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The atmosphere of Mars is the layer of gases surrounding Mars composed mostly of carbon dioxide. The atmospheric pressure on the Martian surface averages , about 0.6% of Earth's mean sea level pressure of . It ranges from a low of on Olympus Mons's peak to over 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 versus Earth's . 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,〔 for a mean molar mass of 43.34 g/mol. There has been renewed interest in its composition since the detection of traces of methane in 2003〔(Interplanetary Whodunit – Methane on Mars ), David Tenenbaum, ''Astrobiology Magazine'', NASA, July 20, 2005. (Note: part one of a four-part series.)〕〔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.〕 that may indicate life but may also be produced by a geochemical process, volcanic or hydrothermal activity.〔(Making Sense of Mars Methane (June 2008) )〕 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 that suspended dust particles of roughly 1.5 micrometres diameter.〔(Lemmon et al., "Atmospheric Imaging Results from the Mars Exploration Rovers: Spirit and Opportunity" )〕 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. 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. 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. 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.〔〔 == Structure == Mars's atmosphere is composed of the following layers: * Exosphere: Typically stated to start at 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,〔(ESA – New views of the Martian ionosphere )〕 and a seasonal ozone layer over the south pole.〔(ESA – A seasonal ozone layer over the Martian south pole )〕 The ''MAVEN'' spacecraft determined in 2015 that there is a substantial layered structure present in both neutral gases and ion densities. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Atmosphere of Mars」の詳細全文を読む スポンサード リンク
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