The Martian atmosphere contains about 95.3% carbon dioxide (CO2) and 2.7% nitrogen, with the remainder a mixture of trace gases. However, it is a very thin atmosphere, roughly 100 times less dense than the Earth's.

The atmosphere of a planet is made up of volatile substances that readily form a gas or vapor. Mars acquired volatiles from planetesimals that coalesced and assembled the original bulk of the planet. However, later Mars lost volatiles when the atmosphere was removed by various physical and chemical processes.

When Mars formed, impacting planetesimals produced much heat on the surface and released their volatiles, especially water. Consequently, the very earliest atmosphere probably was steam. When the Martian surface cooled sufficiently, the water condensed out onto the surface. At this stage, the very early atmosphere would have briefly included much hydrogen because iron in the mantle reacts with water to produce huge amounts of hydrogen that vents to the surface. Hydrogen is a very light gas that escapes to space. Other gases released from volcanoes on early Mars included carbon dioxide, nitrogen, sulfur dioxide, and perhaps relatively large amounts of methane. The atmosphere would have been thick, perhaps denser than the Earth's present atmosphere, and made up mostly of outgassed carbon dioxide and nitrogen. Three key processes would have been effective at removing the early thick atmosphere:

1. when atmospheric CO2 reacted with surface rocks and water to form a solid carbonate mineral
2. when impacts from asteroids or comets blew away the atmosphere
3. when the solar wind stripped away the upper atmosphere

* Dried up river valleys indicate that Mars had liquid water on its surface about 3.6 to 4 billion years ago. Because liquid water cannot exist at today's sub-freezing temperature on Mars (about -60 deg C on average), except as a very transient phenomenon, this implies a thicker greenhouse atmosphere. On Earth, the CO2 released to the atmosphere from volcanoes dissolves in water, reacts with silicate rocks, and forms carbonate minerals that sink to the bottom of the ocean. At the ocean floor, over many thousands of years, this solid material gets squashed down to form sedimentary rock. A similar process would have occurred on an early, wet Mars. Then later on, in the absence of Martian volcanic activity to re-inject the CO2 into the atmosphere, atmospheric CO2 levels would have declined, greenhouse warming would have diminished, and Mars would have become frozen. Luckily, Earth has not frozen because volcanic activity resulting from tectonics continually recycles CO2 back into the atmosphere on geological timescales. Despite the expectation of carbonate minerals on the surface of Mars, NASA's Mars Global Surveyor spacecraft has failed to detect carbonate deposits unambiguously. Ancient carbonate deposits, if they exist at all, must be buried.

* But perhaps carbonate deposits are relatively sparse. It is likely that much of the very early Martian atmosphere, perhaps more than 99% of the CO2 and nitrogen, got blown away to space. After all, why is there so little nitrogen now? This would have happened when large asteroids or comets crashed into Mars and atmospheric gases were blasted away into space beyond the influence of Mars' weak gravity. We know many objects collided with Mars because of all the craters on its surface dating from the period before 3.8 billion years ago, called the Period of Heavy Bombardment. Evidence also comes from isotopes of the gas xenon. The mixture of xenon isotopes in the Martian atmosphere today contains a much greater proportion of xenon-129 (129Xe) than in the Earth's atmosphere or in the Sun. 129Xe is produced by the decay of radioactive iodine-129 in the interior of the planet. Half of the iodine-129 decays in a period of 17 million years. For 129Xe to predominate, the primordial Martian atmosphere, which contained a xenon isotope mixture similar to that in the rest of the solar system, must have been largely removed before most of the radioactive iodine inside the planet decayed to 129Xe.

* Another process that removes the atmosphere is sputtering by the solar wind. The upper atmosphere of a planet is constantly bombarded by the solar wind, a fast stream of very light particles emanating from the sun. This wind itself is fairly benign, but it also carries a magnetic field. This picks up ions from the upper atmosphere, accelerates them, and then smashes them back into other ions at several hundred kilometers per second, knocking ions out to space. If the planet itself has a magnetic field, it can shield the upper atmosphere from the solar wind. However, because it is a small planet, Mars cooled rapidly so that its inner dynamo disappeared and it lost its original magnetic field quickly. Certainly, the magnetic field had disappeared by about 4 billion years ago when the Hellas impact basin formed because this punched through remnant magnetic field lineations detected by Mars Global Surveyor. Sputtering may have been responsible for removing up to 1 bar of the early Martian atmosphere.

Whatever the dominant atmospheric removal process, when the atmospheric density got below about 50 times more than it is now, the planet would have become cold enough for CO2 to form ice caps at the poles. When this happened, much of the atmosphere would have collapsed into the polar ice caps to leave the cold, arid Mars that we see today.

Finally, the fact that the atmospheric surface pressure is today remarkably close to a value of 6.1 millibars, called the "triple point" of water below which liquid water is unstable, suggests that the atmosphere is self-limiting. If the atmosphere were to become thick enough for sufficient greenhouse warming to facilitate liquid water periodically at the surface, the CO2 would get removed as carbonates and tend to draw the atmosphere gradually back down to 6.1 millibars. This may be the reason for the particular value of the atmospheric pressure (and hence density) on Mars that we observe today.