| Chemical species | mole fraction |
| Carbon dioxide | 95.32% |
| Nitrogen | 2.7% |
| Argon | 1.6% |
| Oxygen | 0.13% |
| Carbon monoxide | 0.07% |
| Water vapor | 0.03% |
| Nitric oxide | 0.013% |
| Neon | 2.5 μmol/mol |
| Krypton | 300 nmol/mol |
| Formaldehyde | 130 nmol/mol |
| Xenon | 80 nmol/mol |
| Ozone | 30 nmol/mol |
| Methane | 10.5 nmol/mol |
The atmosphere of Mars is relatively thin and is composed mostly of carbon dioxide (95.32%). There has been interest in studying its composition since the detection of trace amounts of methane,[1][2] which may indicate the presence of life on Mars, but may also be produced by a geochemical process, volcanic or hydrothermal activity.[3]
The atmospheric pressure on the surface of Mars varies from around 30 pascals (0.0044 psi) on Olympus Mons's peak to over 1,155 pascals (0.1675 psi) in the depths of Hellas Planitia, with a mean surface level pressure of 600 pascals (0.087 psi), compared to Earth's sea level average of 101.3 kilopascals (14.69 psi), and a total mass of 25 teratonnes, compared to Earth's 5148 teratonnes. However, the scale height of the atmosphere is about 11 kilometres (6.8 mi), somewhat higher than Earth's 7 kilometres (4.3 mi). The atmosphere on Mars consists of 95% carbon dioxide, 3% nitrogen, 1.6% argon, and contains traces of oxygen, water, and methane, for a mean molar mass of 43.34 g/mol.[4][5] The atmosphere is quite dusty, giving the Martian sky a light brown or orange color when seen from the surface; data from the Mars Exploration Rovers indicate that suspended dust particles within the atmosphere are roughly 1.5 micrometres across.[6]
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Mars' atmosphere is believed to have changed over the course of the planet's lifetime, with evidence suggesting the possibility that Mars had large oceans a few billion years ago.[7] As stated in the Mars Ocean Hypothesis, atmospheric pressure on the present day Martian surface only exceeds that of the triple point of water (6.11 hectopascals (0.0886 psi)) in the lowest elevations; at higher elevations water can exist only in solid or vapor form. Annual mean temperatures at the surface are currently less than 210 K (−63 °C; −82 °F), significantly lower than what is needed to sustain liquid water. However, early in its history Mars may have had conditions more conducive to retaining liquid water at the surface.
Possible causes for the depletion of a previously thicker Martian atmosphere include:
Mars' atmosphere is composed of the following layers:
The main component of the atmosphere of Mars is carbon dioxide (CO2). During the Martian winter the poles are in continual darkness and the surface gets so cold that as much as 25% of the atmospheric CO2 condenses at the polar caps into solid CO2 ice (dry ice). When the poles are again exposed to sunlight during the Martian summer, the CO2 ice sublimates back into the atmosphere. This process leads to a significant annual variation in the atmospheric pressure and atmospheric composition around the Martian poles.
The atmosphere of Mars is enriched considerably with the noble gas argon, in comparison to the atmosphere of the other planets within the Solar System. Unlike carbon dioxide, the argon content of the atmosphere does not condense, and hence the total amount of argon in the Mars atmosphere is constant. However, the relative concentration at any given location can change as carbon dioxide moves in and out of the atmosphere. Recent satellite data shows an increase in atmospheric argon over the southern pole during its autumn, which dissipates the following spring.[10]
Other aspects of the Martian atmosphere vary significantly. As carbon dioxide sublimates back into the atmosphere during the Martian summer, it leaves traces of water. Seasonal winds sweep off the poles at speeds approaching 400 kilometres per hour (250 mph) and transport large amounts of dust and water vapor giving rise to Earth-like frost and large cirrus clouds. These clouds of water-ice were photographed by the Opportunity rover in 2004.[11] NASA scientists working on the Phoenix Mars mission confirmed on July 31, 2008 that they had indeed found subsurface water ice at Mars' northern polar region. Further analysis by the Phoenix lander will confirm whether the water was ever liquid and if it contains organic materials necessary for life.
Trace amounts of methane (CH4), at the level of several nmol/mol (parts per billion, ppb), were first reported in Mars's atmosphere by a team at the NASA Goddard Space Flight Center in 2003.[2][12] In March 2004 the Mars Express Orbiter[13] and ground based observations from Canada-France-Hawaii Telescope[14] also suggested the presence of methane in the atmosphere with a mole fraction of about 10 nmol/mol.[15]
Because methane on Mars would quickly break down due to ultraviolet radiation from the Sun and chemical reactions with other gases, its reported persistent presence in the atmosphere also necessitates the existence of a source to continually replenish the gas.[16] Current photochemical models alone can not explain neither the fast appearance nor the disappearance of the methane, or its reported variations in space and time.[17] It had been proposed that the methane might be replenished by meteorites entering the atmosphere of Mars, but researchers from Imperial College London found that the volumes of methane released this way are too low to sustain the measured levels of the gas.[18]
The methane occurs in extended plumes, and their profiles imply that the gas was released from sources in three discrete regions. In northern midsummer, the principal plume contained 19,000 metric tons of methane, with an estimated source strength of 0.6 kilogram per second.[19][20] The profiles suggest that there may be two local source regions, the first centered near 30° N, 260° W and the second near 0°, 310° W.[19] It is estimated that Mars must produce 270 ton/year of methane.[19][21][22]
Research suggests that the implied methane destruction lifetime is as long as ~4 Earth years and as short as ~0.6 Earth years.[19][23] This lifetime is short enough for the atmospheric circulation to yield the observed uneven distribution of methane across the planet. In either case, the destruction lifetime for methane is much shorter than the timescale (~350 years) estimated for photochemical (UV radiation) destruction.[19] The rapid destruction of methane suggests another process must dominate removal of atmospheric methane on Mars and it must be more efficient than destruction by light by a factor of 100x to 600x.[19][23] This unexplained fast destruction rate also suggests a very active replenishing source.[24] A possibility is that the methane is not consumed at all, but rather condenses and evaporates seasonally from clathrates.[25]
Although the methane could stem from a geological source, the lack of current volcanism, hydrothermal activity or hotspots are not favorable for a geological explanation. Living microorganisms, such as methanogens, are another possible source, but no evidence exists for the presence of such organisms anywhere on Mars. NASA and ESA are planning to look for companion gases that may suggest which sources are most likely;[26][27] in the Earth's oceans, biological methane production tends to be accompanied by ethane, while volcanic methane is accompanied by sulfur dioxide.[27]
The principal candidates for the origin of Mars methane include non-biological processes such as water-rock reactions, radiolysis of water, and pyrite formation, all of which produce H2 that could then generate methane and hydrocarbons via Fischer-Tropsch synthesis with CO and CO2.[28] It was also recently shown that methane could be produced by a process involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.[29] The required conditions for this reaction (i.e. high temperature and pressure) do not exist on the surface, but may exist within the crust.[30] To prove this process is occurring, serpentinite, a mineral by-product of the process would be detected. Another possible geophysical source could be clathrate hydrates.[31]
The European Space Agency (ESA) found that the concentrations of methane in the Martian atmosphere were not even, but coincided with the presence of water vapor. In the upper atmosphere these two gases are uniformly distributed, but near the surface they concentrate in three equatorial regions, namely Arabia Terra, Elysium Planitia, and Arcadia Memnonia. Planetary scientist David H. Grinspoon of the Southwest Research Institute believes the coincidence of water vapor and methane increases the chance that the methane is of biological origin, but he cautions that it is uncertain how life could have survived so long on a planet as inhospitable as Mars.[12] It has been suggested that caves may be the only natural structures capable of protecting primitive life forms from micrometeoroids, UV radiation, solar flares and high energy particles that bombard the planet's surface.[32][33][34]
In contrast to the findings described above, studies by Kevin Zahnle, a planetary scientist at NASA's Ames Research Center, and two colleagues, conclude that "there is as yet no compelling evidence for methane on Mars". They argue that the strongest reported observations of the gas to date have been taken at frequencies where interference from methane in the Earth's atmosphere is particularly difficult to remove, and are thus unreliable. Additionally, they claim that the published observations most favorable to interpretation as indicative of Martian methane are also consistent with no methane being present on Mars.[35][36][37]
Ultimately, to determine the provenance of the Martian methane findings, a future probe or lander hosting a mass spectrometer must be sent to Mars.[38] Efforts to identify the sources of terrestrial methane have found that measurements of different methane isotopologues do not necessarily distinguish between possible geologic and biogenic sources, but the abundances of other cogenerated gases, such as ethane (C2H6), relative to methane do; the ethane/methane abundance ratio is<0.001 for biogenic sources, while other sources produce nearly equivalent amounts of methane and ethane.[39]
The Mars Science Laboratory rover, scheduled to land on Mars in 2012, will be able to make measurements that distinguish between different isotopologues of methane,[40] but even if the mission is to determine that microscopic Martian life is the source of the methane, the lifeforms likely resides far below the surface, outside of the rover's reach.[41] The Mars Trace Gas Mission orbiter planned to launch in 2016 would further study the methane,[42][43] as well as its decomposition products such as formaldehyde and methanol.
The atmosphere of Mars is a resource of known composition available at any landing site on Mars. It has been proposed that human exploration of Mars could use carbon dioxide (CO2) from Martian atmosphere to make rocket fuel for the return mission. Mission studies that propose using the atmosphere in this way include the Mars Direct proposal of Robert Zubrin and the NASA Design reference mission study. Two major chemical pathways for use of the carbon dioxide are the Sabatier reaction, converting atmospheric carbon dioxide along with additional hydrogen (H2), to produce methane (CH4) and oxygen (O2), and electrolysis, using a zirconia solid oxide electrolyte to split the carbon dioxide into oxygen (O2) and carbon monoxide (CO).
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