Atmosphere

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 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 Atmospheric loss for what happened to this thick atmosphere.

Composition (gaseous parts)

Composition of Mars atmosphere by volume

Percentage	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 vapor (H2O)
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.)

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 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 usually 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. So 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.

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.