Composition of Mars' Atmosphere

The composition of the Martian atmosphere by volume, as measured by NASA missions. Carbon dioxide accounts for 95.0 percent, nitrogen 2.8 percent and argon 2.0 percent. Oxygen makes up 0.174 percent, carbon monoxide 0.0747 percent and water vapour about 0.03 percent.

The three main gases account for 99.8 percent of the atmosphere between them, which makes Mars far simpler in composition than Earth, where nitrogen and oxygen alone make up 99 percent in almost the opposite proportion. Oxygen on Mars is present at roughly one five-hundredth of its share in Earth's air.

The percentages describe composition, not quantity, and that is the crucial thing this chart cannot show on its own. Mars has an extremely thin atmosphere: surface pressure averages under one percent of Earth's at sea level. So while carbon dioxide makes up 95 percent of Martian air against 0.04 percent of Earth's, the total amount of CO2 above a square metre of Mars is far smaller than the figure implies. A 95 percent share of very little is still very little.

Water vapour at 0.03 percent is likewise a share of a very thin atmosphere, which is why liquid water cannot persist on the surface: at that pressure water passes directly between ice and vapour across most of the planet.

The minor constituents vary with season and location. Carbon dioxide freezes out onto the polar caps in winter and sublimates back in summer, which changes total atmospheric pressure by around 25 percent over a Martian year and shifts the proportions of everything else accordingly.