Mars has not been able to retain more than a very thin atmosphere for most of its history. The atmosphere escaped the low surface gravity of the planet at least 3 billions years ago. With a mass only about 10% that of Earth, Mars has only one third the Earth's gravity. The atmospheric pressure is only about 0.006-0.008 bars--roughly the pressure at an altitude of 35 km here on Earth. That is about the altitude at which U2 spy planes and stratospheric balloons cruise.

The composition of Mars's atmosphere is quite similar to that of Venus, at 95% carbon dioxide (CO₂), 2.5% nitrogen (N₂) and 1.5% argon (Ar). The chemistry of Earth's atmosphere is quite the exception among the inner planets, due to the presence of oceans and of life. These factors radically altered our atmosphere, which would have otherwise been dominated by carbon dioxide as it is on Mars and Venus.

The temperature on Mars's surface is generally very low and ranges between extreme values. While at the planet's equator the temperature can reach about 300 K (70 F) at midday, the same region of the planet can plunge to 250 K (- 20 F) the following night! At most locations on the planet the temperature hardly reaches the freezing point of water (273 K, or 32 F) any time during the year.

An exception is the southern polar region, which is permanently far below the freezing point of water. Its temperature is even below the freezing point of carbon dioxide, and the southern polar cap contains a lot of 'dry ice'. The carbon dioxide from the southern cap is partly released during the summer months, contributing to a 25% periodic oscillation of the atmospheric pressure between 0.006 and 0.008 bars.

Even though we expect Mars to possess a lot of frozen groundwater, the atmospheric pressure on Mars does not allow the water to remain in liquid form. In fact, as soon as any geologic event takes place on Mars that is able to liquefy ice, the running liquid water quickly boils away into vapor. Such events seem to have happened quite recently in Martian history, and probably are still occurring (see the section on Mars's geology).

Most of water vapor does not remain in the atmosphere for long. It either re-freezes, gradually escapes the planet's low gravitational pull, or gets dissociated into hydrogen and oxygen by ultraviolet radiation from the Sun, free to reach the Martian ground. The probes that landed of the planet took photos showing that Mars does have wispy clouds, similar in appearance to the cirrus clouds on Earth. The temperature is so low that often these clouds are made of carbon dioxide ice, rather than ice crystals.

Wind has had a surprisingly great effect on the modeling of Mars's landscape. Mariner 9 and various later probes (recently even the Hubble Space Telescope) observed huge sandstorms when Mars was at its perihelion, sometimes covering the entire planet for a few days. The red desert dust kicked up by these storms makes Mars's sky reddish in color for a while as the smallest particles remain in suspension. Dust storms seem to result from the fact that Mars has an eccentric orbit and receives 40% more light at perihelion than at the aphelion, giving rise to large climatic variations.