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The Noachian is a geologic system and early time period on the planet Mars characterized by high rates of meteorite and asteroid impacts and the possible presence of abundant surface water. The absolute age of the Noachian period is uncertain but probably corresponds to the lunar Pre-Nectarian to Early Imbrian periods of 4100 to 3700 million years ago, during the interval known as the Late Heavy Bombardment.〔 Many of the large impact basins on the Moon and Mars formed at this time. The Noachian Period is roughly equivalent to the Earth’s Hadean and early Archean eons when the first life forms likely arose. Noachian-aged terrains on Mars are prime spacecraft landing sites to search for fossil evidence of life.〔Grant, J.A. et al. (2010). The Science Process for Selecting the Landing Site for the 2011 Mars Science Laboratory. ''Planet. Space Sci.,'' (press ), . http://www.geo.utep.edu/pub/hurtado/planetary/lectures/week8/papers2/grant_etal_2010_pss_mslsiteselection.pdf.〕 During the Noachian, the atmosphere of Mars was denser than it is today, and the climate possibly warm enough to allow rainfall. Large lakes and rivers were present in the southern hemisphere,〔Irwin, R.P. ''et al.'' (2002). A Large Paleolake Basin at the Head of Ma’adim Vallis, Mars. ''Science,'' 296 2209; .〕 and an ocean may have covered the low-lying northern plains.〔Di Achille, G.; Hynek, B.M. (2010). Ancient Ocean on Mars Supported by Global Distribution of Deltas and Valleys. ''Nature Geoscience,'' 1–5, .〕 Extensive volcanism occurred in the Tharsis region, building up enormous masses of volcanic material (the Tharsis bulge) and releasing large quantities of gases into the atmosphere. Weathering of surface rocks produced a diversity of clay minerals (phyllosilicates) that formed under chemical conditions conducive to microbial life. ==Description and name origin== The ''Noachian'' System and Period is named after Noachis Terra (lit. "Land of Noah"), a heavily cratered highland region west of the Hellas basin. The type area of the Noachian System is in the Noachis quadrangle (MC-27) around . At a large scale (>100 m), Noachian surfaces are very hilly and rugged, superficially resembling the lunar highlands. Noachian terrains consist of overlapping and interbedded ejecta blankets of many old craters. Mountainous rim materials and uplifted basement rock from large impact basins are also common.〔 (See Anseris Mons, for example.) The number-density of large impact craters is very high, with about 400 craters greater than 8 km in diameter per million km2.〔Strom, R.G.; Croft, S.K.; Barlow, N.G. (1992) The Martian Impact Cratering Record in ''Mars,'' H.H. Kieffer ''et al.,'' Eds.; University of Arizona Press: Tucson, AZ, pp. 383–423.〕 Noachian-aged units cover 45% of the Martian surface;〔Tanaka, K.L. et al. (2014). Geologic Map of Mars. U.S. Geological Survey Scientific Investigations Map 3292, pamphlet〕 they occur mainly in the southern highlands of the planet, but are also present over large areas in the north, such as in Tempe and Xanthe Terrae, Acheron Fossae, and around the Isidis basin (Libya Montes).〔Scott, D.H.; Tanaka, K.L. (1986). Geologic Map of the Western Equatorial Region of Mars. U.S. Geological Survey Miscellaneous Investigations Series Map I–1802–A.〕〔Greeley, R.; Guest, J.E. (1987). Geologic Map of the Eastern Equatorial Region of Mars. U.S. Geological Survey Miscellaneous Investigations Series Map I–1802–B.〕 ImageSize = width:800 height:50 PlotArea = left:15 right:15 bottom:20 top:5 AlignBars = early Period = from:-4500 till:0 TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:500 start:-4500 ScaleMinor = unit:year increment:100 start:-4500 Colors= id:prenoachicol value:rgb(0.7,0.4,1) id:noachicol value:rgb(0.5,0.5,0.8) id:hespericol value:rgb(1,0.2,0.2) id:amazonicol value:rgb(1,0.5,0.2) PlotData= align:center textcolor:black fontsize:8 mark:(line,black) width:25 shift:(0,-5) text:Amazonian from:-3000 till:0 color:amazonicol text:Hesperian from:-3700 till:-3000 color:hespericol text:Noachian from:-4100 till:-3700 color:noachicol text:Pre-Noachian from:start till:-4100 color:prenoachicol 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「Noachian」の詳細全文を読む スポンサード リンク
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