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Water on Mars exists today almost entirely as ice, though it also exists in small quantities as vapour in the atmosphere and occasionally as low-volume liquid brines in shallow Martian soil. The only place where water ice is visible at the surface is at the north polar ice cap. Abundant water ice is also present beneath the permanent carbon dioxide ice cap at the Martian south pole and in the shallow subsurface at more temperate latitudes.〔(【引用サイトリンク】title=Mars Odyssey: Newsroom )〕 More than five million cubic kilometers of ice have been identified at or near the surface of modern Mars, enough to cover the whole planet to a depth of .〔 Even more ice is likely to be locked away in the deep subsurface.〔Carr, 2006, p. 173.〕 Some liquid water may occur transiently on the Martian surface today, but only under certain conditions.〔 No large standing bodies of liquid water exist, because the atmospheric pressure at the surface averages just —about 0.6% of Earth's mean sea level pressure—and because the global average temperature is far too low (), leading to either rapid evaporation (sublimation) or rapid freezing. Before about 3.8 billion years ago, Mars may have had a denser atmosphere and higher surface temperatures, allowing vast amounts of liquid water on the surface,〔(【引用サイトリンク】title=releases/2015/03/150305140447 )〕 possibly including a large ocean〔(【引用サイトリンク】title=PSRD: Ancient Floodwaters and Seas on Mars )〕〔(【引用サイトリンク】title=Gamma-Ray Evidence Suggests Ancient Mars Had Oceans )〕 that may have covered one-third of the planet.〔〔(【引用サイトリンク】title=Ancient ocean may have covered third of Mars )〕 Water has also apparently flowed across the surface for short periods at various intervals more recently in Mars' history.〔Carr, 2006, pp 144–147.〕〔(【引用サイトリンク】title=Flashback: Water on Mars Announced 10 Years Ago )〕 On December 9, 2013, NASA reported that, based on evidence from the ''Curiosity'' rover studying Aeolis Palus, Gale Crater contained an ancient freshwater lake that could have been a hospitable environment for microbial life. Many lines of evidence indicate that water is abundant on Mars and has played a significant role in the planet's geologic history. The present-day inventory of water on Mars can be estimated from spacecraft imagery, remote sensing techniques (spectroscopic measurements, radar, etc.), and surface investigations from landers and rovers. Geologic evidence of past water includes enormous outflow channels carved by floods, ancient river valley networks, deltas, and lakebeds;〔 and the detection of rocks and minerals on the surface that could only have formed in liquid water.〔(【引用サイトリンク】title=New Signs That Ancient Mars Was Wet )〕 Numerous geomorphic features suggest the presence of ground ice (permafrost) and the movement of ice in glaciers, both in the recent past and present. Gullies and slope lineae along cliffs and crater walls suggest that flowing water continues to shape the surface of Mars, although to a far lesser degree than in the ancient past. Although the surface of Mars was periodically wet and could have been hospitable to microbial life billions of years ago, the current environment at the surface is dry and subfreezing, probably presenting an insurmountable obstacle for living organisms. In addition, Mars lacks a thick atmosphere, ozone layer, and magnetic field, allowing solar and cosmic radiation to strike the surface unimpeded. The damaging effects of ionizing radiation on cellular structure is another one of the prime limiting factors on the survival of life on the surface. Therefore, the best potential locations for discovering life on Mars may be in subsurface environments.〔 Understanding water on Mars is vital to assess the planet’s potential for harboring life and for providing usable resources for future human exploration. For this reason, 'Follow the Water' was the science theme of NASA's Mars Exploration Program (MEP) in the first decade of the 21st century. Discoveries by the 2001 Mars Odyssey, Mars Exploration Rovers (MERs), Mars Reconnaissance Orbiter (MRO), and Mars Phoenix Lander have been instrumental in answering key questions about water's abundance and distribution on Mars. The ESA's Mars Express orbiter has also provided essential data in this quest.〔NASA Mars Exploration Program Overview. http://www.nasa.gov/mission_pages/mars/overview/index.html.〕 The Mars Odyssey, Mars Express, MER ''Opportunity'' rover, MRO, and Mars Science Lander ''Curiosity'' rover are still sending back data from Mars, and discoveries continue to be made. == Historical background == (詳細はspace age by hundreds of years. Early telescopic observers correctly assumed that the white polar caps and clouds were indications of water's presence. For many years, the dark regions visible on the surface were interpreted as oceans.〔Hartmann, 2003, p. 11.〕 These observations, coupled with the fact that Mars has a 24-hour day, led astronomer William Herschel to declare in 1784 that Mars probably offered its inhabitants "a situation in many respects similar to ours."〔Sheehan, 1996, p. 35.〕 By the start of the 20th century, most astronomers recognized that Mars was far colder and drier than Earth. The presence of oceans was no longer accepted, so the paradigm changed to an image of Mars as a "dying" planet with only a meager amount of water. The dark areas, which could be seen to change seasonally, were now thought to be tracts of vegetation. The man most responsible for popularizing this view of Mars was Percival Lowell (1855–1916), who imagined a race of Martians constructing a network of canals to bring water from the poles to the inhabitants at the equator. Although generating tremendous public enthusiasm, Lowell's ideas were rejected by most astronomers. The consensus of the scientific establishment at the time is probably best summarized by English astronomer Edward Walter Maunder (1851–1928) who compared the climate of Mars to conditions atop a twenty-thousand-foot peak on an arctic island〔hartmann, 2003, p. 20.〕 where only lichen might be expected to survive. In the meantime, many astronomers were refining the tool of planetary spectroscopy in hope of determining the composition of the Martian atmosphere. Between 1925 and 1943, Walter Adams and Theodore Dunham at the Mount Wilson Observatory tried to identify oxygen and water vapor in the Martian atmosphere, with generally negative results. The only component of the Martian atmosphere known for certain was carbon dioxide (CO2) identified spectroscopically by Gerard Kuiper in 1947.〔Sheehan, 1996, p. 150.〕 Water vapor was not unequivocally detected on Mars until 1963. The composition of the polar caps, assumed to be water ice since the time of Cassini (1666), was questioned by a few scientists in the late 1800s who favored CO2 ice, because of the planet's overall low temperature and apparent lack of appreciable water. This hypothesis was confirmed theoretically by Robert Leighton and Bruce Murray in 1966. Today we know that the winter caps at both poles are primarily composed of CO2 ice, but that a permanent (or perennial) cap of water ice remains during the summer at the northern pole. At the southern pole, a small cap of CO2 ice remains during summer, but this cap too is underlain by water ice. The final piece of the Martian climate puzzle was provided by Mariner 4 in 1965. Grainy television pictures from the spacecraft showed a surface dominated by impact craters, which implied that the surface was very old and had not experienced the level of erosion and tectonic activity seen on Earth. Little erosion meant that liquid water had probably not played a large role in the planet's geomorphology for billions of years. Furthermore, the variations in the radio signal from the spacecraft as it passed behind the planet allowed scientists to calculate the density of the atmosphere. The results showed an atmospheric pressure less than 1% of Earth’s at sea level, effectively precluding the existence of liquid water, which would rapidly boil or freeze at such low pressures. Thus, a vision of Mars was born of a world much like the Moon, but with just a wisp of an atmosphere to blow the dust around. This view of Mars would last nearly another decade until Mariner 9 showed a much more dynamic Mars with hints that the planet’s past environment was more clement than the present one. On January 24, 2014, NASA reported that current studies on Mars by the ''Curiosity'' and ''Opportunity'' rovers will now be searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic and/or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. For many years it was thought that the observed remains of floods were caused by the release of water from a global water table, but research published in 2015 reveals regional deposits of sediment and ice emplaced 450 million years earlier to be the source. "Deposition of sediment from rivers and glacial melt filled giant canyons beneath primordial ocean contained within the planet's northern lowlands. It was the water preserved in these canyon sediments that was later released as great floods, the effects of which can be seen today."〔〔 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「Water on Mars」の詳細全文を読む スポンサード リンク
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