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Mars from Hubble Space Telescope October 28, 2005 with sandstorm visible.
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 much interest in studying its composition since the recent 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] 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.[5]

Contents

History

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.[6] 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:

  • Catastrophic collision by a body large enough to blow away a significant percentage of the atmosphere;[7]
  • Gradual erosion of the atmosphere by solar wind;[8] and
  • On-going removal of atmosphere due to electromagnetic field and solar wind interaction.[7]
Martian sunset by Spirit rover at Gusev crater, May 2005.

Structure

Mars' atmosphere is composed of the following layers:

  • 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. Atmospheric gases start to separate from each other at these altitudes, rather than forming the even mix found in the lower atmospheric layers.
  • Exosphere: Typically stated to start at 200 kilometres (120 mi) and higher, this region is where the last wisps of atmosphere merge into the vacuum of 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 sublimates 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 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.[9]

Water

Main article: Water on Mars
Mars Pathfinder image of Martian sky with water ice clouds

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.[10] 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.

Methane

Trace amounts of methane, 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][11] In March 2004 the Mars Express Orbiter[12] and ground based observations from Canada-France-Hawaii Telescope[13] also suggested the presence of methane in the atmosphere with a mole fraction of about 10 nmol/mol.[14] The presence of methane indicates that there must be an active source on the planet, as it is an unstable gas. 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.[15]

Distribution of methane in the atmosphere of Mars during what is summertime in its Northern Hemisphere

It had been proposed that the methane was being replenished by meteorites entering the Martian atmosphere, but researchers from Imperial College London have found that the volumes of methane released this way are too low to maintain the measured levels of the gas.[16]

Methane occurs in extended plumes, and the profiles imply that the methane was released from 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.[17][18] 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.[17] It is estimated that Mars must produce 270 ton/year of methane.[17][19][20]

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.[17][21] 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.[17] 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, such as strong oxidants like peroxides and perchlorates in the soil.[17][21] A possibility is that the methane is not consumed at all, but rather condenses and evaporates seasonally from clathrates.[22] This unexplained fast destruction rate also suggests a very active replenishing source.[23]

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.

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.[24] 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.[25] The required conditions for this reaction (i.e. high temperature and pressure) do not exist on the surface, but may exist within the crust.[26] 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.[27]

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 (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,[11] although 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.[28][29][30]

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.[31] However, efforts to identify the sources of terrestrial methane have found that measurements of methane (CH4) isotopologues do not necessarily distinguish between possible geologic and 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 <0.001 for biogenic sources, while other sources produce nearly equivalent amounts of methane and ethane.[32]

In 2012, the Mars Science Laboratory rover will measure such isotopes.[33] 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.[34] NASA has revealed the goal of launching the Mars Trace Gas Mission orbiter on 2016 to further study the methane,[35][36] as well as its decomposition products such as formaldehyde and methanol.

Potential for use by humans

Main article: In-situ resource utilization
See also: Terraforming of Mars

The atmosphere of Mars is a resource of known composition available at any landing site on Mars. It has been used to slow down landers and de-accelerate orbiters (see aerocapture). It has 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.

See also

Mars portal
Solar System portal
Atmospheric sciences portal
Space portal
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References

  1. ^ Interplanetary Whodunit - Methane on Mars, David Tenenbaum, Astrobiology Magazine, NASA, July 20, 2005. (Note: part one of a four-part series.)
  2. ^ a b 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.
  3. ^ Making Sense of Mars Methane (June 2008)
  4. ^ 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.
  5. ^ Lemmon et al., "Atmospheric Imaging Results from the Mars Exploration Rovers: Spirit and Opportunity"
  6. ^ "Gamma-Ray Evidence Suggests Ancient Mars Had Massive Oceans". Science Daily. University of Arizona. http://www.sciencedaily.com/releases/2008/11/081117212321.htm. Retrieved October 31, 2009. 
  7. ^ a b "Solar Wind Rips Up Martian Atmosphere". NASA Headlines. NASA. http://science.nasa.gov/headlines/y2008/21nov_plasmoids.htm. Retrieved October 31, 2009. 
  8. ^ "The Solar Wind at Mars". Science @ NASA. NASA. http://science.nasa.gov/headlines/y2001/ast31jan_1.htm. Retrieved October 31, 2009. 
  9. ^ Francois Forgot. "Alien Weather at the Poles of Mars". Science. http://www-mars.lmd.jussieu.fr/mars/publi/forget_science2004.pdf. Retrieved February 25, 2007. 
  10. ^ Clouds - December 13, 2004 NASA Press release. URL accessed March 17, 2006.
  11. ^ a b Michael J. Mumma. "Mars Methane Boosts Chances for Life". Skytonight.com. http://www-mgcm.arc.nasa.gov/MGCM.html. Retrieved February 23, 2007. 
  12. ^ V. Formisano, S. Atreya T. Encrenaz, N. Ignatiev, M. Giuranna (2004). "Detection of Methane in the Atmosphere of Mars". Science 306 (5702): 1758–1761. Bibcode 2004Sci...306.1758F. doi:10.1126/science.1101732. PMID 15514118. 
  13. ^ V. A. Krasnopolskya, J. P. Maillard, T. C. Owen (2004). "Detection of methane in the martian atmosphere: evidence for life?". Icarus 172 (2): 537–547. Bibcode 2004Icar..172..537K. doi:10.1016/j.icarus.2004.07.004. 
  14. ^ ESA Press release. "Mars Express confirms methane in the Martian atmosphere". ESA. http://www.esa.int/SPECIALS/Mars_Express/SEMZ0B57ESD_0.html. Retrieved March 17, 2006. 
  15. ^ Mars Trace Gas Mission (September 10, 2009)
  16. ^ "Life on Mars theory boosted by new methane study". Imperial College London. December 8, 2009. http://www3.imperial.ac.uk/newsandeventspggrp/imperialcollege/newssummary/news_8-12-2009-10-5-57#fni-2. Retrieved December 9, 2009. 
  17. ^ a b c d e f Mumma, Michael J.; et al. (February 20, 2009). "Strong Release of Methane on Mars in Northern Summer 2003". Science 323 (5917): pp. 1041–1045. Bibcode 2009Sci...323.1041M. doi:10.1126/science.1165243. PMID 19150811. http://images.spaceref.com/news/2009/Mumma_et_al_Methane_Mars_wSOM_accepted2.pdf. 
  18. ^ Hand, Eric (October 21, 2008). "Plumes of methane identified on Mars". Nature News. http://esse.engin.umich.edu/psl/PRESS/Mars/NatureNews_2008.pdf. Retrieved August 2, 2009. 
  19. ^ Vladimir A. Krasnopolsky (February 2005). "Some problems related to the origin of methane on Mars". Icarus 180 (2): 359–367. doi:10.1016/j.icarus.October+1,+20055. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-4HTCW36-2&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=a614a9e35a422b94cc2611ccdc4bf180. 
  20. ^ Planetary Fourier Spectrometer website (ESA, Mars Express)
  21. ^ a b Franck, Lefèvre; François Forget (August 6, 2009). "Observed variations of methane on Mars unexplained by known atmospheric chemistry and physics". Nature 460 (7256): 720–723. Bibcode 2009Natur.460..720L. doi:10.1038/nature08228. PMID 19661912. http://www.nature.com/nature/journal/v460/n7256/abs/nature08228.html. Retrieved October 23, 2009. 
  22. ^ Zahnle, Kevin; Richard Freedman and David Catling (2010). "Is there Methane on Mars?". 41st Lunar and Planetary Science Conference. http://www.lpi.usra.edu/meetings/lpsc2010/pdf/2456.pdf. Retrieved 2010-07-26 
  23. ^ Burns, Judith (August 5, 2009). "Martian methane mystery deepens". BBC News. http://news.bbc.co.uk/2/hi/science/nature/8186314.stm. Retrieved August 7, 2009. 
  24. ^ "THE ASTROBIOLOGY OF MARS: METHANE AND OTHER CANDIDATE BIOMARKER GASES, AND RELATED INTERDISCIPLINARY STUDIES ON EARTH AND MARS.". Astrobiology Science Conference 2010. Goddard Space Flight Center. Greenbelt, MD, USA.: Astrophysics Data System. 2010. http://www.lpi.usra.edu/meetings/abscicon2010/pdf/5590.pdf. Retrieved 2010-07-24 
  25. ^ Oze, C., M. Sharma (2005). "Have olivine, will gas: Serpentinization and the abiogenic production of methane on Mars". Geophys. Res. Lett. 32 (10): L10203. Bibcode 2005GeoRL..3210203O. doi:10.1029/2005GL022691. http://www.agu.org/journals/gl/gl0510/2005GL022691/. 
  26. ^ Rincon, Paul (March 26, 2009). "Mars domes may be 'mud volcanoes'". BBC News. http://news.bbc.co.uk/2/hi/science/nature/7966437.stm. Retrieved April 2, 2009. 
  27. ^ Thomas, Caroline; et al. (January 2009). "Variability of the methane trapping in martian subsurface clathrate hydrates". Planetary and Space Science 57 (1): 42–47. Bibcode 2009P&SS...57...42T. doi:10.1016/j.pss.2008.10.003. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V6T-4TPHRT5-2&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=e64bf861e4ef71f5336823d5dc1b9e12. Retrieved August 2, 2009. 
  28. ^ Langhoff, Stephanie, ed (June 2008). "Workshop Report On Deep Mars" (PDF). Accessing The Subsurface Of Mars On Near Term Missions. NASA. PMID NASA/CP–2008-214586. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090026215_2009024820.pdf. Retrieved September 15, 2009 
  29. ^ Rincon, Paul (March 17, 2007). "'Cave entrances' spotted on Mars". BBC News. http://news.bbc.co.uk/2/hi/science/nature/6461201.stm. Retrieved September 15, 2009. 
  30. ^ Than, Ker (April 2, 2007). "Possible New Mars Caves Targets in Search for Life". Space,com. http://www.space.com/scienceastronomy/070402_mm_mars_caves.html. Retrieved September 15, 2009. 
  31. ^ Remote Sensing Tutorial, Section 19-13a - Missions to Mars during the Third Millennium, Nicholas M. Short, Sr., et al., NASA
  32. ^ "Report from the 2013 Mars Science Orbiter (MSO) - Second Science Analysis Group". Mars Exploration Program Analysis Group (MEPAG MSO-SAG-2). 2007. pp. 16. http://mepag.jpl.nasa.gov/reports/MSO_SAG2_Report_MEPAG_29may1.pdf. Retrieved November 9, 2009 
  33. ^ Tenenbaum, David (June 9, 2008):). "Making Sense of Mars Methane". Astrobiology Magazine. http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=2765&mode=thread&order=0&thold=0. Retrieved October 8, 2008. 
  34. ^ Steigerwald, Bill (January 15, 2009). "Martian Methane Reveals the Red Planet is not a Dead Planet". NASA's Goddard Space Flight Center (NASA). http://www.nasa.gov/mission_pages/mars/news/marsmethane.html. Retrieved January 24, 2009. 
  35. ^ Rincon, Paul (July 9, 2009). "Agencies outline Mars initiative". BBC News. http://news.bbc.co.uk/2/hi/science/nature/8130393.stm. Retrieved July 26, 2009. 
  36. ^ "NASA orbiter to hunt for source of Martian methane in 2016". Thaindian News. March 6, 2009. http://www.thaindian.com/newsportal/health/nasa-orbiter-to-hunt-for-source-of-martian-methane-in-2016_100163335.html. Retrieved July 26, 2009. 

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