Ancient Atmosphere

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.

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.

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.

During this time, nitrogen was lost to lighting which would build up nitrate soil deposits. 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".