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Ritchey Crater is a crater in the Coprates quadrangle on Mars, located at 28.8° South and 51° West. It is 79 km in diameter and was named after George W. Ritchey, an American astronomer (1864-1945).〔http://planetarynames.wr.usgs.gov/〕 There is strong evidence that it was once a lake.〔Sun, V., R. Milliken. 2014. The geology and mineralogy of Ritchey crater, Mars: Evidence for post-Noachian clay formation. Journal of Geophysical Research:119(4), 810–836〕 〔http://marsoweb.nas.nasa.gov/landingsites/msl/workshops/2nd_workshop/talks/Milliken_Ritchey.pdf〕 It formed later than the Noachian period. Pictures form HiRISE show that the central peak has massive bedrock and megabreccia with large clasts.〔Ding, N., V. Brayb, A. McEwen, S. Mattson, C. Okubo, M. Chojnacki, L. Tornabene, 2015. The central uplift of Ritchey crater, Mars. Icarus: 252, 255-270.〕 Wikiritchey.jpg|Western side of Ritchey Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Wikiritchyfan.jpg|Fan along western wall of Ritchey Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of previous image. Ritchey Crater is interesting to scientists because it displays several different layers. A dark layer at the top forms a cap rock that protects the underlying layers from erosion. Under this hard, dark layer is a softer, light-toned rock, that breaks into small boulders. The layers might be formed of volcanic ash, lake or stream deposits, or sand dunes.〔http://hirise.lpl.arizona.edu/PSP_003249_1510〕 Fluvial channels and fan deposit are common on and along the walls.〔Sun, V., and R. Milliken. 2014. The geology and mineralogy of Ritchey crater, Mars: Evidence for post-Noachian clay formation, J. Geophys. Res:119, 810-836, doi: 10.1002/2013JE004602.〕 Clay minerals have been found in Ritchy.〔Milliken, R., et al. 2010. The case for mixed-layered clays on Mars, Lunar Planet. Sci. XLI, Abstract 2030〕 These minerals indicate that water was present for a time. Evidence of smectite clay was found around the central uplift of the crater in a geologic unit that was probably formed as a consequence of the impact. The heat of the impact allowed liquid water to be present long enough to turn minerals into clay. Because Ritchey is Hesperian or younger in age this means that liquid water could have existed at various times in Martian history. Clay minerals were also found in the crater wall, crater floor, and fan deposits but it is not clear when they might have been produced. These minerals could have formed in place, transported from another location, or have been come from erosion of preexisting clays. However, there is spectral evidence that at least some of the clays in the crater floor and fan deposits came from the crater wall.〔Sun, V., and R. Milliken. 2014. The geology and mineralogy of Ritchey crater, Mars: Evidence for post-Noachian clay formation, J. Geophys. Res:119, 810-836, doi: 10.1002/2013JE004602.〕 Hydrated silica, maybe in the form of hydrated opaline silica has been detected at the central uplift.〔Milliken, R., et al. 2008. Opaline silica in young deposits on Mars, Geology, 36(11), 847–850, doi:10.1130/G24967A.1.〕 Also, Olivine, Low calcium pyroxene and plagioclase have recently been detected there, as well. 〔Ding, N., V. Brayb, A. McEwen, S. Mattson, C. Okubo, M. Chojnacki, L. Tornabene, 2015. The central uplift of Ritchey crater, Mars. Icarus: 252, 255-270.〕 Ritchey Crater has been suggested as a landing site for a Mars Rover.〔http://marsoweb.nas.nasa.gov/landingsites/msl/workshops/2nd_workshop/talks/Milliken_Ritchey.pdf〕 It may have once held a large lake. A thick sequence of sedimentary deposits that include clay is found in the crater.〔Sun, V., R. Milliken. 2014. The geology and mineralogy of Ritchey crater, Mars: Evidence for post-Noachian clay formation. Journal of Geophysical Research:119(4), 810–836〕 The presence of fluvial features along crater wall and rim, as well as alluvial/fluvial deposits are strong evidence of lots of water being present at some time in the past. ==See also== * Climate of Mars * Geology of Mars * Impact crater * List of craters on Mars * List of quadrangles on Mars * Hesperian * Geological history of Mars * Water on Mars 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「Ritchey (Martian crater)」の詳細全文を読む スポンサード リンク
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