Photo of Mars's southern ice cap. Image taken from JPL Mars site
To understand the atmosphere of Mars, we must know what volatiles are present and in what quantities. Not only are the volatiles in the atmosphere important, but those trapped in the ice caps and in the subsurface are important as potential sources for a new atmosphere.
The composition of Mars's atmosphere is farily well understood, both from spectroscopic analyses and the Viking and Mars Pathfinder missions. The table below summarizes the major components.
| Volatile | Percentage | Surface Partial Pressure | Mass in atmosphere |
| CO2 | 95% | 6.56 mbar | 3.40e11 kg |
| N2 | 2.7% | 0.186 mbar | 9.68e9 kg |
| Ar | 1.6% | 0.110 mbar | 5.74e9 kg |
| While there may be no appreciable water vapor in the atmosphere, the northern polar ice cap is dominated by it. If melted, the northern ice cap would release enough water for a 2 m-deep global ocean. The southern ice cap is mostly carbon dioxide, as are the perennial caps. Recent observations show that these caps are dissipating. There is enough carbon dioxide in the southern cap to double the atmospheric pressure on Mars. | Photo of Mars's southern ice cap. Image taken from JPL Mars site |
 Image taken from JPL's Mars Odyssey Page | While the atmosphere and ice caps have been widely studied, the composition of the subsurface is poorly known. Recent observations by the Mars Odyssey spacecraft indicate large amounts of water in the regolith. However, the neutron spectrometer aboard the spacecraft only tells us of its present and its distribution, with no information regarding the form. The water may be frozen as ice or may be bound up with CO2 in a clathrate. |
Much of what we know about the volatile content of the martian interior comes from analysis of the SNC meteorites, meteorites that are believed to have come from Mars and be representative of the martian mantle. Such analyses indicate a very dry mantle, only 36 ppm of water, less than a tenth that of the Earth's mantle. The crust, however, seems to be quite rich in water and geomorphological evidence (channels and valleys) indicate that a 500 meter deep ocean once flowed there. One interpretation is that the lack of plate tectonics prevented the water from being recycled. [Moersch, 1996] More about this can be found on the atmospheric evolution page.