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Composition of Earth's atmosphereEdit

Filtered air includes at least trace amounts of ten (or more) of the chemical elements. Substantial amounts of argon, nitrogen, and oxygen are present as elementary gases, as well as hydrogen (and additional oxygen) in water vapor (H2O). Much smaller or trace amounts of elementary helium, hydrogen, iodine, krypton, neon, and xenon are also present, as well as carbon in carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO). Many additional elements from natural sources may be present in tiny amounts in an unfiltered air sample, including contributions from dust, pollen and spores, sea spray, vulcanism, and meteoroids. Various industrial pollutants are also now present in the air, such as chlorine (elementary or in compounds), fluorine (in compounds), elementary mercury, and sulfur (in compounds such as sulfur dioxide [SO2]).

Atmosphere gas proportions

Composition of Earth's atmosphere as of Dec. 1987. The lower pie represents the least common gases that compose 0.038% of the atmosphere. Values normalized for illustration.

Atmospheric Water Vapor Mean.2005.030

Mean atmospheric water vapor

Composition of
dry atmosphere, by volume
[1]
ppmv: parts per million by volume
Gas Volume
Nitrogen (N2) 780,840 ppmv (78.084%)
Oxygen (O2) 209,460 ppmv (20.946%)
Argon (Ar) 9,340 ppmv (0.9340%)
Carbon dioxide (CO2) 383 ppmv (0.0383%)
Neon (Ne) 18.18 ppmv (0.001818%)
Helium (He) 5.24 ppmv (0.000524%)
Methane (CH4) 1.745 ppmv (0.0001745%)
Krypton (Kr) 1.14 ppmv (0.000114%)
Hydrogen (H2) 0.55 ppmv (0.000055%)
Not included in above dry atmosphere:
Water vapor (H2O) ~0.40% over full atmosphere, typically 1% to 4% near surface
Minor components of air not listed above include
See also: Error: Template must be given at least one article name
Gas Volume
nitrous oxide 0.3 ppmv (0.00003%)
xenon 0.09 ppmv (9x10-6%)
ozone 0.0 to 0.07 ppmv (0%-7x10-6%)
nitrogen dioxide 0.02 ppmv (2x10-6%)
iodine 0.01 ppmv (1x10-6%)
carbon monoxide trace
ammonia trace

ppmv

The composition figures above are by volume-fraction (V%), which for ideal gases is equal to mole-fraction (that is, the fraction of total molecules). Although the atmosphere is not an ideal gas, nonetheless the atmosphere behaves enough like an ideal gas that the volume-fraction is the same as the mole-fraction for the precision given.

By contrast, mass-fraction abundances of gases will differ from the volume values. The mean molar mass of air is 28.97 g/mol, while the molar mass of helium is 4.00, and krypton is 83.80. Thus helium is 5.2 ppm by volume-fraction, but 0.72 ppm by mass-fraction ([4/29] × 5.2 = 0.72), and krypton is 1.1 ppm by volume-fraction, but 3.2 ppm by mass-fraction ([84/29] × 1.1 = 3.2).

HeterosphereEdit

Below the turbopause at an altitude of about 100 km (not far from the mesopause), the Earth's atmosphere has a more-or-less uniform composition (apart from water vapor) as described above; this constitutes the homosphere.[2] However, above about 100 km, the Earth's atmosphere begins to have a composition which varies with altitude. This is essentially because, in the absence of mixing, the density of a gas falls off exponentially with increasing altitude but at a rate which depends on the molar mass. Thus higher mass constituents, such as oxygen and nitrogen, fall off more quickly than lighter constituents such as helium, molecular hydrogen, and atomic hydrogen. Thus there is a layer, called the heterosphere, in which the earth's atmosphere has varying composition. As the altitude increases, the atmosphere is dominated successively by helium, molecular hydrogen, and atomic hydrogen. The precise altitude of the heterosphere and the layers it contains varies significantly with temperature.

In pre-history, the Sun's radiation caused a loss of the hydrogen, helium and other hydrogen-containing gases from early Earth, and Earth was devoid of an atmosphere. The first atmosphere was formed by outgassing of gases trapped in the interior of the early Earth, which still goes on today in volcanoes.[3]

Composition of Atmosphere of MarsEdit

Mars atmosphere

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

Carbon dioxideEdit

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.

ArgonEdit

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.[4]

WaterEdit

Main article: Water on Mars

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 Template:Convert/km/h 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.[5] 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.

MethaneEdit

Martian Methane Map

Distribution of the methane in the atmosphere of Mars in the summer of its Northern Hemisphere

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.[6][7] In March 2004 the Mars Express Orbiter [8] and ground based observations from Canada-France-Hawaii Telescope[9] also suggested the presence of methane in the atmosphere with a concentration of about 10 ppb by volume.[10] 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.[11]

It had been proposed that the methane were 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.[12]

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.[13] [14] 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.[13] It is estimated that Mars must produce 270 ton/year of methane.[15][16][13]

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.[13][17] 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.[13] 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.[13][17] This unexplained faster destruction rate also suggests a very active replenishing source.[18]

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.[19] The required conditions for this reaction (i.e. high temperature and pressure) do not exist on the surface, but may exist within the crust.[20] 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.[21]

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 gasses 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,[7] 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.[22][23][24]

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.[25] 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 <10-3 for the former, while other sources produce nearly equivalent amounts of methane and ethane.[26]

In 2012, the Mars Science Laboratory rover will measure such isotopes.[27] 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.[28] NASA has revealed the goal of launching the Mars Trace Gas Mission orbiter on 2016 to further study the methane,[29][30] as well as its decomposition products such as formaldehyde and methanol.

Composition of Atmosphere of the MoonEdit

The elements sodium (Na) and potassium (K) have been detected using Earth-based spectroscopic methods, whereas the isotopes radon-222 and polonium-210 have been inferred from data obtained by the Lunar Prospector alpha particle spectrometer.[31] Argon-40, helium-4, oxygen and/or methane (CH4), nitrogen gas (N2) and/or carbon monoxide (CO), and carbon dioxide (CO2) were detected by in-suit detectors placed by the Apollo astronauts.[32]

The average daytime abundances of the elements known to be present in the lunar atmosphere, in atoms per cubic centimeter, are as follows:

  • Argon: 40,000
  • Helium: 2,000-40,000
  • Sodium: 70
  • Potassium: 17
  • Hydrogen: less than 17

This yields approximately 80,000 total atoms per cubic centimeter, marginally higher than the quantity posited to exist in the atmosphere of Mercury.[33] While this greatly exceeds the density of the solar wind, which is usually on the order of just a few protons per cubic centimeter, it is virtually a vacuum in comparison with the atmosphere of the Earth.

In fact, the Moon is often considered to not have an atmosphere, as it cannot absorb measurable quantities of radiation, does not appear layered or self-circulating, and requires constant replenishment given the high rate at which the atmosphere is lost to space (solar wind and outgasing are not primary components of the Earth's, or any stable atmosphere yet known).

The Moon may also have a tenuous "atmosphere" of electrostatically-levitated dust. See moon dust for more details.

Composition of Atmosphere of VenusEdit

The atmosphere of Venus is composed mainly of carbon dioxide, along with a small amount of nitrogen and other trace elements. The amount of nitrogen in the atmosphere is relatively small compared to the amount of carbon dioxide, but as the atmosphere is so much thicker than that on Earth, its total nitrogen content is roughly four times higher than Earth's, even though on Earth nitrogen makes up about 78% of the atmosphere.[34][35]

The atmosphere contains a range of interesting compounds in small quantities, including some based on hydrogen, such as hydrogen chloride (HCl) and hydrogen fluoride (HF). There are carbon monoxide, water vapor and molecular oxygen as well.[36][37] A large amount of the planet's hydrogen is theorised to have been lost to space,[38] with the remainder being mostly bound up in sulfuric acid (H2SO4) and hydrogen sulfide. Subsequently, hydrogen is in relatively short supply in the Venusian atmosphere. The loss of significant amounts of hydrogen is proved by a very high D/H ratio measured in the Venusian atmosphere.[37] The ratio is about 0.025, which is much higher than the terrestrial value of 1.6×10−4.[36] In addition, in the upper atmosphere of Venus D/H ratio is 1.5 higher than in the bulk atmosphere.[36]

Chemical composition of Atmosphere of JupiterEdit

Elemental abundances relative to hydrogen
in Jupiter and Sun[39]
ElementSunJupiter/Sun
He/H 0.0975 0.807 ± 0.02
Ne/H 1.23 × 10−4 0.10 ± 0.01
Ar/H 3.62 × 10−6 2.5 ± 0.5
Kr/H 1.61 × 10−9 2.7 ± 0.5
Xe/H 1.68 × 10−10 2.6 ± 0.5
C/H 3.62 × 10−4 2.9 ± 0.5
Template:Nitrogen/H 1.12 × 10−4 3.6 ± 0.5 (8 bar)

3.2 ± 1.4 (9–12 bar)

Template:Oxygen/H 8.51 × 10−4 0.033 ± 0.015 (12 bar)

0.19–0.58 (19 bar)

Template:Phosphorus/H 3.73 × 10−7 0.82
Template:Sulfur/H 1.62 × 10−45 2.5 ± 0.15
Isotopic ratios in Jupiter and Sun[39]
Ratio Sun Jupiter
13C/12C 0.011 0.0108 ± 0.0005
15N/14N <2.8 × 10−3 2.3 ± 0.3 × 10−3

(0.08–2.8 bar)

36Ar/38Ar 5.77 ± 0.08 5.6 ± 0.25
20Ne/22Ne 13.81 ± 0.08 13 ± 2
3He/4He 1.5 ± 0.3 × 10−4 1.66 ± 0.05 × 10−4
D/H 3.0 ± 0.17 × 10−5 2.25 ± 0.35 × 10−5

The composition of Jupiter's atmosphere is similar to that of the planet as a whole.[39] Jupiter's atmosphere is the most comprehensively understood of those of all the gas giants because it was observed directly by the Galileo atmospheric probe when it entered the Jovian atmosphere on December 7, 1995.[40] Other sources of information about Jupiter's atmospheric composition include the Infrared Space Observatory (ISO),[41] the Galileo and Cassini orbiters,[42] and Earth-based observations.[39]

The two main constituents of the Jovian atmosphere are molecular hydrogen (H2) and helium.[39] The helium abundance is 0.157 ± 0.0036 relative to molecular hydrogen by number of molecules, and its mass fraction is 0.234 ± 0.005, which is slightly lower than the solar system's primordial value.[39] The reason for this low abundance is not entirely understood, but, being denser than hydrogen, some of the helium may have condensed into the core of Jupiter.[43] The atmosphere contains various simple compounds such as water, methane (CH4), hydrogen sulfide (H2S), ammonia (NH3) and phosphine (PH3).[39] Their abundances in the deep (below 10 bar) troposphere imply that the atmosphere of Jupiter is enriched in the elements carbon, nitrogen, sulfur and possibly oxygen[b] by factor of 2–4 relative to the Sun.[c][39] The noble gases argon, krypton and xenon appear to be enriched relative to solar abundances as well (see table), while neon is scarcer.[39] Other chemical compounds such as arsine (AsH3) and germane (GeH4) are present only in trace amounts.[39] The upper atmosphere of Jupiter contains small amounts of simple hydrocarbons such as ethane, acetylene, and diacetylene, which form from methane under the influence of the solar ultraviolet radiation and charged particles coming from Jupiter's magnetosphere.[39] The carbon dioxide, carbon monoxide and water present in the upper atmosphere are thought to originate from impacting comets, such as Shoemaker-Levy 9. The water cannot come from the troposphere because the cold tropopause acts like a cold trap, effectively preventing water from rising to the stratosphere (see Vertical structure above).[39]

Earth- and spacecraft-based measurements have led to improved knowledge of the isotopic ratios in Jupiter's atmosphere. As of July 2003, the accepted value for the deuterium abundance is 2.25 ± 0.35 × 10−5,[39] which probably represents the primordial value in the protosolar nebula that gave birth to the Solar System.[41] The ratio of nitrogen isotopes in the Jovian atmosphere, 15N to 14N, is 2.3 × 10−3, a third lower than that in the Earth's atmosphere (3.5 × 10−3).[39] The latter discovery is especially significant since the previous theories of Solar System formation considered the terrestrial value for the ratio of nitrogen isotopes to be primordial.[41]

See alsoEdit

ReferencesEdit

  1. Source for figures: Carbon dioxide, NASA Earth Fact Sheet, (updated 2007.01). Methane, IPCC TAR table 6.1, (updated to 1998). The NASA total was 17 ppmv over 100%, and CO2 was increased here by 15 ppmv. To normalize, N2 should be reduced by about 25 ppmv and O2 by about 7 ppmv.
  2. homosphere—AMS Glossary
  3. Vercheval, J. "The thermosphere: a part of the heterosphere". (offline, see Internet Archive copy)
  4. Francois Forgot. "Alien Weather at the Poles of Mars". Science. Retrieved on 2007-02-25.
  5. Clouds - Dec. 13, 2004 NASA Press release. URL accessed March 17, 2006.
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  7. 7.0 7.1 Michael J. Mumma. "Mars Methane Boosts Chances for Life". Skytonight.com. Retrieved on 2007-02-23.
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  9. 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. doi:10.1016/j.icarus.2004.07.004. 
  10. ESA Press release. "Mars Express confirms methane in the Martian atmosphere". ESA. Retrieved on 2006-03-17.
  11. Mars Trace Gas Mission (10 September 2009)
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  13. 13.0 13.1 13.2 13.3 13.4 13.5 Mumma, Michael J.; et al. (20 February 2009). "Strong Release of Methane on Mars in Northern Summer 2003". Science 323 (5917): pp. 1041–1045. doi:10.1126/science.1165243, http://images.spaceref.com/news/2009/Mumma_et_al_Methane_Mars_wSOM_accepted2.pdf. 
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  16. Planetary Fourier Spectrometer website (ESA, Mars Express)
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  19. Oze, C., M. Sharma (2005). "Have olivine, will gas: Serpentinization and the abiogenic production of methane on Mars". Geophys. Res. Lett. 32: L10203. doi:10.1029/2005GL022691, http://www.agu.org/journals/gl/gl0510/2005GL022691/. 
  20. Rincon, Paul (26 March 2009). "Mars domes may be 'mud volcanoes'", BBC News. Retrieved on 2 April 2009. 
  21. Thomas, Caroline; et al. (January 2009). "Variability of the methane trapping in martian subsurface clathrate hydrates". Planetary and Space Science 57 (1): 42–47. 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 on 2 August 2009. 
  22. 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 
  23. Rincon, Paul (17 March 2007). "'Cave entrances' spotted on Mars", BBC News. Retrieved on 15 September 2009. 
  24. Than, Ker (2 April 2007). "Possible New Mars Caves Targets in Search for Life", Space,com. Retrieved on 15 September 2009. 
  25. Remote Sensing Tutorial, Section 19-13a - Missions to Mars during the Third Millennium, Nicholas M. Short, Sr., et al., NASA
  26. "Report from the 2013 Mars Science Orbiter (MSO) - Second Science Analysis Group", Mars Exploration Program Analysis Group (MEPAG MSO-SAG-2), 2007, pp. 16 
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  28. Steigerwald, Bill (January 15, 2009). "Martian Methane Reveals the Red Planet is not a Dead Planet", NASA's Goddard Space Flight Center, NASA. Retrieved on 24 January 2009. 
  29. Rincon, Paul (July 9, 2009). "Agencies outline Mars initiative", BBC News. Retrieved on 26 July 2009. 
  30. "NASA orbiter to hunt for source of Martian methane in 2016", Thaindian News (March 6, 2009). Retrieved on 26 July 2009. 
  31. S. Lawson, W. Feldman, D. Lawrence, K. Moore, R. Elphic, and R. Belian (2005). "Recent outgassing from the lunar surface: the Lunar Prospector alpha particle spectrometer". J. Geophys. Res. 110 (E9): E9009. doi:10.1029/2005JE002433, http://www.agu.org/pubs/crossref/2005/2005JE002433.shtml. 
  32. S. Alan Stern (1999). "The Lunar atmosphere: History, status, current problems, and context". Rev. Geophys. 37 (4): 453–491. doi:10.1029/1999RG900005, http://www.agu.org/pubs/crossref/1999/1999RG900005.shtml. 
  33. Adapted from Stern, S.A. (1999) Rev. Geophys. 37, 453
  34. Cite error: Invalid <ref> tag; no text was provided for refs named Basilevsky2003
  35. "Clouds and atmosphere of Venus". Institut de mécanique céleste et de calcul des éphémérides. Retrieved on 2008-01-22.
  36. Cite error: Invalid <ref> tag; no text was provided for refs named Bertaux2007
  37. Cite error: Invalid <ref> tag; no text was provided for refs named Svedhem2007
  38. Lovelock, James (1979). Gaia: A New Look at Life on Earth, Oxford University Press. ISBN 0-19-286218-9. 
  39. 39.00 39.01 39.02 39.03 39.04 39.05 39.06 39.07 39.08 39.09 39.10 39.11 39.12 39.13 Atreya et al. (2003)
  40. McDowell, Jonathan (1995-12-08). "Jonathan's Space Report, No. 267". Harvard-Smithsonian Center for Astrophysics. Retrieved on 2007-05-06.
  41. 41.0 41.1 41.2 Encrenaz (2003)
  42. Kunde et al. (2004)
  43. Atreya et al. (1999)

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