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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 ppm
Krypton 300 ppb
Formaldehyde 130 ppb [267008]
Xenon 80 ppb
Ozone 30 ppb
Methane 10.5 ppb
Mars, the fourth planet from the Sun, has a very different atmosphere from that of the Earth. There has been much interest in studying its composition since the recent detection of a small amount of methane, which may signal life on Mars; it could also be a geochemical process or the result of volcanic or hydrothermal activity.

The atmosphere of Mars is relatively thin, and the atmospheric pressure on the surface varies from around 30 Pa (0.03 kPa) on Olympus Monsmarker's peak to over 1155 Pa (1.155 kPa) in the depths of Hellas Planitiamarker, with a mean surface level pressure of 600 Pa (0.6 kPa, or 6 millibars, or 0.087 psi), compared to Earth's 101.3 kPa, and a total mass of 25 teratonnes, compared to Earth's 5148 teratonnes. However, the scale height of the atmosphere is about 11 km, somewhat higher than Earth's 7 km. The atmosphere on Mars consists of 95% carbon dioxide, 3% nitrogen, and 1.6% argon, and contains traces of oxygen, water, and methane, for a mean molecular weight of 43.34 g/mole. The atmosphere is quite dusty, giving the Martian sky a tawny colour when seen from the surface; data from the Mars Exploration Rovers indicate that the suspended dust particles are roughly 1.5 micrometres across.

History

Mars' atmosphere is believed to have changed over the course of the planet's lifetime. Evidence suggests the possibility that Mars had large oceans a few billion years ago. 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 hPa) 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, significantly less 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 the following:
  • Catastrophic collision by a body large enough to blow away a significant percentage of the atmosphere;
  • Gradual erosion of the atmosphere by solar wind; and
  • On-going removal of atmosphere due to electromagnetic field and solar wind interaction.


Structure

Mars' atmosphere is composed of the following major divisions:
  • Lower Atmosphere: This is a warm region affected by heat from airborne dust and from the ground.
  • Middle Atmosphere: Mars has a jetstream which flows in this region.
  • Upper Atmosphere, or Thermosphere: This region has very high temperatures caused by heating from the Sun. Here, atmospheric gases start to separate from each other rather than forming the even mix found in the lower atmospheric layers.
  • Exosphere: 200 kilometers and higher. This region is where the last wisps of atmosphere merge into space. There is no distinct boundary where the atmosphere ends; it just tapers away.


Composition

Mars' thin atmosphere, visible on the horizon in this low orbit image.


Carbon dioxide

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 sublimes back into the atmosphere. This process leads to a significant annual variation in the atmospheric pressure and atmospheric composition around the Martian poles.

Argon

The atmosphere of Mars is considerably enriched 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 in autumn, which dissipates the following spring.

Water

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 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. NASAmarker 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.

Methane

Trace amounts of methane, at the level of several parts per billion (ppb), were first reported in Mars's atmosphere by a team at the NASA Goddard Space Flight Center in 2003.In March 2004 the Mars Express Orbiter and ground based observations from Canada-France-Hawaii Telescopemarker also suggested the presence of methane in the atmosphere with a concentration of about 10 ppb by volume. The presence of methane on Mars is very intriguing, since as an unstable gas it indicates that there must be an active source of the gas on the planet. Furthermore, current photochemical models cannot explain the presence of methane in the atmosphere of Mars and its reported rapid variations in space and time. Neither its fast appearance nor disappearance can be explained yet.

Methane occurres in extended plumes, and the profiles imply that the methane was released from 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. 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.It is estimated that Mars must produce 270 ton/year of methane.

The latest research suggests that the implied methane destruction lifetime is as long as ~4 Earth years and as short as ~0.6 Earth years. This lifetime is short enough for the atmospheric circulation to yield an uneven distribution of methane across the planet, which is what is observed. In either case, the destruction lifetime for methane is much shorter than the timescale (~350 years) estimated for photochemical (UV radiation) destruction. The rapid destruction of methane suggests another process must dominate removal of atmospheric methane on Mars and it must be more efficient than photochemistry by a factor of 100 to 600, such as strong oxidants like peroxides and perchlorates in the soil.This unexplained faster destruction rate also suggests a very active replenishing source.

Although geologic sources of methane are possible, the lack of current volcanism, hydrothermal activity or hotspots are not favorable for geologic methane. The existence of life in the form of microorganisms such as methanogens are among possible, but as yet unproven sources. Plans are now being made to look for other companion gases that may suggest which sources are most likely; in the Earth's oceans, biological methane production tends to be accompanied by ethane, while volcanic methane is accompanied by sulfur dioxide.

It was also recently shown that methane could be produced by a non-biological process involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars. The required conditions for this reaction (i.e. high temperature and pressure) do not exist on the surface, but may exist within the crust. To prove this process is occurring, serpentine, a mineral by-product of the process would be detected. Another possible geophysical source could be clathrate hydrates.

The European Space Agencymarker (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 gasses are uniformly distributed, but near the surface they concentrate in three equatorial regions, namely Arabia Terramarker, Elysium Planitiamarker, and Arcadia Memnonia. Planetary scientist David H. Grinspoon (Southwest Research Institute) feels the coincidence of water vapor and methane increases the chance of a biological source, but cautions that it is uncertain how life could have survived so long on a planet as inhospitable as Mars, although it has been sugested 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.

Ultimately, to rule out a biogenic origin for the methane, a future probe or lander hosting a mass spectrometer will be needed, since the isotopic proportions of carbon-12 to carbon-14 in methane could distinguish between a biogenic and non-biogenic origin. However, efforts to identify the sources of terrestrial methane have found that measurements of methane (CH4) isotopologues do not necessarily distinguish between possible geologicand biogenic sources, and it has been found that the abundances of other cogenerated gas, such as ethane (C2H6), relative to methane can distinguish between a source from active biology and other potential sources; the ethane/methane abundance ratio is <10-3 for="" the="" former,="" while="" other="" sources="" produce="" nearly="" equivalent="" amounts="" of="" methane="" and="" ethane.=""></10-3>

In 2012, the Mars Science Laboratory rover will measure such isotopes. If microscopic Martian life is producing the methane, it likely resides far below the surface, where it is still warm enough for liquid water to exist. NASA has revealed the goal of launching the Mars Trace Gas Mission orbiter on 2016 to further study the methane in Mars's atmosphere.

Potential for use by humans

The atmosphere of Mars is a resource of known composition available at any landing site on Mars. It has therefore been proposed that human exploration of Mars could use carbon dioxide from Martian atmosphere as feedstock 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 to produce methane and oxygen, and electrolysis, using a zirconia solid oxide electrolyte to split the carbon dioxide into oxygen and carbon monoxide.

However, if humans are to colonize Mars in the future, they will likely need as many greenhouse gases as they can get in order to maintain a warm climate. So using the Martian atmosphere as a consumable resource with no intentions of replenishing it may be considered dubious.

See also



References

  1. Life on Mars? - Geological and biological processes observed on Earth provide hunky-dory explanations for methane on Mars, Martin Baucom, American Scientist, March-April 2006.
  2. Interplanetary Whodunit - Methane on Mars, David Tenenbaum, Astrobiology Magazine, NASA, 20 Jul 2005. (Note: part one of a four-part series.)
  3. Mumma, M. J.; Novak, R. E.; DiSanti, M. A.; Bonev, B. P., "A Sensitive Search for Methane on Mars" (abstract only). American Astronomical Society, DPS meeting #35, #14.18.
  4. Making Sense of Mars Methane (June 2008)
  5. Seiff, A. and Kirk, D. (1977). "Structure of the atmosphere of Mars in summer at mid-latitudes" (abstract only). Journal of Geophysical Research, 82(28):4364–4378.
  6. Lemmon et al., "Atmospheric Imaging Results from the Mars Exploration Rovers: Spirit and Opportunity"
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  11. Clouds - Dec. 13, 2004 NASA Press release. URL accessed March 17, 2006.
  12. Mars Trace Gas Mission (10 September 2009)
  13. Planetary Fourier Spectrometer website (ESA, Mars Express)
  14. Remote Sensing Tutorial, Section 19-13a - Missions to Mars during the Third Millennium, Nicholas M. Short, Sr., et al., NASA


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