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The most conspicuous feature of Mars is a sharp contrast, known as the Martian dichotomy, between the Southern hemisphere and the Northern. The two hemispheres' geography differ in elevation by 1 to 3 km. The average thickness of the Martian crust is 45 km, with 32 km in the northern lowlands region, and 58 km in the southern highlands. The boundary between the two regions is quite complex in places. One distinctive type of topography is called fretted terrain.〔Greeley, R. and J. Guest. 1987. Geological map of the eastern equatorial region of Mars, scale 1:15,000,000. U. S. Geol. Ser. Misc. Invest. Map I-802-B, Reston, Virginia〕〔Sharp, R. 1973. Mars Fretted and chaotic terrains. J. Geophys. Res: 78. 4073-4083〕〔ISBN 0-8165-1247-7〕 It contains mesas, knobs, and flat-floored valleys having walls about a mile high. Around many of the mesas and knobs are lobate debris aprons that have been shown to be rock-covered glaciers.〔Plaut, J. et al. 2008. Radar Evidence for Ice in Lobate Debris Aprons in the Mid-Northern Latitudes of Mars. Lunar and Planetary Science XXXIX. 2290.pdf〕〔Carr, M. 2006. The Surface of Mars. Cambridge University Press. ISBN 978-0-521-87201-0〕〔Squyres, S. 1978. Martian fretted terrain: Flow of erosional debrid. Icarus: 34. 600-613.〕〔ISBN 0-8165-1257-4〕 Many large river valleys cut through the dichotomy.〔Watters, T. et al. 2007. Hemispheres Apart: The Crustal Dichotomy on Mars. Annual Review Earth Planet Science: 35. 621-652〕〔Irwin III, R. et al. 2004. Sedimentary resurfacing and fretted terrain development along the crustal dichotomy boundary, Aeolis Mensae, Mars.: 109. E09011〕〔Tanaka, K. et al. 2003. Resurfacing history of the northern plains of Mars based on geologic mapping of Mars Global surveyor data. Journal of Geophical Research: 108. 8043〕 The Martian dichotomy boundary includes the regions called Deuteronilus Mensae, Protonilus Mensae, and Nilosyrtis Mensae. All three regions have been studied extensively because they contain landforms believed to have been produced by the movement of ice.〔Baker, D. et al. 2010. Flow patterns of lobate debris aprons and lineated valley fill north of Ismeniae Fossae, Mars: Evidence for extensive mid-latitude glaciation in the Late Amazonian. Icarus: 207. 186-209.〕〔http://www.uahirise.org/ESP_018857_2225〕 The northern lowlands comprise about one-third of the surface of Mars and are relatively flat, with occasional impact craters. The other two-thirds of the Martian surface are the highlands of the southern hemisphere. The difference in elevation between the hemispheres is dramatic. Three major hypotheses have been proposed for the origin of the crustal dichotomy: endogenic (by mantle processes), single impact, or multiple impact. Both impact-related hypotheses involve processes that could have occurred before the end of the primordial bombardment, implying that the crustal dichotomy has its origins early in the history of Mars. ==Geography== 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「Martian dichotomy」の詳細全文を読む スポンサード リンク
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