Atmosphere

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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.

Composition (gaseous parts)

Composition of Mars atmosphere by volume[1][2]

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.

Air Pressure

1-9 millibars (depending on altitude) or 600 Pa, average. This is 0.6% of Earth's air pressure at sea level.

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 240K while at 80 to 120 km the temperature is about 130K (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 190K to 245K in the lower latitudes.

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.

References

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: 987-1-107-01618-7.