Composition and surface pressure

Carbon dioxide constitutes 95.3 percent of the atmosphere by weight (see the table), nine times the quantity now in Earth’s much more massive atmosphere. Much of Earth’s carbon dioxide, however, is chemically locked in sedimentary rocks; the amount in the Martian atmosphere is less than a thousandth of the terrestrial total. The balance of the Martian atmosphere consists of molecular nitrogen, water vapor, and noble gases (argon, neon, krypton, and xenon). There are also trace amounts of gases that have been produced from the primary constituents by photochemical reactions, generally high in the atmosphere; these include molecular oxygen, carbon monoxide, nitric oxide, and small amounts of ozone.

The lower atmosphere supplies gas to the planet’s ionosphere, where densities are low, temperatures are high, and components separate by diffusion according to their masses. Various constituents in the top of the atmosphere are lost to space, which affects the isotopic composition of the remaining gases. For example, because hydrogen is lost preferentially over its heavier isotope deuterium, Mars’s atmosphere contains five times more deuterium than Earth’s.

Although water is only a minor constituent of the Martian atmosphere (a few molecules per 10,000 at most), primarily because of low atmospheric and surface temperatures, it plays an important role in atmospheric chemistry and meteorology. The Martian atmosphere is effectively saturated with water vapor, yet there is no liquid water present on the surface. The temperature and pressure of the planet are so low that water molecules can exist only as ice or as vapor. Little water is exchanged daily with the surface despite the very cold nighttime surface temperatures.

Water vapor is mixed uniformly up to altitudes of 10–15 km (6–9 miles) and shows strong latitudinal gradients that depend on the season. The largest changes occur in the northern hemisphere. During summer in the north, the complete disappearance of the carbon dioxide cap leaves behind a water-ice cap. Sublimation of water from the residual cap results in a strong north-to-south concentration gradient of water vapor in the atmosphere. In the south, where a small carbon dioxide cap remains in summer and only a small amount of water ice has been detected, a strong water vapor gradient does not normally develop in the atmosphere.

The atmospheric water vapor is believed to be in contact with a much larger reservoir in the Martian soil. Subsurface layers of ice seem to be ubiquitous on Mars at latitudes poleward of 40°; the very low subsurface temperatures would prevent the ice from subliming. The 2001 Mars Odyssey spacecraft confirmed that ice is present within a meter of the surface at latitudes higher than 60°, and the Phoenix lander found ice below the surface at 68° N, but it is not known how deep the ice layer extends. Images taken by the Mars Reconnaissance Orbiter showed new impact craters at latitudes between 40° N and 60° N that had exposed the subsurface water ice up to a depth of 74 cm (29 inches). In contrast, at low latitudes ice is unstable, and any ice present in the ground would tend to sublime into the atmosphere.

Isotopic measurements suggest that larger amounts of carbon dioxide, nitrogen, and argon were present in the atmosphere in the past and that Mars may have lost much of its inventory of volatile substances early in its history, either to space or to the ground (i.e., locked up chemically in rocks). Mars once had a much thicker atmosphere that was mostly lost to space through the solar wind and the Sun’s ultraviolet radiation, which were much more intense in the early solar system.

Methane has also been detected in Mars’s atmosphere. The Curiosity rover observed a seasonal variation in methane, but measurements from orbiters have shown only sporadic variations or even a complete absence of methane. This contradiction suggests that some process removes the methane observed near the surface before it spreads through the atmosphere. Volcanoes and meteorites have been ruled out as origins for the methane, which leaves chemical reactions between rock and water or metabolism by possible Martian microorganisms as possible sources.