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Atmosphere of Mars
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Further information: Climate of Mars
Atmosphere of Mars
Image of Mars with sandstorm visible, taken by the Hubble Space Telescope on 28 October 2005
General information[1]
Chemical species
Mole fraction
Composition[1]
Carbon dioxide
95.97%
Argon
1.93%
Nitrogen
1.89%
Oxygen
0.146%
Carbon monoxide
0.0557%
The atmosphere of Mars is the layer of gases surrounding Mars. It is composed mostly of carbon dioxide. The atmospheric pressure on the Martian surface averages 600 pascals (0.087 psi; 6.0 mbar), about 0.6% of Earth's mean sea level pressure of 101.3 kilopascals (14.69 psi; 1.013 bar). It ranges from a low of 30 pascals (0.0044 psi; 0.30 mbar) on Olympus Mons's peak to over 1,155 pascals (0.1675 psi; 11.55 mbar) in the depths of Hellas Planitia. This pressure is well below the Armstrong limit for the unprotected human body. Mars's atmospheric mass of 25 teratonnes compares to Earth's 5148 teratonnes with a scale height of about 11 kilometres (6.8 mi) versus Earth's 7 kilometres (4.3 mi).
The Martian atmosphere consists of approximately 96% carbon dioxide, 1.9% argon, 1.9% nitrogen, and traces of free oxygen, carbon monoxide, water and methane, among other gases,[1] for a mean molar mass of 43.34 g/mol.[2][3] There has been renewed interest in its composition since the detection of traces of methane in 2003[4][5] that may indicate life but may also be produced by a geochemical process, volcanic or hydrothermal activity.[6]
The atmosphere is quite dusty, giving the Martian sky a light brown or orange-red color when seen from the surface; data from the Mars Exploration Rovers indicate suspended particles of roughly 1.5 micrometres in diameter.[7]
On 16 December 2014, NASA reported detecting an unusual increase, then decrease, in the amounts of methane in the atmosphere of the planet Mars. Organic chemicals have been detected in powder drilled from a rock by the Curiosity rover. Based on deuterium to hydrogen ratio studies, much of the water at Gale Crater on Mars was found to have been lost during ancient times, before the lakebed in the crater was formed; afterwards, large amounts of water continued to be lost.[8][9][10]
On 18 March 2015, NASA reported the detection of an aurora that is not fully understood and an unexplained dust cloud in the atmosphere of Mars.[11]
On 4 April 2015, NASA reported studies, based on measurements by the Sample Analysis at Mars (SAM) instrument on the Curiosity rover, of the Martian atmosphere using xenon and argon isotopes. Results provided support for a "vigorous" loss of atmosphere early in the history of Mars and were consistent with an atmospheric signature found in bits of atmosphere captured in some Martian meteorites found on Earth.[12] This was further supported by results from the MAVEN orbiter circling Mars, that the solar wind is responsible for stripping away the atmosphere of Mars over the years.[13][14][15]
In September 2017, NASA reported radiation levels on the surface of the planet Mars were temporarily doubled, and were associated with an aurora 25-times brighter than any observed earlier, due to a massive, and unexpected, solar storm in the middle of the month.[16]
Contents
[hide]
1 Structure
2 Observations and measurement from Earth
3 Composition
3.1 Carbon dioxide
3.2 Argon
3.3 Water
3.4 Methane
3.5 Sulfur dioxide
3.6 Ozone
3.7 Oxygen
4 Potential for use by humans
5 History
6 Images
7 Interactive Mars map
8 See also
9 References
10 Further reading
11 External links
Structure[edit]
Pressure comparison
Where
Pressure
Olympus Mons summit
0.03 kilopascals (0.0044 psi)
Mars average
0.6 kilopascals (0.087 psi)
Hellas Planitia bottom
1.16 kilopascals (0.168 psi)
Armstrong limit
6.25 kilopascals (0.906 psi)
Mount Everest summit[17]
33.7 kilopascals (4.89 psi)
Earth sea level
101.3 kilopascals (14.69 psi)
Play media
The solar wind accelerates ions from the Mars upper atmosphere into space
(video (01:13); 5 November 2015)
Mars's atmosphere is composed of the following layers:
Exosphere: Typically stated to start at 200 km (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.
Upper atmosphere, or thermosphere: A region with 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.
Middle atmosphere: The region in which Mars's jetstream flows.
Lower atmosphere: A relatively warm region affected by heat from airborne dust and from the ground.
There is also a complicated ionosphere,[18] and a seasonal ozone layer over the south pole.[19] The MAVEN spacecraft determined in 2015 that there is a substantial layered structure present in both neutral gases and ion densities.[20]
Initial analyses by the MAVEN orbiter[21] and the ExoMars Trace Gas Orbiter, have shown high thermal and density variability in the atmosphere with a slightly lower average density than predicted by existing models.[22]
Observations and measurement from Earth[edit]
Comparison of the atmospheric compositions of Venus, Mars, and Earth.
In 1864, William Rutter Dawes observed "that the ruddy tint of the planet does not arise from any peculiarity of its atmosphere seems to be fully proved by the fact that the redness is always deepest near the centre, where the atmosphere is thinnest."[23] Spectroscopic observations in the 1860s and 1870s[24][25] led many to think the atmosphere of Mars is similar to Earth's. In 1894, though, spectral analysis and other qualitative observations by William Wallace Campbell suggested Mars resembles the Moon, which has no appreciable atmosphere, in many respects.[24]
In 1926, photographic observations by William Hammond Wright at the Lick Observatory allowed Donald Howard Menzel to discover quantitative evidence of Mars's atmosphere.[26][27]
Composition[edit]
Most abundant gases on Mars.
Carbon dioxide[edit]
The main component of the atmosphere of Mars is carbon dioxide (CO2) at 95.9%. Each pole is in continual darkness during its hemisphere's winter, 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 pole is again exposed to sunlight during 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.
It has been suggested that Mars had a much thicker, warmer, and wetter atmosphere early in its history.[28] Much of this early atmosphere would have consisted of carbon dioxide. Such an atmosphere would have raised the temperature, at least in some places, to above the freezing point of water.[29] With the higher temperature, running water could have carved out the many channels and outflow valleys that are common on the planet. It also might have gathered to form lakes and maybe an ocean.[30] Some researchers have suggested that the atmosphere of Mars may have been many times as thick as the present one of Earth; however, research published in fall 2015 advanced the idea that perhaps the early Martian atmosphere was not as thick as previously thought.[31] Currently, the atmosphere is very thin. For many years, it was assumed that as with Earth, most of the early carbon dioxide was locked up in minerals, called carbonates. However, despite the use of many orbiting instruments that looked for carbonates, very few carbonate deposits have been found.[31][32] Today, it is thought that much of the carbon dioxide in the Martian air was removed by the solar wind. Researchers have discovered a two-step process that sends the gas into space.[33] Ultraviolet light from the sun could strike a carbon dioxide molecule, breaking it into carbon monoxide and oxygen. A second photon of ultraviolet light could subsequently break the carbon monoxide into oxygen and carbon which would receive enough energy to escape the planet. In this process the light isotope of carbon (12C) is most likely to leave the atmosphere. Hence, the carbon dioxide left in the atmosphere would be enriched with the heavy isotope (13C).[34] This higher level of the heavy isotope is what was recently found by the Curiosity Rover that sits on the surface of Mars.[35][36]
Argon[edit]
Argon isotope ratios are a signature of atmospheric loss on Mars.[37][38]
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.[39]
Water[edit]
Main article: Water on Mars
Some aspects of the Martian atmosphere vary significantly. As carbon dioxide sublimes back into the atmosphere during the Martian summer, it leaves traces of water. Seasonal winds 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.[40] NASA scientists working on the Phoenix Mars mission confirmed on July 31, 2008 that they had indeed found subsurface water ice at Mars's northern polar region.
Methane[edit]
See also: Climate of Mars § Methane presence, and Timeline of Mars Science Laboratory § Detection of organics
Volatile gases on Mars.
Trace amounts of methane (CH4), 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.[5][41] In March 2004, the Mars Express Orbiter and ground-based observations by three groups also suggested the presence of methane in the atmosphere at a concentration of about 10 ppb (parts per billion).[42][43][44] Large differences in the abundances were measured between observations taken in 2003 and 2006, which suggested that the methane was locally concentrated and probably seasonal.
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 implies the existence of a source to continually replenish the gas. Current photochemical models alone can not explain the rapid variability of the methane levels.[45][46] It had been proposed that the methane might be replenished by meteorites entering the atmosphere of Mars,[47] 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.[48]
Research suggests that the implied methane destruction lifetime is as long as ~4 Earth years and as short as ~0.6 Earth years.[49][50] 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.[49] The rapid destruction (or "sink") of methane suggests that another process must dominate removal of atmospheric methane on Mars, and it must be more efficient than destruction by light by a factor of 100 to 600.[50][49] This unexplained fast destruction rate also suggests a very active replenishing source.[51] In 2014 it was concluded that presence of strong methane sinks are not subject to atmospheric oxidation.[52] A possibility is that the methane is not consumed at all, but rather condenses and evaporates seasonally from clathrates.[53] Another possibility is that methane reacts with tumbling surface sand quartz (SiO
2) and olivine to form covalent Si–CH
3 bonds.[54]
Possible methane sources and sinks on Mars.
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 other hydrocarbons via Fischer–Tropsch synthesis with CO and CO2.[55] It has also been 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.[56] The required conditions for this reaction (i.e. high temperature and pressure) do not exist on the surface, but may exist within the crust.[57][58] A detection of the mineral by-product serpentinite would suggest that this process is occurring. An analog on Earth suggests that low-temperature production and exhalation of methane from serpentinized rocks may be possible on Mars.[59] Another possible geophysical source could be clathrate hydrates.[60] Under the assumption of a cold early Mars environment, a cryosphere could trap such methane as clathrates in stable form at depth, that might exhibit sporadic release.[61]
A group of Mexican scientists performed plasma experiments in a synthetic Mars atmosphere and found that bursts of methane can be produced when a discharge interacts with water ice. A potential source of the discharges can be the electrification of dust particles from sand storms and dust devils. The ice can be found in trenches or in the permafrost. The electrical discharge ionizes gaseous CO2 and water molecules and their byproducts recombine to produce methane. The results obtained show that pulsed electrical discharges over ice samples in a Martian atmosphere produce about 1.41×1016 molecules of methane per joule of applied energy.[62][63]
Living microorganisms, such as methanogens, are another possible source, but no evidence exists for the presence of such organisms anywhere on Mars. In Earth's oceans, biological methane production tends to be accompanied by ethane, whereas volcanic methane is accompanied by sulfur dioxide.[64] Several studies of trace gases in the Martian atmosphere have found no evidence for sulfur dioxide in the Martian atmosphere, which makes volcanism unlikely to be the source of methane.[65][66]
In 2011, NASA scientists reported a comprehensive search using ground-based high-resolution infrared spectroscopy for trace species (including methane) on Mars, deriving sensitive upper limits for methane (<7 ppbv), ethane (<0.2 ppbv), methanol (<19 ppbv) and others (H2CO, C2H2, C2H4, N2O, NH3, HCN, CH3Cl, HCl, HO2 – all limits at ppbv levels).[67] The data were acquired over a period of 6 years and span different seasons and locations on Mars, suggesting that if organics are being released into the atmosphere, these events were extremely rare or currently non-existent, considering the expected long lifetimes for some of these species.[67]
Methane measurements in the atmosphere of Mars by the Curiosity rover.
In August 2012, the Curiosity rover landed on Mars. The rover's instruments are capable of making precise abundance measurements, which can be used to distinguish between different isotopologues of methane.[68][69] The first measurements with Curiosity's Tunable Laser Spectrometer (TLS) in 2012 indicated that there was no methane or less than 5 ppb of methane at the landing site.[70][71][72][73][74] On 2013, NASA scientists again reported no detection of methane beyond a baseline.[75][76][77] But on 2014, NASA reported that the Curiosity rover detected a tenfold increase ('spike') in methane in the atmosphere around it in late 2013 and early 2014. Four measurements taken over two months in this period averaged 7.2 ppb, implying that Mars is episodically producing or releasing methane from an unknown source.[78] Before and after that, readings averaged around one-tenth that level.[8][9][78]
The Indian Mars Orbiter Mission, which entered orbit around Mars on 24 September 2014, is equipped with a Fabry–Pérot interferometer to measure atmospheric methane at a level of several ppb, which is systematically gathering data as of September 2015[update].[79][80] The ExoMars Trace Gas Orbiter, which entered orbit on 19 October 2016, will further study the methane, as well as its decomposition products such as formaldehyde and methanol.[64][81][82][83]
Measuring the ratio of hydrogen and methane levels on Mars may help determine the likelihood of life on Mars.[84][85] According to the scientists, "...low H2/CH4 ratios (less than approximately 40) indicate that life is likely present and active."[84]
Sulfur dioxide[edit]
Sulfur dioxide in the atmosphere is thought to be a tracer of current volcanic activity. It has become especially interesting due to the long-standing controversy of methane on Mars. If methane on Mars were being produced by volcanoes (as it is in part on Earth) we would expect to find sulfur dioxide in large quantities. Several teams have searched for sulfur dioxide on Mars using the NASA Infrared Telescope Facility. No sulfur dioxide was detected in these studies, but they were able to place stringent upper limits on the atmospheric concentration of 0.2 ppb.[65][66] In March 2013, a team led by scientists at NASA Goddard Space Flight Center reported a detection of SO2 in Rocknest soil samples analyzed by the Curiosity rover.[86]
Ozone[edit]
Rotation of Mars near opposition. Ecliptic south is up.
As reported by the European Space Agency (ESA) on September 29, 2013,[citation needed] a new comparison of spacecraft data with computer models explains how global atmospheric circulation creates a layer of ozone (O
3) above Mars's southern pole in winter. Ozone was most likely difficult to detect on Mars because its concentration is typically 300 times lower than on Earth, although it varies greatly with location and time. The SPICAM —an UV/IR spectrometer— on board Mars Express has shown the presence of two distinct ozone layers at low-to-mid latitudes. These comprise a persistent, near-surface layer below an altitude of 30 km, a separate layer that is only present in northern spring and summer with an altitude varying from 30 to 60 km, and another separate layer that exists 40–60 km above the southern pole in winter, with no counterpart above the Mars's north pole. This third ozone layer shows an abrupt decrease in elevation between 75 and 50 degrees south. SPICAM detected a gradual increase in ozone concentration at 50 km until midwinter, after which it slowly decreased to very low concentrations, with no layer detectable above 35 km. The reporting scientists think that the observed polar ozone layers are the result of the same atmospheric circulation pattern that creates a distinct oxygen emission identified in the polar night and also present in Earth's atmosphere. This circulation takes the form of a huge Hadley cell in which warmer air rises and travels toward the south pole before cooling and sinking at higher latitudes. Mars is on a quite elliptical orbit and has a large axial tilt, which causes extreme seasonal variations in temperature amongst the northern and southern hemispheres. Mars's temperature difference greatly influences the amount of water vapor in the atmosphere, because warmer air can contain more moisture. This, in turn, affects the production of ozone-destroying hydrogen radicals.[citation needed]
Oxygen[edit]
In early 2016, Stratospheric Observatory for Infrared Astronomy (SOFIA) detected atomic oxygen in the atmosphere of Mars.[87] This was the first time in forty years it was detected, the last time being the Viking and Mariner missions in the 1970s.[87]
Potential for use by humans[edit]
Main articles: In-situ resource utilization, Terraforming of Mars, and Colonization of Mars
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 the 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).
History[edit]
Mars's atmosphere is thought 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.[88] 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 < 210 K (−63 °C; −82 °F), significantly lower than that needed to sustain liquid water. However, early in its history Mars may have had conditions more conducive to retaining liquid water at the surface. In 2013, scientists published that Mars once had "oxygen-rich" atmosphere billions of years ago.[89][90]
Possible causes for the depletion of a previously thicker Martian atmosphere include:
Gradual erosion of the atmosphere by solar wind. On 5 November 2015, NASA announced that data from MAVEN shows that the erosion of Mars' atmosphere increases significantly during solar storms. This shift took place between about 4.2 to 3.7 billion years ago, as the shielding effect of the global magnetic field was lost when the planet's internal dynamo cooled.[13][14][91][92]
Catastrophic collision by a body large enough to blow away a significant percentage of the atmosphere;[93]
Mars’ low gravity allowing the atmosphere to "blow off" into space by Jeans escape.[94]
Mars's escaping atmosphere—carbon, oxygen, hydrogen—made by MAVEN UV spectrograph).[95]
Images[edit]
Mars's thin atmosphere, visible on the horizon.
Mars Pathfinder – Martian sky with water ice clouds.
A storm front moves in
Martian sunset by Spirit rover at Gusev crater (May, 2005).
Martian sunset by Pathfinder at Ares Vallis (July, 1997).
Interactive Mars map[edit]
Interactive imagemap of the global topography of Mars. Hover your mouse to see the names of over 25 prominent geographic features, and click to link to them. Coloring of the base map indicates relative elevations, based on data from the Mars Orbiter Laser Altimeter on NASA's Mars Global Surveyor. Reds and pinks are higher elevation (+3 km to +8 km); yellow is 0 km; greens and blues are lower elevation (down to −8 km). Whites (>+12 km) and browns (>+8 km) are the highest elevations. Axes are latitude and longitude; Poles are not shown.
(See also: Mars Rovers map) (view • discuss)

See also[edit]
Book: Solar System
Climate of Mars
In situ resource utilization
Life on Mars
Mars MetNet – proposed surface observation network
Mars regional atmospheric modeling system
MAVEN orbiter
Seasonal flows on warm Martian slopes
Terraforming of Mars
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^ Jump up to: a b Franck, Lefèvre; Forget, François (6 August 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.
Jump up ^ Burns, Judith (5 August 2009). "Martian methane mystery deepens". BBC News. Archived from the original on 6 August 2009. Retrieved 7 August 2009.
Jump up ^ Aoki, Shohei; Guiranna, Marco; Kasaba, Yasumasa; Nakagawa, Hiromu; Sindoni, Giuseppe (1 January 2015). "Search for hydrogen peroxide in the Martian atmosphere by the Planetary Fourier Spectrometer onboard Mars Express". Icarus. 245: 177–183. Bibcode:2015Icar..245..177A. doi:10.1016/j.icarus.2014.09.034.
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Jump up ^ Jensen, Svend J. Knak; Skibsted, Jørgen; Jakobsen, Hans J.; Kate, Inge L. ten; Gunnlaugsson, Haraldur P.; Merrison, Jonathan P.; Finster, Kai; Bak, Ebbe; Iversen, Jens J.; Kondrup, Jens C.; Nørnberg, Per (2014). "A sink for methane on Mars? The answer is blowing in the wind". Icarus. 236: 24–27. Bibcode:2014Icar..236...24K. doi:10.1016/j.icarus.2014.03.036.
Jump up ^ Mumma, Michael; et al. (2010). "The Astrobiology of Mars: Methane and Other Candinate Biomarker Gases, and Related Interdisciplinary Studies on Earth and Mars" (PDF). Astrobiology Science Conference 2010. Astrophysics Data System. Greenbelt, MD: Goddard Space Flight Center. Retrieved 24 July 2010.
Jump up ^ Oze, C.; Sharma, M. (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.
Jump up ^ Rincon, Paul (26 March 2009). "Mars domes may be 'mud volcanoes'". BBC News. Archived from the original on 29 March 2009. Retrieved 2 April 2009.
Jump up ^ Team Finds New Hope for Life in Martian Crust. Astrobiology.com. Western University. June 16, 2014.
Jump up ^ Etiope, Giuseppe; Ehlmannc, Bethany L.; Schoell, Martin (2013). "Low temperature production and exhalation of methane from serpentinized rocks on Earth: A potential analog for methane production on Mars". Icarus. 224 (2): 276–285. Bibcode:2013Icar..224..276E. doi:10.1016/j.icarus.2012.05.009. Online 14 May 2012
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Jump up ^ Atkinson, Nancy. "Could Dust Devils Create Methane in Mars' Atmosphere?". Universe Today. Retrieved 2016-11-29.
Jump up ^ Robledo-Martinez, A.; Sobral, H.; Ruiz-Meza, A. (2012). "Electrical discharges as a possible source of methane on Mars: lab simulation". Geophys. Res. Lett. 39 (17): L17202. Bibcode:2012GeoRL..3917202R. doi:10.1029/2012gl053255.
^ Jump up to: a b Docksai, Rick (July 2011). "A Chemical Mission to Mars". CBS. Retrieved 5 August 2011.
^ Jump up to: a b Krasnopolsky, Vladimir A (2012). "Search for methane and upper limits to ethane and SO2 on Mars". Icarus. 217: 144–152. Bibcode:2012Icar..217..144K. doi:10.1016/j.icarus.2011.10.019.
^ Jump up to: a b Encrenaz, T.; Greathouse, T. K.; Richter, M. J.; Lacy, J. H.; Fouchet, T.; Bézard, B.; Lefèvre, F.; Forget, F.; Atreya, S. K. (2011). "A stringent upper limit to SO2 in the Martian atmosphere". Astronomy and Astrophysics. 530: 37. Bibcode:2011A&A...530A..37E. doi:10.1051/0004-6361/201116820.
^ Jump up to: a b Villanueva, G. L.; Mumma, M. J.; Novak, R. E.; Radeva, Y. L.; Käufl, H. U.; Smette, A.; Tokunaga, A.; Khayat, A.; Encrenaz, T.; Hartogh, P. (2013). "A sensitive search for organics (CH4, CH3OH, H2CO, C2H6, C2H2, C2H4), hydroperoxyl (HO2), nitrogen compounds (N2O, NH3, HCN) and chlorine species (HCl, CH3Cl) on Mars using ground-based high-resolution infrared spectroscopy". Icarus. 223 (1): 11–27. Bibcode:2013Icar..223...11V. doi:10.1016/j.icarus.2012.11.013.
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Jump up ^ Calvin, Wendy; et al. (2007). "Report from the 2013 Mars Science Orbiter (MSO) – Second Science Analysis Group" (PDF). Mars Exploration Program Analysis Group (MEPAG): 16. Retrieved 9 November 2009.
Jump up ^ "Mars Curiosity Rover News Telecon -November 2, 2012".
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Jump up ^ Mann, Adam (18 July 2013). "Mars Rover Finds Good News for Past Life, Bad News for Current Life on Mars". Wired. Retrieved 19 July 2013.
Jump up ^ Webster, Christopher R.; Mahaffy, Paul R.; Atreya, Sushil K.; Flesch, Gregory J.; Farley, Kenneth A. (19 September 2013). "Low Upper Limit to Methane Abundance on Mars". Science. 342 (6156): 355–357. Bibcode:2013Sci...342..355W. doi:10.1126/science.1242902. PMID 24051245. Retrieved 19 September 2013.
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^ Jump up to: a b Webster, Christopher R. (23 January 2015). "Mars methane detection and variability at Gale crater". Science. 347 (6220): 415–417. Bibcode:2015Sci...347..415W. doi:10.1126/science.1261713. PMID 25515120. Retrieved 2015-04-15.
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Jump up ^ "NASA orbiter to hunt for source of Martian methane in 2016". Thaindian News. 6 March 2009. Retrieved 26 July 2009.
Jump up ^ Inside ExoMars – Quarterly Newsletter (May 2012)
^ Jump up to: a b Oze, Christopher; Jones, Camille; Goldsmith, Jonas I.; Rosenbauer, Robert J. (7 June 2012). "Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces". PNAS. 109 (25): 9750–9754. Bibcode:2012PNAS..109.9750O. doi:10.1073/pnas.1205223109. PMC 3382529  . PMID 22679287. Retrieved 27 June 2012.
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Jump up ^ McAdam, A. C.; Franz, H.; Archer, P. D.; Freissinet, C.; Sutter, B.; Glavin, D. P.; Eigenbrode, J. L.; Bower, H.; Stern, J.; Mahaffy, P. R.; Morris, R. V.; Ming, D. W.; Rampe, E.; Brunner, A. E.; Steele, A.; Navarro-González, R.; Bish, D. L.; Blake, D.; Wray, J.; Grotzinger, J.; MSL Science Team (2013). "Insights into the Sulfur Mineralogy of Martian Soil at Rocknest, Gale Crater, Enabled by Evolved Gas Analyses". 44th Lunar and Planetary Science Conference, held March 18–22, 2013 in The Woodlands, Texas. LPI Contribution No. 1719, p. 1751
^ Jump up to: a b [1]
Jump up ^ "Gamma-Ray Evidence Suggests Ancient Mars Had Massive Oceans". Science Daily. University of Arizona. Archived from the original on 14 December 2009. Retrieved 31 October 2009.
Jump up ^ "Mars Atmosphere was Oxygen-Rich 4 Billion Years Ago, Astrobio.net, 28 June 2013". Astrobio.net. Retrieved 2016-11-29.
Jump up ^ Tuff, J.; Wade, J.; Wood, B. J. (20 June 2013). "Volcanism on Mars controlled by early oxidation of the upper mantle". Nature – Letters. 498 (7454): 342–345. Bibcode:2013Natur.498..342T. doi:10.1038/nature12225.
Jump up ^ Northon, Karen (5 November 2015). "NASA Mission Reveals Speed of Solar Wind Stripping Martian Atmosphere". NASA. Retrieved 5 November 2015.
Jump up ^ Wall, Mike (5 November 2015). "Mars Lost Atmosphere to Space as Life Took Hold on Earth". Space.com. Retrieved 6 November 2015.
Jump up ^ "Solar Wind Rips Up Martian Atmosphere". NASA Headlines. NASA. Retrieved 31 October 2009.
Jump up ^ Planetary Science. Astronomynotes.com. Retrieved 19 August 2013.
Jump up ^ Jones, Nancy; Steigerwald, Bill; Brown, Dwayne; Webster, Guy (14 October 2014). "NASA Mission Provides Its First Look at Martian Upper Atmosphere". NASA. Retrieved 15 October 2014.
Further reading[edit]
"Mars Clouds Higher Than Any On Earth". Space.com.
Mikulski, Lauren (2000). "Pressure on the Surface of Mars". The Physics Factbook.
"Atmospheric pressure – summary of the Viking lander results".
Khan, Michael (4 December 2009). "The Low Down on Methane on Mars". Archived from the original on 7 December 2009. Retrieved 8 December 2009.
"A seasonal ozone layer over the Martian south pole". ESA. 29 September 2013. Retrieved 17 December 2013.
External links[edit]
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Pure function
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In computer programming, a function may be considered a pure function if both of the following statements about the function hold:
The function always evaluates the same result value given the same argument value(s). The function result value cannot depend on any hidden information or state that may change while program execution proceeds or between different executions of the program, nor can it depend on any external input from I/O devices (usually—see below).
Evaluation of the result does not cause any semantically observable side effect or output, such as mutation of mutable objects or output to I/O devices (usually—see below).
The result value need not depend on all (or any) of the argument values. However, it must depend on nothing other than the argument values. The function may return multiple result values and these conditions must apply to all returned values for the function to be considered pure. If an argument is passed by reference, any parameter mutation will alter the value of the argument outside the function, which will render the function impure.[citation needed]
Contents
[hide]
1 Examples
1.1 Pure functions
1.2 Impure functions
2 Pure expressions
3 I/O in pure functions
4 Impure functions in pure expressions
5 See also
6 References
7 External links
Examples[edit]
Pure functions[edit]
sin(x), returning the sine of a number x
length(s), returning the size of a string s
encrypt(k,d), running a deterministic encryption algorithm on a piece of data d using a key k (Such as AES)
Impure functions[edit]
Any function that uses a non-local variable is potentially impure. I.e., if there are any free variables in the function definition. For example inc(x): x + a returns the value of x incremented by the free variable a, and thus depends on the value of a.
A function that returns the current time or date, is impure because at different times it will yield different results—it refers to some global state.
random() is impure because each call potentially yields a different value. This is because pseudorandom generators use and update a global "seed" state. If we modify it to take the seed as an argument, i.e. random(seed); then random becomes pure, because multiple calls with the same seed value return the same random number.
printf() and similar functions are impure because it causes output to an I/O device as a side effect.
Pure expressions[edit]
Pure functions are required to construct pure expressions. Constant expressions are pure by definition. An expression consisting of a function subexpression applied to one or more argument subexpressions is pure if both these statements about the subexpressions hold:
The function and argument subexpressions are pure expressions.
The function subexpression yields a pure function.
Typically the function subexpression is simply a function identifier. Pure expressions are often referred to as being referentially transparent.
Evaluation of a given pure expression will yield the same result regardless of when or how many times evaluation occurs during program execution. This property is what makes it meaningful to talk about an expression's "value". It also makes it possible to replace an expression with the corresponding value (or it with an equivalent alternative expression) without changing the meaning of a program.
I/O in pure functions[edit]
A function can perform input or output and still be pure if the sequence of operations on the relevant Input/Output devices is modeled explicitly as both an argument and a result, and I/O operations are taken to fail when the input sequence does not describe the operations actually taken since the program began execution.
The second point ensures that the only sequence usable as an argument must change with each I/O action; the first allows different calls to an I/O-performing function to return different results on account of the sequence arguments having changed.[1][2]
The I/O monad is a programming idiom typically used to perform input/output in pure functional languages.
Impure functions in pure expressions[edit]
The definitions above still allow some laxity with regard to purity. It is possible for a pure expression to yield an impure function (or more generally a value which contains one or more impure functions).
It is also possible for an expression to be pure even if one or more of the argument subexpressions yields an impure function (or a value which contains one or more impure functions). In this case the impure function(s) in the argument must not be applied during evaluation (but may be incorporated in the result somehow). However, dealing with programs that allow impure and pure functions to be mixed like this can be quite difficult in practice, thus purely functional programming languages do not allow impure functions to be defined. Effect systems, among other things, allow one to reason precisely and formally about the purity of certain expressions even in the presence of higher-order functions etc.; they even allow to prove that a certain function does not have any side effects although it uses impure operations (for example, uses a mutable array for computation internally, but does not expose it to the outer world or maintain its state between invocations).
See also[edit]
Compile time function execution: the evaluation of pure functions at compile time
Purely functional data structure
Referential transparency (computer science)
Lambda calculus
Side effect (computer science)
Pure procedure
Idempotence
pure keyword in Fortran annotating pure functions
constexpr keyword in C++ annotating pure functions usable at compile-time
References[edit]
Jump up ^ Peyton Jones, Simon L. (2003). Haskell 98 Language and Libraries: The Revised Report (PDF). Cambridge, United Kingdom: Cambridge University Press. p. 95. ISBN 0-521 826144. Retrieved 17 July 2014.
Jump up ^ Hanus, Michael. "Curry: An Integrated Functional Logic Language" (PDF). http://www-ps.informatik.uni-kiel.de/currywiki/. Institut für Informatik, Christian-Albrechts-Universität zu Kiel. p. 33. Retrieved 17 July 2014. External link in |website= (help)
External links[edit]
Pure attribute in Fortran
constexpr attribute in C++
Pure attribute in D language
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