>Schimmerling W. (2011, Feb 5). The Space Radiation Environment: An Introduction. <nowiki>https://three.jsc.nasa.gov/concepts/SpaceRadiationEnviron.pdf</nowiki></ref> Solar radiation includes the same radiation types, but it a higher proportion of protons and its heavy primaries have lower energy levels. The high-energy heavy primaries in cosmic radiation can penetrate materials that effectively block lower-energy radiation<ref name=":1">Rapp D. (2006). Radiation Effects and Shielding Requirements in Human Missions to the Moon and Mars. Mars 2:46-71. <nowiki>https://doi.org/10.1555/mars.2006.0004</nowiki></ref>.

==Astronaut Exposures==
The following astronauts have experienced radiations doses equal to, or greater than, the radiation doses in a 2.5 year Mars mission. (Assuming a 520 mSv dose for this mission. Different missions have different amounts of radiation protection.) In three cases the astronauts took more than double the amount of radiation than what is expected this Mars mission.

They are: Carl Waltz, Colin Foale, Peggy Whitson, Jeffrey Williams, Oleg Kononenko, Sergei Krikalyov, Vladimir Solovyov, Valeri Polyakov, Fyodor Yurchikhin, Aleksandr Kaleri, Sergei Krikalyov, Yurki Malenchenko, Gennadi Padalka, & Sergei Avdeyev.

None have exhibited ANY radiation health effects.

Note that NASA has stricter radiation limits than Russia (or the Soviet Union), so as time passes the number of Russian names on this list will become more pronounced.

See [[Long duration space flight]] for more information.

==Exposure limits==

===Limits for humans===
Exposure to dangerous levels of radiation causes [[radiation sickness]] and cancer. The average exposure to radiation on Earth due to natural sources is 6.2 mSv per year<ref name=":3">http://www.ans.org/pi/resources/dosechart/msv.php</ref>. The highest natural exposure is recorded in Ramsar, Iran, where people are exposed up to 260 mSv/y for many generations, with no reported harmful effects<ref name="Ghiassi-Nejad et al 2002">Ghiassi-Nejad et al, Very high background radiation areas of Ramsar, Iran: Preliminary biological studies, Health Physics 82(1):87-93 (February 2002), DOI: 10.1097/00004032-200201000-00011 [https://www.researchgate.net/publication, /11588980_Very_high_background_radiation_areas_of_Ramsar_Iran_Preliminary_biological_studies abstract]</ref>. This is 13 times the maximum exposure allowed radiation workers each year. Importantly, this level of radiation is what Mars settlers (living inside shelters of reasonable cost) would expect. So the 'high' levels of the back ground radiation at Ramsar is good news for Mars settlement.<ref>https://pubmed.ncbi.nlm.nih.gov/17563407/</ref> A recent study showed that the people living in this city showed increased immunity to gamma ray exposure, tho if this is from evolutionary adaption over many generations, or from the immune system being 'exercised' regularly is not known. <ref>https://pubmed.ncbi.nlm.nih.gov/11769138/</ref>

It should be emphasized, that low level radiation doses spread over a long period of time (long enough that the bodies natural functions have time to repair the damage), are far less dangerous than large doses received in a short amount of time. (In fact numerous studies show health benefits from extremely low levels of radiation.) <ref>https://jnm.snmjournals.org/content/59/12/1786</ref> See [Radiation Hormesis]. <ref>https://pmc.ncbi.nlm.nih.gov/articles/PMC2477686/</ref>
See also: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6149023/. https://www.sciencedaily.com/releases/2017/09/170913104428.htm. https://www.ajronline.org/doi/full/10.2214/ajr.179.5.1791137. https://en.wikipedia.org/wiki/Radiation_hormesis

Some people believe that ANY exposure to radiation, no matter how slight, poses some risk. Small dose - small risk of cancer. High dose - high risk of cancer. This is the No Minimum Threshold theory of radiation dosage. This works quite well with fast radiation doses high enough to cause cancers later in life, but the evidence is much weaker for low level does over a long period of time. No Minimum Threshold is used by regulatory agencies when they wish to be extremely conservative about radiation risks.<ref>https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6043938/</ref> <ref>https://www.federalregister.gov/documents/2021/08/17/2021-17475/linear-no-threshold-model-and-standards-for-protection-against-radiation</ref> <ref>https://pubs.rsna.org/doi/10.1148/radiol.2511080671</ref>

Nevertheless, there are defined legal limits for exposure during work for several professional activities, such as for X-ray assistants, airplane personnel, etc. The International Commission on Radiation Protection recommends that occupational (work-related) radiation exposure be limited to 50 millisieverts (mSv) per year, and limited to 100 mSv over any 5-year period<ref>http://www.icrp.org/publication.asp?id=ICRP%20Publication%20103</ref>. NASA's radiation dose limits for astronauts are established in NASA-STD-3001<ref>NASA. (2015). NASA Space Flight Human-System Standard Volume 1, Revision A: Crew Health. Retrieved from https://standards.nasa.gov/standard/nasa/nasa-std-3001-vol-1</ref>.

There is scientific uncertainty surrounding the health hazard from cosmic and solar radiation, because most past research on the health effects of radiation studied only x-rays and gamma rays<ref name=":1" />.

===Limits for plants===
[[File:Plant radiation.PNG|thumb|Table 2: Need to find source for this table]]
"In general, plants are relatively radiation resistant when growing and extremely resistant as dormant seeds." Radiation should not interfere with raising plants as food sources, at least not on the time scales of exploration missions.<ref>National Research Council 1996. Radiation Hazards to Crews of Interplanetary Missions: Biological Issues and Research Strategies. Washington, DC: The National Academies Press. <nowiki>https://doi.org/10.17226/5540</nowiki>.</ref>

Table 2 shows the limits for plants(source of table required, in space or on mars?). It shows that in practically all cases plants can survive radiations events that are likely for Mars without any kind of protection.

However, a 2021 study in the Netherlands, conducted on two types of plants, has shown that radiation at the Mars surface may reduce yields substantially<ref>TACK, Nynke, WAMELINK, G. W. W., DENKOVA, A. G., ''et al.'' Influence of Martian Radiation-like Conditions on the Growth of Secale cereale and Lepidium sativum. ''Frontiers in Astronomy and Space Sciences'', 2021, p. 127.</ref>. Germination of plants in a protected environment before setting them out in greenhouses might be a potential mitigation measure but further research is needed.

==Martian Environment==

===Effects of the Martian atmosphere===
Most Solar Proton Events (SPE) particles are low energy & will be stopped by the atmosphere before they reach the surface. However, interactions with atmospheric particles can produce neutrons; those neutrons can reach the surface, so the health hazard is not eliminated. However, the rare, very powerful SPE have energies of medium energy [[Cosmic radiation| cosmic rays]] and are a dangerous hazard.

Cosmic radiation protons are likely to penetrate the atmosphere. Cosmic ray heavy ions may fragment in the atmosphere, producing lower-mass ions that can still harm astronauts on the surface.

Mars' thin atmosphere allows more [[Ultraviolet]] light to reach the surface, compared to Earth. However, habitat structural materials and standard space suits should be sufficient to protect humans from UV radiation.<ref name=":4">Beaty DW, Snook K, Carlton A, Eppler D, Farrell B, Heldmann J,...Zeitlin C, on behalf of the Mars Human Precursor Science Steering Group. (2005). An Analysis of the Precursor Measurements of Mars Needed to Reduce the Risk of the First Human Mission to Mars. Available at <nowiki>https://mepag.jpl.nasa.gov/reports/MHP_SSG_(06-02-05).pdf</nowiki></ref>

Mars atmosphere effect depends on the considered inclination, as the incoming radiations will cross more matter if it's coming from the horizon rather than from the zenith. For inclination angles greater than ~45°, the atmospheric thickness is in the range from 20-30 g/cm2, and for lower inclination angles, the atmospherie thickness can exceed 100 g/cm2<ref name=":2" />. In other words, solar radiation is less dangerous when the sun in near the horizon.

===Low gravity effects on atmospheric thickness===
Note that the radiation protection given by the Martian atmosphere is higher than would be expected considering the air pressure. Pressure can be thought of the weight of the air above you in the atmosphere. Mars' gravity is 38% of Earths. So the weight of that air is less than it would be on Earth. On other words, more air must be above you on Mars to give the same pressure, compared to Earth. For example, you might think that since Mars' air pressure is 0.6% of Earth's, the radiation protection would also be 0.6%. However, the mass of air above you on Mars is 1/38% or 2.6 times thicker than that pressure on Earth. So even tho Mars' air pressure is 0.6% of Earth's, it gives 1.56% of Earth's atmospheric protection. Likewise, if we were to terraform Mars to have 10% Earth's pressure, the radiation protection by that atmosphere would be 26%. If we were to give Mars an atmosphere of 50% Earth's air pressure, then the Martian atmosphere's radiation protection would be 132% that of Earth.

===Effects of cosmic rays striking the regolith===
When [[cosmic rays]] strikes regolith, it can cause the impacted atoms to emit their own radiation. Surrounding regolith particles absorb much of this radiation, with the exception of neutrons. Neutrons generated in this way are called albedo neutrons. These neutrons have the potential to add substantially to the radiation dose for astronauts on the surface.<ref name=":4" />

Neutrons are well absorbed by water, so blocks of ice or water around habitats would be useful radiation protection. Lithium hydride is thought to be the most effective neutron absorber ever discovered, and it might be built into the floor of long term habitats. <ref>https://www.tandfonline.com/doi/abs/10.1179/1743284715Y.0000000105?journalCode=ymst20
</ref> (Lithium hydroxide reacts with water, so it must be kept away from the humid interior of the settlement.) If a long duration habitat is has a space under it, blocks of local ice could protect against secondary neutrons.

===Dose received by an unprotected human on Mars===

====Cosmic radiation====
The equivalent dose rate from cosmic radiation on Earth's surface at sea level is .26 mSv per year<ref name=":3" /> <ref>http://nuclearsafety.gc.ca/eng/resources/radiation/introduction-to-radiation/radiation-doses.cfm</ref> (this makes up ~10% of the annual natural radiation dose, and it increase with altitude). Based on measurements made by the Curiosity rover, the corresponding figure for the surface of Mars is approximately 230 mSv/year<ref name=":0">Hassler DM, Zeitlin C, Wimmer-Schweingruber RF, Ehresmann B, Rafkin S, Eigenbrode JL,...MSL Science Team. (2014). Mars' Surface Radiation Environment Measured with the Mars Science Laboratory's Curiosity Rover. Science 343(6169). <nowiki>https://doi.org/10.1126/science.1244797</nowiki></ref>. More generally, one model estimated that the dose equivalent rate on the surface of Mars ranges from 156.4 mSv/year (at solar maximum) to 273.8 mSv/year (at solar minimum)<ref name=":2" /><ref name=":5">McKenna-Lawlor S, Goncalves P, Keating A, Reitz G, Matthia D. (2012). Overview of Energetic Particle Hazards During Prospective Manned Missions to Mars. Planetary and Space Science. 63: 123-132.</ref>. A 2005 report by the Mars Human Precursor Science Steering Group estimated that (at solar minimum) the dose from cosmic radiation would be 1.2 +/- 0.5 mSv/day; this includes 0.4 +/- 0.4 mSv/day from albedo neutrons.<ref name=":4" />

====Solar Proton Events====
These are also known a [[Coronal Mass Ejections]]. Curiosity also measured the temporary increase in radiation during a single SPE. The results indicate an increase in equivalent dose rate of approximately 25% over a 1-day interval<ref name=":0" />. This figure will vary depending on the intensity of a particular SPE. See [[Solar Cosmic Rays]] for further discussion.

==Effect on material==
Radiation can change the properties of [[plastics]] and metals, making them brittle after a period of time.

As semi-conductor computer chips have become smaller and smaller, they become more likely to be confused or damaged by radiation. Electronic equipment sent to Mars, or built on Mars should be radiation hardened.

==Protection==
Long term [[Habitat|habitats]] should be equipped with a [[radiation shielding]], thick enough to reduce the radiation to a level equal to Earth, that is, almost zero. Best protection may be achieved with houses built in natural [[caves]] or set into cliffs or hillsides. (If, however, it is decided that radiation levels equal to the city of Ramsar, Iran, are safe enough, then the thickness of the radiation shielding suggested below can be reduced up to 13 fold.)

Early exploration habitats could have water tanks, or sand bags above where people live. When radiation goes thru water, every 18 cm reduces the radiation by half. So a water tank 108 cm thick (6 halvings) will reduce the radiation level by 64 times. (As a bonus, water is a good neutron absorber.) Packed soil has a halving-distance of 9.1 cm, so 55 cm of hard soil would provide a similar level of protection. In general, it is far better to use local materials for radiation protection, rather than hauling them from Earth. See: https://www.imagesco.com/geiger/lead-shielding-guide.html.

[[Space suit]]s must be designed with radiation in mind. The suit should provide adequate shielding for the occupant. It may be necessary to design suits with several grades of protection. Suits designed for short-term use can carry lighter shielding which would reduce weight and improve maneuverability. That said, radiation protection from suits will be much less than inside habitats, so minimizing time on the surface will be the largest protection. Going outside during solar storms would likely be banned. Jobs that require regular EVA's (such as cleaning solar cells of dust) should be avoided.

During severe radiation events, such as [[solar flare|solar flares]], surface [[settlement|settlements]] may use [[storm shelter|storm shelters]] with heavier than normal shielding.

"In this work, it is shown that on the Martian surface, almost any amount of aluminum shielding increases exposure levels for humans. The increased exposure levels are attributed to neutron production in the shield and Martian regolith as well as the electromagnetic cascade induced in the Martian atmosphere. This result is significant for optimization of vehicle and shield designs intended for the surface of Mars." <ref name=":2">NASA, Tony C. Slaba, Christopher J. Mertens, and Steve R. Blattnig '' Radiation Shielding Optimization on Mars '', https://spaceradiation.larc.nasa.gov/nasapapers/NASA-TP-2013-217983.pdf, Apr 2013.</ref>

"An in-situ shielding strategy will also be of little help unless several hundred g/cm2 of regolith is utilized. Such a strategy would probably require large scale excavation making it an unlikely candidate. Instead, the shielding strategy would rely primarily on material optimization. Options, such as replacing aluminum structures with high hydrogen content carbon composites, could be pursued." <ref name=":2" /> This opinion is open to argument as in-situ resources utilization for any type of settlement should make large amounts of regolith available for construction. It mainly is true for the very first level of habitats.

The use of g/cm2 can be translated into an equivalent thickness that depends on the material density. For martian regolith at 2000 kg/m3, a thickness of 1m of regolith is 200 g/cm2. Water (or ice) is 100 g/cm2. So the minimum covering for a long term settlement would be 5m or more. For water, although the radiation absorption is better the density is lower, so about the same thickness would be required for protection. (to be discussed)

For people living on Mars for many years, they may prefer to have habitats with more than the minimum radiation protection. For example, if people decide that 2 meters of packed soil is sufficient as minimum protection, they may wish to have 4 or 5 meters for most of the base. They may spend a significant amount of time outside the habitat, so to balance the low radiation protection when outside they have with higher radiation protection inside.

==References==
<references />

==External links==

*[http://www.ips.gov.au/ IPS:] [http://www.ips.gov.au/Category/Educational/Space%20Weather/Space%20Weather%20Effects/guide-to-space-radiation.pdf A Guide to Space Radiation]
*[http://www.niauk.org/radiation-and-safety.html Nuclear Industry Association: Radiation, health and nuclear safety]
*[https://hesperia.gsfc.nasa.gov/sspvse/posters/DF_Smart/poster.pdf The frequency distribution of solar proton events: 5 solar cycles and 45 solar cycles]
*[[w:Radiation_hormesis|Low levels of radiation can be stimulatory: Radiation Hormesis]]

[[Category:Radiation Protection]]

Solar Wind Sputtering

2025-08-16T09:25:17Z

RichardWSmith: /* Atmospheric Loss of Light Gases */ Fixed typo.

Solar Wind Sputtering is a process where the solar wind slowly strips away the atmosphere of a planet without a strong magnetic field. It is thought to have removed about 1/3 of Mars' early atmosphere.

See [[Ancient Atmosphere]] and [[Atmospheric loss]] for a full description of what has happened to Mars' originally thick atmosphere.

==Overview==
===Atmospheric Loss of Light Gases===
Atoms and molecules with low mass move faster at a given temperature. On worlds with lower gravity, gases with low masses such as hydrogen, helium, neon, may be moving fast enough that those moving the fastest exceed the escape velocity of the planet and thus are lost to space.<ref>https://en.wikipedia.org/wiki/Atmospheric_escape</ref>

Thus, hot planets will lose gases faster than cold ones, and planets with low surface gravity will lose gases faster than ones with higher gravity.

Mars has a surface gravity that it should be able to hang on to heavier molecules such as [[oxygen]] (O2), [[nitrogen]] (N2), [[carbon monoxide]] (CO), [[carbon dioxide]] (CO2), and heavier gases.

Mars originally had a thick atmosphere of 3 or 4 bar, but now is a near vacuum (0.006 bar). What caused the atmosphere to be stripped away? (Note: some scientists think that Mars could not have such a thick early atmosphere, but it is hard to explain how Mars could have had running water ~4 to 5 billion years ago without it. Note that then, the Sun was only 70% as warm as it is now, so running water on an early Mars is doubly hard to explain.)

Note that Owen & Tobias calculated that Mars had a 10 bar CO2 atmosphere in a paper in Icarus #166, but this is an outlier, most scientists prefer thinner atmospheres.<ref>Owen, Tobias; Bar-Nun, Akiva (1 August 1995). "Comets, impacts, and atmospheres". Icarus. 116 (2): 215–226.</ref>

===Gases Absorbed Into the Crust===
See [[Atmospheric Loss]] for more details, but it is clear that most of Mars' early atmosphere has been absorbed into the crust, but this does not account for all of the missing atmosphere.

===Solar Wind Sputtering===
When high energy electromagnetic radiation (such as X-rays or [[Ultraviolet]] radiation hits the top of the Martian atmosphere, it can ionize gases. High energy impact of particles such as [[Cosmic Radiation|cosmic rays]] or [[Solar Radiation]] can also cause ionization.

Ionized gases are strongly effected by electric charges and magnetic fields. They can get caught up by the magnetic fields of the solar wind, and be pulled away from the planet, despite the planet's gravity being strong enough to hold on to them. It is thought that Mars has lost at least 1/3 of its early atmosphere via this method. Satellites have detected this loss around Mars, and tho slow, it is significant over hundred of millions of years.

Sputtering should allow gases to escape from heavy planets, but Venus (also with no magnetic field) has a solar wind more than 4.4 times that of Mars and has not lost much atmosphere to solar wind sputtering. This is far less than predicted, so our models need to be adjusted.

Sputtering results in a slight increase in the rate of atmospheric loss, but over hundreds of millions or billions of years it becomes significant.

==Detailed Discussion==
===Paleomagnetism===
When molten rock cools, magnetic particles inside it (such as iron, nickel, or cobalt) align with the planet's magnetic field, and as the rock freezes, these particle are locked in place. This gives the rock a weak permanent magnetic field. Martian rocks exhibit this, so we know that early Mars did have a magnetic field. By looking at areas with such 'paleomagnetism' scientists can determine the strength and direction of the magnetic field around the rocks at the time they solidified.<ref>https://www.sciencedirect.com/topics/earth-and-planetary-sciences/paleomagnetism</ref>

It has been found that this very weak paleomagnetic field provides enough of a magnetic field to protect the atmosphere above some regions on Mars. So some areas above large, ancient lava flows suffer less gas loss to sputtering.

These Martian crustal fields cover ~2/3 of the Martian surface, but the strongest are in the Southern highlands, particularly in the Sirenum and Terra Cimmerian regions. Above these regions, the magnetic field is about 1,000 nanoTeslas, where as the induced magnetic field at the top of the atmosphere is typically 30nT. (The Earth's magnetic field is about 30,000 nT in comparison.)

Originally it was thought that these regions had only local influence, but we are finding that even these extremely weak fields are enough to effect the global dynamics. They are strongest in the Southern Hemisphere, which is the side of Mars which points at the Sun when the planet is closest to the Sun. Thus, it gets the most protection from these paleomagnetic fields, when it is feeling the strongest effects from the solar wind.

The topology of these magnetic fields reacts in complex ways with the Interplanetary Magnetic Field, which may shield the planet, or direct streams of particles into the atmosphere. They also increase the turbulence and complexity of the magnetic fields lines flowing around the planet. Generally, when they are facing the sun, they push the Bow Shock (see below) further from the planet.

In places where the magnetic field lines are vertical, they promote the solar wind hitting the atmosphere, and allow easier mixing of the upper and lower levels of the ionosphere.

It was surprising how strongly these weak magnetic fields effect the solar wind.

===Isotope Ratios===
Many gases are made of atoms with more than one [[Isotopes|Isotope]]. Since these differ in mass, lighter isotopes are less dense and tend to rise to the top of the Martian atmosphere. These are more subject to sputtering, and thus, lighter isotopes tend to be lost more quickly than heavier ones. By measuring the ratios between light and heavier isotopes, scientists have estimated that about 1/3 of the Martian carbon dioxide has been lost to space in the last 5.5 billion years.

Measurements of the isotope ratios of noble gases (which will not chemically combine into the crust) find that it is hard to account for atmosphere loss with solar wind sputtering. The amount lost to sputtering has been overestimated. Much of Mars' early atmosphere must have been lost from impact erosion (giant asteroids hitting the planet and blowing off a portion of the atmosphere).

(Some recent studies suggest that this might be higher, over 50%. Note that molecules and atoms with lower mass, will have a higher % of them lost to space. Mars will have kept a higher % of heavier molecules like CO2)

These studies are difficult because we do not know the initial isotope ratios, and because we do not know if the 'clock' was reset by comet impacts refilling the atmosphere with new batches of volatiles. Also if Mars had an atmospheric pressure of 0.5 to 1.0 bar in the Noachian, then sputtering and other forms of atmospheric escape can account for the current pressure. If Mars had 4 or 5 bar then, then sputtering is not capable to removing that much (so it must have been absorbed into the crust).

===Direct Measurements by Orbiting Spacecraft===
Four spacecraft have collected most of the data on this subject, namely: Phobos 2, Mars Global Surveyor, Mars Express, and most recently MAVEN.<ref>https://en.wikipedia.org/wiki/MAVEN</ref> Other space craft have provided some data. The Pioneer Venus Orbiter, and the Venus Express have looked at Venus' solar wind interactions and given data that can be contrasted with Martian conditions.

They have found that Mars (and Venus) has no large magnetosphere, but interactions with the solar wind on the upper atmosphere of the planets create an induced magnetic field. (The upper atmosphere becomes ionized, which can then carry electric currents.) Such currents are inducted by the magnetic field, which create their own magnetic field in opposition to the Interplanetary Magnetic Field. These induced fields are much weaker than Earth's magnetic field, but still help to protect the atmosphere. To create this induced field, the magnetic field must vary. The Interplanetary Magnetic Field (IMF) does in fact vary a great deal, so such protective fields are induced, but at random intervals, the IMF is steady enough that the induced fields collapse regionally, so the solar wind CAN entrain gas particles and lift them away from the planet.

The interactions of the solar wind this Mars' upper atmosphere are complex, varying with time, region, and altitude. Numerous models have been created and while no single model can describe the whole of this system, they are very successful in describing subsets of the environment.

===Bow Shock===
The bow shock is where the Interplanetary Magnetic Field (IMF) hits this induced magnetic field and is directed around Mars. It marks where the flow of the solar wind goes from supersonic to subsonic flow. It lies closest to Mars under the sub-solar point, about 1.6 Mars radii, from the planet.

Similar to Venus, the shock distance and the flow of plasma around the planet is close relative to the size of the planet. (Earth's powerful magnetic field keeps this flow much further from us.) Models have so far done a poor job of modelling the details of the flow around the planet; the flow around Mars (and Venus) show various turbulence and asymmetries, which the models do not capture.

As the particles of the solar wind hit the bow shock they are suddenly slowed and heat up. Thus they are considerably hotter than particles further out from the planet. As the solar wind picks up atmospheric particles it is further slowed, and extends partly around the planet into the night side. The details of the region where the solar wind dominates and where the planetary ions dominate have proved hard to model. Scientists are unsure if there is a single boundary, or several closely spaced ones, and instrument readings are inconsistent in time.

===Flow of Ions Around and Away From Mars===
Some ions are directed towards the planet and impact the exosphere heating it. Others become magnetically entrained with the solar wind and are directed away from the planet. Note, that if an ion missing an electron, then absorbs one, it decouples from the magnetic field and is strongly effected by gravity. If it is not moving too fast, it will fall back to Mars.

The solar side of Mars' atmosphere is ionized by solar radiation. However, the night side has very few ions. This effects the dynamics of the plasma flow as it moves into the Martian night side, cooling it and decoupling it from the Martian atmosphere. The magnetic field of this plasma flow is very weak on the Martian night side, being about 10 nanoTeslas. (Tho this is still stronger than the Interplanetary Magnetic Field (IMF) far from Mars.)

The induced magnetic lines (in the atmosphere) and the IMF interact with each other in very complex, turbulent ways. But they can locally connect, which results in a great deal of magnetic energy being transferred to the plasma, heating it. This typically happens at the terminator, or at the near night side of the planet.

The magnetic fields entrained in the plasma flow around Mars, and move away from the planet into deep space behind it. This flow induces an 'induced magnetic tail' behind Mars, which is very weak as there are few ions in this region.

===Solar Storms===
Observations of young stars suggest that [[Coronal Mass Ejections]] (CME) or solar storms were more common, and more powerful with our young sun. However, early in Mars' history, it had a magnetic field protecting it. It is debated if the early solar storms would have had an outsized effect on the rate of Martian atmospheric loss, or if the magnetic field lasted long enough to cancel this effect.

Modern solar storms are expected to increase the rate of atmospheric loss, but the rate of the increase has not been well measured.

The sun produces very few x-rays. Most of the ionization of the atmosphere is caused by UV rays. However in a large CME, there is a significant increase in UV and x-rays, which means the upper atmosphere is more strongly ionized.

While a CME (on the side of the sun facing Mars) will always cause more ionization, the flood of solar particles usually will miss Mars. In the case when the energetic solar particles hit the planet, there is far more energy available to pick up Martian ions. (And some of these high energy solar particles will collide with the ionosphere, ionizing it more, and heating it.)

It is estimated that a CME hitting Mars will increase the rate of atmospheric loss by 2 to 100 times (depending on the CME intensity) for the couple hour duration of the solar storm.

===Rate of Escape to Space===
Studies suggest Mars' magnetic field protected the atmosphere for the first 1.8 billion years of Mars lifetime. (This is longer than previous estimates based on MAVEN data.). Some studies suggest that it may have lasted 500 million years longer, or stopped then restarted at least once.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Recent measurements estimate that the rate of Martian atmospheric loss (from all sources, not just sputtering), is about 2.5 kg / second or ~79,000 tonnes per year. (For comparison, the mass of the Earth's atmosphere is 5,150,000,000,000,000 tonnes.)

This low rate of atmospheric loss does not seem to be enough to explain Mars' current near vacuum, so models usually predict that Mars lost its magnetic field early, and or the rate of loss was higher in the past, and or the Martian atmosphere started with much less than 4 bar. Possibly, they are underestimating the amount of atmosphere that has been absorbed into the crust.<ref>https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018JE005727</ref>

This recent review paper suggests that the need of a magnetic field to preserve atmospheres has been over estimated.<ref>https://arxiv.org/pdf/2003.03231</ref>

This paper suggests that while a strong magnetic field protects the atmosphere, a very weak one is actually worse than no magnetic field at all. Thus, if Mars' magnetic field collapses partially and remained weak for a long time, it could explain the higher than expected atmospheric loss. <ref>https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019JA026945</ref>

Note that Earth and Venus are both losing a bit under a kg of mass per second from atmosphere escape, and Mars is losing about 2.5 kg/s. So Mars is losing mass about 3 times faster than Earth, but not orders of magnitude more quickly.

If terraformers were to somehow give Mars a 1 bar atmosphere (and didn't give it an artificial magnetic field which is much easier) it would take at least 500 million years for sputtering to reduce the atmosphere back down to its current near vacuum.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 15: "Solar Wind Interaction & Atmospheric Escape". See also Chapter 17: "Evidence for a Different Atmosphere & Climate".

==References==

Ancient Atmosphere

2025-08-12T00:13:35Z

RichardWSmith: /* The Faint Young Sun - Warm Mars Paradox */ Added reference.

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
[[File:Noachian_Mars.png]]
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

==Events in the Pre-Noachian Period==
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2) could be pumped into the atmosphere to raise the temperature to where brines would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming. Note, that once the planet is warm enough to have common rainfall, the SO2 is washed out of the air faster. So this gas can bring Mars up to the melting point of water, but won't raise it far above that point.There is another problem, if SO2 is present in a wet atmosphere, it will react in light to eventually form sulphuric acid(H2SO4) which condenses into sulphate aerosols which reflect visible light, cooling the planet. Overall, SO2 will help warm the planet, but by itself won't do the job alone to bring the temperature up to the melting point of water (273K).

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler. If the sun was 10% more massive than we think it was, then it would actually be warmer 4 Ga than it is now. Then liquid water flowing on Mars (and Earth) is much easier to model. However, studies of similar stars to our sun, which are much younger, suggest that such stars might lose 0.1% of their mass in 4.5 Ga. So how the Sun could lose 10% is very hard to explain.<ref>https://www.space.com/14565-earth-climate-young-sun-paradox.html</ref>

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 ice would likely be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

===Transient Impact Climate Change===
This idea is that the impact of a large sized body (~100 km in diameter or a bit larger), throws up a huge amount of dust which would warm the planet and vaporize water. This would warm the planet for hundreds to a couple thousand years. While such events certainly happened from time to time, this would not explain the long term water erosion features.

===Cold Wet Mars===
One idea is that if Mars had a thick atmosphere, it could be overall, below the freezing point of water (273K), but still have local areas of melting which would explain the erosion features. With a thicker atmosphere, higher altitudes would be colder (not the case in modern Mars) and winds blowing down hill will convert potential energy into heat, which could melt ice fields. Impacts, geothermal warming, and local weather events could then melt surface ice and snow, causing erosion features or refilling aquifers which could contribute to seep erosion.

This may be true in a small way, but as we have found sedimentary beds that would take a long time to lay down (hundreds of thousands or millions of years), this idea seems increasingly less likely.

==The Atmosphere During the Hesperian==
[[File:Post_Noachian_Mars.png]]
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==

Ancient Atmosphere

2025-08-11T23:57:48Z

RichardWSmith: Was the sun hotter?

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
[[File:Noachian_Mars.png]]
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

==Events in the Pre-Noachian Period==
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2) could be pumped into the atmosphere to raise the temperature to where brines would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming. Note, that once the planet is warm enough to have common rainfall, the SO2 is washed out of the air faster. So this gas can bring Mars up to the melting point of water, but won't raise it far above that point.There is another problem, if SO2 is present in a wet atmosphere, it will react in light to eventually form sulphuric acid(H2SO4) which condenses into sulphate aerosols which reflect visible light, cooling the planet. Overall, SO2 will help warm the planet, but by itself won't do the job alone to bring the temperature up to the melting point of water (273K).

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler. If the sun was 10% more massive than we think it was, then it would actually be warmer 4 Ga than it is now. Then liquid water flowing on Mars (and Earth) is much easier to model. However, studies of similar stars to our sun, which are much younger, suggest that such stars might lose 0.1% of their mass in 4.5 Ga. So how the Sun could lose 10% is very hard to explain.

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 ice would likely be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

===Transient Impact Climate Change===
This idea is that the impact of a large sized body (~100 km in diameter or a bit larger), throws up a huge amount of dust which would warm the planet and vaporize water. This would warm the planet for hundreds to a couple thousand years. While such events certainly happened from time to time, this would not explain the long term water erosion features.

===Cold Wet Mars===
One idea is that if Mars had a thick atmosphere, it could be overall, below the freezing point of water (273K), but still have local areas of melting which would explain the erosion features. With a thicker atmosphere, higher altitudes would be colder (not the case in modern Mars) and winds blowing down hill will convert potential energy into heat, which could melt ice fields. Impacts, geothermal warming, and local weather events could then melt surface ice and snow, causing erosion features or refilling aquifers which could contribute to seep erosion.

This may be true in a small way, but as we have found sedimentary beds that would take a long time to lay down (hundreds of thousands or millions of years), this idea seems increasingly less likely.

==The Atmosphere During the Hesperian==
[[File:Post_Noachian_Mars.png]]
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==

Ancient Atmosphere

2025-08-11T23:50:46Z

RichardWSmith: Cold wet mars is unlikely.

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
[[File:Noachian_Mars.png]]
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

==Events in the Pre-Noachian Period==
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2) could be pumped into the atmosphere to raise the temperature to where brines would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming. Note, that once the planet is warm enough to have common rainfall, the SO2 is washed out of the air faster. So this gas can bring Mars up to the melting point of water, but won't raise it far above that point.There is another problem, if SO2 is present in a wet atmosphere, it will react in light to eventually form sulphuric acid(H2SO4) which condenses into sulphate aerosols which reflect visible light, cooling the planet. Overall, SO2 will help warm the planet, but by itself won't do the job alone to bring the temperature up to the melting point of water (273K).

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler, which is scoffed at by those who know stellar evolution.

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 ice would likely be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

===Transient Impact Climate Change===
This idea is that the impact of a large sized body (~100 km in diameter or a bit larger), throws up a huge amount of dust which would warm the planet and vaporize water. This would warm the planet for hundreds to a couple thousand years. While such events certainly happened from time to time, this would not explain the long term water erosion features.

===Cold Wet Mars===
One idea is that if Mars had a thick atmosphere, it could be overall, below the freezing point of water (273K), but still have local areas of melting which would explain the erosion features. With a thicker atmosphere, higher altitudes would be colder (not the case in modern Mars) and winds blowing down hill will convert potential energy into heat, which could melt ice fields. Impacts, geothermal warming, and local weather events could then melt surface ice and snow, causing erosion features or refilling aquifers which could contribute to seep erosion.

This may be true in a small way, but as we have found sedimentary beds that would take a long time to lay down (hundreds of thousands or millions of years), this idea seems increasingly less likely.

==The Atmosphere During the Hesperian==
[[File:Post_Noachian_Mars.png]]
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==

Ancient Atmosphere

2025-08-11T23:39:13Z

RichardWSmith: Large impactors could temp. warm planet.

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
[[File:Noachian_Mars.png]]
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

==Events in the Pre-Noachian Period==
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2) could be pumped into the atmosphere to raise the temperature to where brines would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming. Note, that once the planet is warm enough to have common rainfall, the SO2 is washed out of the air faster. So this gas can bring Mars up to the melting point of water, but won't raise it far above that point.There is another problem, if SO2 is present in a wet atmosphere, it will react in light to eventually form sulphuric acid(H2SO4) which condenses into sulphate aerosols which reflect visible light, cooling the planet. Overall, SO2 will help warm the planet, but by itself won't do the job alone to bring the temperature up to the melting point of water (273K).

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler, which is scoffed at by those who know stellar evolution.

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 ice would likely be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

===Transient Impact Climate Change===
This idea is that the impact of a large sized body (~100 km in diameter or a bit larger), throws up a huge amount of dust which would warm the planet and vaporize water. This would warm the planet for hundreds to a couple thousand years. While such events certainly happened from time to time, this would not explain the long term water erosion features.

==The Atmosphere During the Hesperian==
[[File:Post_Noachian_Mars.png]]
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==

Ancient Atmosphere

2025-08-11T22:11:18Z

RichardWSmith:

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
[[File:Noachian_Mars.png]]
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

==Events in the Pre-Noachian Period==
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2) could be pumped into the atmosphere to raise the temperature to where brines would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming. Note, that once the planet is warm enough to have common rainfall, the SO2 is washed out of the air faster. So this gas can bring Mars up to the melting point of water, but won't raise it far above that point.There is another problem, if SO2 is present in a wet atmosphere, it will react in light to eventually form sulphuric acid(H2SO4) which condenses into sulphate aerosols which reflect visible light, cooling the planet. Overall, SO2 will help warm the planet, but by itself won't do the job alone to bring the temperature up to the melting point of water (273K).

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler, which is scoffed at by those who know stellar evolution.

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 ice would likely be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

==The Atmosphere During the Hesperian==
[[File:Post_Noachian_Mars.png]]
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==

Ancient Atmosphere

2025-08-11T22:10:59Z

RichardWSmith: /* The Noachian Period */

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

==Events in the Pre-Noachian Period==
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2) could be pumped into the atmosphere to raise the temperature to where brines would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming. Note, that once the planet is warm enough to have common rainfall, the SO2 is washed out of the air faster. So this gas can bring Mars up to the melting point of water, but won't raise it far above that point.There is another problem, if SO2 is present in a wet atmosphere, it will react in light to eventually form sulphuric acid(H2SO4) which condenses into sulphate aerosols which reflect visible light, cooling the planet. Overall, SO2 will help warm the planet, but by itself won't do the job alone to bring the temperature up to the melting point of water (273K).

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler, which is scoffed at by those who know stellar evolution.

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 ice would likely be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

==The Atmosphere During the Hesperian==
[[File:Post_Noachian_Mars.png]]
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==

Ancient Atmosphere

2025-08-11T22:09:54Z

RichardWSmith: /* The Atmosphere During the Hesperian */ Adding a picture.

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

==Events in the Pre-Noachian Period==
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
[[File:Noachian_Mars.png]]
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2) could be pumped into the atmosphere to raise the temperature to where brines would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming. Note, that once the planet is warm enough to have common rainfall, the SO2 is washed out of the air faster. So this gas can bring Mars up to the melting point of water, but won't raise it far above that point.There is another problem, if SO2 is present in a wet atmosphere, it will react in light to eventually form sulphuric acid(H2SO4) which condenses into sulphate aerosols which reflect visible light, cooling the planet. Overall, SO2 will help warm the planet, but by itself won't do the job alone to bring the temperature up to the melting point of water (273K).

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler, which is scoffed at by those who know stellar evolution.

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 ice would likely be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

==The Atmosphere During the Hesperian==
[[File:Post_Noachian_Mars.png]]
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==

Ancient Atmosphere

2025-08-11T22:08:55Z

RichardWSmith: /* The Noachian Period */ Adding a picture.

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

==Events in the Pre-Noachian Period==
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
[[File:Noachian_Mars.png]]
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2) could be pumped into the atmosphere to raise the temperature to where brines would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming. Note, that once the planet is warm enough to have common rainfall, the SO2 is washed out of the air faster. So this gas can bring Mars up to the melting point of water, but won't raise it far above that point.There is another problem, if SO2 is present in a wet atmosphere, it will react in light to eventually form sulphuric acid(H2SO4) which condenses into sulphate aerosols which reflect visible light, cooling the planet. Overall, SO2 will help warm the planet, but by itself won't do the job alone to bring the temperature up to the melting point of water (273K).

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler, which is scoffed at by those who know stellar evolution.

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 ice would likely be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

==The Atmosphere During the Hesperian==
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==

File:Post Noachian Mars.png

2025-08-11T22:07:04Z

RichardWSmith: Processes that changed Mars during the Post Noachan period.

== Summary ==
Processes that changed Mars during the Post Noachan period.
== Licensing ==
{{PD}}

File:Noachian Mars.png

2025-08-11T22:05:24Z

RichardWSmith: Picture of processes happening during the Noachian

== Summary ==
Picture of processes happening during the Noachian
== Licensing ==
{{PD}}

Ancient Atmosphere

2025-08-11T21:41:07Z

RichardWSmith: /* Events in the Pre-Noachian Period */

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

==Events in the Pre-Noachian Period==
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2) could be pumped into the atmosphere to raise the temperature to where brines would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming. Note, that once the planet is warm enough to have common rainfall, the SO2 is washed out of the air faster. So this gas can bring Mars up to the melting point of water, but won't raise it far above that point.There is another problem, if SO2 is present in a wet atmosphere, it will react in light to eventually form sulphuric acid(H2SO4) which condenses into sulphate aerosols which reflect visible light, cooling the planet. Overall, SO2 will help warm the planet, but by itself won't do the job alone to bring the temperature up to the melting point of water (273K).

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler, which is scoffed at by those who know stellar evolution.

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 ice would likely be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

==The Atmosphere During the Hesperian==
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==

Ancient Atmosphere

2025-08-11T01:31:48Z

RichardWSmith: Added missing word. Added weasel word.

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

===Events in the Pre-Noachian Period===
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2) could be pumped into the atmosphere to raise the temperature to where brines would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming. Note, that once the planet is warm enough to have common rainfall, the SO2 is washed out of the air faster. So this gas can bring Mars up to the melting point of water, but won't raise it far above that point.There is another problem, if SO2 is present in a wet atmosphere, it will react in light to eventually form sulphuric acid(H2SO4) which condenses into sulphate aerosols which reflect visible light, cooling the planet. Overall, SO2 will help warm the planet, but by itself won't do the job alone to bring the temperature up to the melting point of water (273K).

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler, which is scoffed at by those who know stellar evolution.

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 ice would likely be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

==The Atmosphere During the Hesperian==
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==

Ancient Atmosphere

2025-08-11T01:17:23Z

RichardWSmith: Sulphate aerosols cool

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

===Events in the Pre-Noachian Period===
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2) could be pumped into the atmosphere to raise the temperature to where brines would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming. Note, that once the planet is warm enough to have common rainfall, the SO2 is washed out of the air faster. So this gas can bring Mars up to the melting point of water, but won't raise it far above that point.There is another problem, if SO2 is present in a wet atmosphere, it will react in light to eventually form sulphuric acid(H2SO4) which condenses into sulphate aerosols which reflect visible light, cooling the planet. Overall, SO2 will help warm the planet, but by itself won't do the job alone to bring the temperature up to the melting point of water (273K).

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler, which is scoffed at by those who know stellar evolution.

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 would be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

==The Atmosphere During the Hesperian==
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==

Ancient Atmosphere

2025-08-11T01:10:41Z

RichardWSmith: SO2 will be rained out.

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

===Events in the Pre-Noachian Period===
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2) could be pumped into the atmosphere to raise the temperature to where brines would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming. Note, that once the planet is warm enough to have common rainfall, the SO2 is washed out of the air faster. So this gas can bring Mars up to the melting point of water, but won't raise it far above that point.

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler, which is scoffed at by those who know stellar evolution.

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 would be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

==The Atmosphere During the Hesperian==
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==

Ancient Atmosphere

2025-08-11T01:07:12Z

RichardWSmith: spelling error.

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

===Events in the Pre-Noachian Period===
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2) could be pumped into the atmosphere to raise the temperature to where brines would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming.

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler, which is scoffed at by those who know stellar evolution.

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 would be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

==The Atmosphere During the Hesperian==
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==

Ancient Atmosphere

2025-08-11T01:06:27Z

RichardWSmith: fixed formatting.

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

===Events in the Pre-Noachian Period===
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2) could be pumped into the atmosphere to raise the temperature to where brands would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming.

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler, which is scoffed at by those who know stellar evolution.

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 would be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

==The Atmosphere During the Hesperian==
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==

Ancient Atmosphere

2025-08-11T01:06:00Z

RichardWSmith: Volcanos - SO2 is a significant warming gas.

When Mars first formed it had a much denser, very different atmosphere from what it has now. It rapidly lost gases to space. There is a great deal of uncertainty about what the pressure was during the Noachian, with estimates varying from 0.5 Bar to 5 Bar (or more).

Immediately after the solar system creation, the sun was cooler, but subject to powerful flares which caused powerful spikes of extreme [[Ultraviolet]] light. This had a strong effect on atmospheric evolution.

As time passed, Mars lost air pressure as gases were absorbed into the crust and lost to space. See [[Atmospheric loss]] for more information.

Note that 'Ma' stands for Mega-annum (millions of years ago), and that 'Ga' stands for Giga-annum (billions of years ago).

Mars' geologic history is grouped into three main periods: the Noachian (4.1 to 3.7 Ga), the Hesperian (3.7 to 3.1 Ga), and the Amazonian (3.1 Ga to present). This page will concentrate on the pre-Noachian and the Noachian.

==Mars' Formation==
About 4.57 Ga, Mars accreted from the proto-planetary disk, finishing this process about 4.55 Ga. There may have been a lava ocean formed from the energy of impact and radioactive decay (especially from aluminum 26). During this time, Mars formed its core, mantle, and crust.

Hafnium 182 (Hf) decays into Tungsten (W). The half life of Hf182 is ~9 million years. The ratio of Hf/W shows that most of the W was formed AFTER the magma ocean cooled and Mars differentiated into a core, mantle, and crust. (Tungsten is an iron loving element, and will be drawn to the core if it existed at that time.) This suggests that Mars rapidly cooled, and quickly formed its crust. Mars may have accreted and solidified in about 5 million years, which seems fast, but several lines of evidence point in this direction.

Note that before the planet formed its core, molten rock would be outgassing H2, CH4, and NH3 (reducing gases). After the core formed volcanoes would produce H2O, CO2, N2, and minor amounts of H2 & CO (which is a mildly oxidizing mix of gases).

===Mars' First Atmosphere===
The atmosphere was largely composed of steam, with significant amounts of hydrogen, with smaller amounts of nitrogen, and carbon dioxide. Tho speculative, Mars likely started with an atmosphere of 6 to 15 bar of CO2, and H2, and enough water to form an ocean 150 km deep. (Tho almost all of this water would be steam.) This thick wet atmosphere rapidly thinned.

===Hydrodynamic Gas Escape===
Early Mars had an additional way to lose atmosphere, 'hydrodynamic escape'. Hydrogen would not be kept by Mars' gravity and would steadily rise and disperse into space. Heavier atoms which should have been kept by Mars' gravity, would rise with the hydrogen, buoyed up by it so to speak, and move far enough away, to be stripped away by light pressure and the solar wind. This type of gas loss ends when little molecular hydrogen remains.

Hydrodynamic escape would happen from 4.55 Ga until perhaps 4.0 Ga, but would be strongest early in that period.

===Dissociative Recombination Escape===
This is a method of atmospheric escape which allows heavy atoms which normally would be captured by gravity escape. The way the method works is this: Imagine a carbon monoxide atom in the upper atmosphere gets hit by an extreme [[Ultraviolet]] ray. It splits into ionized carbon and oxygen atoms. This oxygen ion eventually recombines with an electron. This releases a good deal of energy, which is expressed as heat (kinetic motion). If the atom (which is moving significantly faster at a given temperature than a larger molecule) is pointing away from the planet and it is in the exosphere (so it is unlikely to hit any other gas for a long distance) it may escape.

This is a very slow process, and has continued from the distance past to the presence. It occurred more commonly in the ancient past since the sun had more (and more powerful) [[Coronal Mass Ejections]] then. This form of atmospheric escape was important until around the start of the Noachian. After that, it declines as the lighter gases were already lost, and the intensity of the solar storms decreased.

===Impact Atmospheric Loss===
Huge impacts will strip away the atmosphere on a tangent to the horizon at impact. This form of atmosphere loss was common during the heavy bombardment (4.55 to 4.1 Ga) and in the late heavy bombardment (4.1 to ~3.9 Ga).

Note that small impacters were depositing volatiles on Mars during this period, so Mars gained and lost gas via impacters.

Melosh and Vickery's "Tangent Plane Model" estimated it takes 500 craters bigger than 100 km diameter to erode a 1 bar atmosphere on Mars. (This rate would only happen just after planetary formation at 4.5 Ga.) It takes roughly 500 craters bigger than 30 km to erode 0.01 bar atmosphere. (Cratering at this rate could only have happened before 3.5 Ga.)

Some estimates suggest that Mars lost 0.35 to 0.6 Bar of pressure from such impacts in its history.

===Events in the Pre-Noachian Period===
The pre-noachian period, ranges from 4.55 Ga until 4.1 Ga as little or no rocks remain from this period. During this period, the crust formed, & the magnetic dynamo turns on protecting Mars' early atmosphere from [[Solar Wind Sputtering]]. Major impact basins form including the formation of the northern lowlands and southern highlands (likely from a giant northern impact). A secondary atmosphere formed as hydrogen escaped into space and water condensed to liquid.

Vulcanism was very common. The Tharsis igneous province started building up.

There was high erosion from impacts and liquid water. The atmosphere thinned to something like 4 to 5 bar.

The pre-noachian period ends at the formation of the Hellas Impact Event, which happened around 4.1 Ga ago. (If our cratering estimates are off, it may have happened as recently as 3.9 Ga.)

===Losing the Magnetic Field===
Sometime around 4.1 to 3.9 Ga (billions of years ago) the magnetic dynamo that maintained Mars' magnetic field turned off. The exact time of this is subject to much debate, but most scientists prefer the earlier date. However, evidence exists for later periods (up to 3.6 Ga). Possibly the magnetic field turned off and on a few times before dying out for good.<ref>https://science.ubc.ca/news/ubc-researchers-establish-new-timeline-ancient-magnetic-field-mars</ref>

Without the magnetic field, the rate of atmospheric loss would increase slightly.

===Mars' Secondary Atmosphere===
Small impacters brought volatiles (gases and ices), and molten rock was outgassing. This formed a secondary atmosphere composed of carbon dioxide (CO2), nitrogen (N2), water (H2O), sulphur dioxide (SO2), neon (Ne), and argon (Ar). If the Martian atmosphere was reducing, then hydrogen (H2), carbon monoxide (CO), and methane (CH4), would be fairly common.

Note that all of these (except nitrogen, neon, and argon) are greenhouse gases.

Any hydrogen (and helium & neon) remaining would be lost to space fairly quickly. So the hydrogen would be gone by now, unless it was being replaced. However, under UV light, water can break up into atomic hydrogen and hydroxide molecules (OH), and the free hydrogen could then be lost. So water can gradually be photo-disassociated and lost to space. This will tend to oxidize the crust, as the extra oxygen binds with rocks.

=== Early Sun===
After the Sun (Sol) first formed, it was spinning much more rapidly than it is now. Magnetic interactions with the ions in the solar wind act as a brake, so star's rate-of-spin gradually slow down as they age.

Fast spinning stars have the magnetic field lines inside them twist up tightly very quickly which form sunspots and solar flares. From observations of young stars in this galaxy, it is clear that [[Coronal Mass Ejections]] (solar storms) and solar flares were more common and more powerful early in Sol's life.

This means that extreme [[Ultraviolet]] radiation was more common, which would ionize planetary atmospheres.

Note that the temperature of the sun was lower. Early in Sol's history it had little helium in its core. As it fuses hydrogen into helium, the helium 'ash' builds up increasing the density of the core. This increases the pressure, and makes hydrogen fusion easier. Thus, as time goes by the sun heats up.

Just after formation, Sol only produced 70% of the energy it does now. By late in the Noachian, it was producing 75% of its heat. So the 'early cool sun' means that Mars should have been cooler long ago.

==The Noachian Period==
This geologic epoch is when the oldest rocks on Mars formed. Mars then was a much warmer planet with many volcanoes, and running water, with snow and rain happening over large portions of the surface. Mars likely had a northern ocean then.

However, exactly HOW Mars remained warm is subject to considerable debate. In particular the question of, were the times of water erosion, lake deposits of silt, & delta formation episodic, or were they of long duration, remains debated.

The recent discover of some layered deposits were likely laid down over millions of years, suggest that the warm period was of long duration. <ref>https://www.imperial.ac.uk/news/168133/nasa-research-martian-lakes-have-survived/</ref>

Around the start of the Noachian, the intense extreme UV radiation (from large and common solar storms) will have decreased, slowing atmospheric escape by UV heating of the exosphere.

===Noachian Geologic Highlights===
The Noachian runs from ~4.1 to 3.7 Ga, and during this period the Tharsis volcanic highlands largely finished building. The late heavy bombardment ended about half way thru this period, with declining cratering rates. Note that the major erosion was driven by wind and impacts. Water (and ice) erosion modified the terrain, but was not the dominate form of landscape shaping.

Water eroding lavas form clays, but evidence suggests that the water gradually became more acidic (likely from sulphur dioxide from volcanos) which encourages the formation of sulphates. This means that early in the Noachian, phyllosilicates and carbonates formed easily, but later sulphates dominated.

During this period there was heavy water erosion, and valley networks formed. This is the period where wide spread layered terrain (sedimentary rocks) were formed. There were lakes, seas, and likely a northern ocean. However, the planet was cool, and these lakes and seas may have been ice covered much of the year. Snow was likely more common than rain.

While a few Martian river systems are over 2,000 km, most are <200 km long. Typically they are V shaped valleys in the upland areas, and more square shaped valleys in their lower reaches. The valleys are often 1 to 4 km wide, and 50 to 200 meters deep. The square shaped lower river valleys suggest that ground water sapping was an important feature in their history.

Glaciers dug out debris fields and made U shaped valleys during this period. See [[Glaciers on Mars]].

For a while it was thought that intense vulcanism locally warmed areas enough to explain water erosion features. But as more evidence has accumulated, this seems ever less likely. It is almost certain that Mars had enough precipitation to allow long flowing large rivers (tho they may have had a surface crust of ice).

However, volcanoes would pump various gases (many of them greenhouse gases) into the air, which would help to warm the planet while they were active.

===The Faint Young Sun - Warm Mars Paradox===
During the pre-noachian and noachian, Mars was almost certainly warm enough for liquid water. (Evidence of liquid water erosion which lasted for long time periods is wide spread.) However, the sun was only 70% as warm as it is now (75% as warm near the end of the Noachian at 3.8 Ga), which causes a major problem in our understanding of Mars' past. It is very hard to make a climate model that allows liquid water when the sun is so cool.

Carbon dioxide and water vapour are powerful greenhouse gases, but there are frequency windows where heat energy can escape. If there were small amounts of sulphur dioxide, methane, carbon monoxide, hydrogen sulphide, nitrous oxide, and or ammonia, then these windows of heat loss would be closed and the atmosphere would stay warmer. Ammonia (NH3) in particular blocks these windows nicely.

However, every one of these minor gases have problems. Nitrous oxide (N2O) forms a haze that blocks light, methane (CH4) breaks down quickly and must be renewed faster than seems likely, hydrogen sulphide(HSO2) & ammonia(NH3) are quickly washed out of the air by rain or snow, etc. Tho trace amounts of these gases might help a little, there is no reasonable candidate to explain the warming.

In times of intense vulcanism, enough sulphur dioxide (SO2</sub) could be pumped into the atmosphere to raise the temperature to where brands would be stable on the surface: ~250K. Since vulcanism was common at this time, this would certainly contribute to Mars' warming.

Much modelling of water and carbon dioxide clouds have been made and they would help to warm the planet slightly. (Clouds reflect heat from the surface back down, but also reflect incoming light back into space.) The warming ability of CO2 is hard to model. Depending on particle sizes in the clouds, they may reflect more energy away from the planet, or significantly warm it. Carbon dioxide clouds and their warming potential is still being researched. Clouds seem to have an overall slight warming effect. Thus, clouds help heating, and if we assume that there are breaks in the clouds (which is a certainty) we do not get enough warming.

Some have suggested that there was much more CO2 than expected. If there were 3 to 5 Bar of CO2 at the end of the Noachian, then this would go a long way to warming the planet. However, we come to different problem: If the partial pressure of CO2 was much higher, then it is easier for the carbon dioxide to frost out or snow at the poles. If we had 5 Bar of CO2, then it is hard to explain why it would not all freeze out and lower the CO2 back down to a near zero partial pressure.

The cool sun - warm Mars problem is currently much debated. Some think that maybe the Sun WAS NOT cooler, which is scoffed at by those who know stellar evolution.

Most scientists feel that the air pressure in the Noachian was from 0.5 to 1 Bar (tho other estimates put it at 2.3 Bar), and do not have an explanation of the faint young sun paradox.

The writer of this essay (Rick) feels that Mars must have had a thick atmosphere, of carbon dioxide and nitrogen. Nitrogen is not a greenhouse gas, but it does have thermal inertia. Perhaps with a thick atmosphere, winds could move enough heat from the equator to the poles, to keep the carbon dioxide from freezing out. Scientists (Manning et al. & Soto et al) have studied this and it is hard to pin down the exact numbers, but they suggest that Mars would have to have at least 0.7 Bar of atmospheric pressure for this to be significant. At pressures higher than this, any polar CO2 would be transitory.

Also, a thicker atmosphere (with plenty of N2) would support more airborne dust, which would darken ice and frost. Dust in the air, warms the atmosphere by slowing radiation loss, and absorbing sunlight and reemitting the energy as heat. (Altho if Mars had common rain and snow, it would have much less dust in the air.)

If Mars had a 4 Bar atmosphere in the Noachian, the slow loss of atmosphere (speeded slightly by solar wind sputtering once the magnetosphere shut down) does not explain the low atmospheric pressure now. However, I feel that they are underestimating the amount of gas absorbed into the crust. See [[Atmospheric Loss]] for more details.

==The Atmosphere During the Hesperian==
The Hesperian period (3.7 to 3.1 Ga) was a period of declining vulcanism, thinning atmosphere, and decreasing erosion.

Tho we see water erosion features, these are smaller and closer to the equator. There are strong indications that water erosion dropped suddenly with the start of the Hesperian. Water erosion was 2 to 5 orders of magnitude less (100 to 100,000 times less) than 300,000,000 years earlier.

It seems that intermittent lakes were common in the early Hesperian. They may have formed and retreated from changes in the height of the water table.

Some gullies could be extended by erosion from ground water, (called 'seep erosion') tho there must be precipitation from time to time to refill the ground water aquifers.

It seems likely that the water on the surface was frozen for increasing amounts of time, tho liquid water aquifers were likely common and fairly shallow.

At times when liquid water was on the surface, it was stable, but the increasingly dryer air would mean it would eventually evaporate. (Tho if covered by surface ice this would be slowed.)

When Mars has a high axil tilt (obliquity) the poles get long months of constant daylight, which totally melts the polar cap. The water would evaporate, and be transported to the cold side of the planet. Much of this would rain or snow out in equatorial regions. Thus periods when Mars had higher obliquity, would be periods when there was much more water movement than when the planet had almost no axil tilt, and both poles were stable year long.<ref>https://www.planetary.org/planetary-radio/2023-mars-gullies</ref>

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits.<ref>https://www.smithsonianmag.com/air-space-magazine/mystery-solved-mars-has-nitrates-180954769/</ref> Various gases were absorbed by the crust, and some of the atmosphere (especially lighter components) was lost to space. The thinning air made the planet steadily cooler. (The slowly warming sun was too slow to offset this loss.)

By the end of the Hesperian, the planet was so cold that carbon dioxide might freeze out in winter, the atmospheric pressure was much less (~0.2 Bar?), and water erosion (except for very rare catastrophic floods) had ended. The atmosphere and planet were approaching modern Mars.

==Bibliography==
"The Atmosphere and Climate of Mars", Edited by Robert M. Haberle, et al, Cambridge Planetary Science, ISBN 978-1-107-01618-7. See Chapter 17: "The Early Mars Climate System".

==References==