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Orson Welles is an impact crater in the Coprates quadrangle of Mars, located at 0.2° S and 45.9° W. It is 124.5 km in diameter and was named after Orson Welles (1915-1985), an American radio and motion picture actor and director. He is famous for, among other things, his radio broadcast of ''The War of the Worlds'' by H. G. Wells in which Martians invade Earth. Wikiorson.jpg|Western side of Orson Welles Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Dark areas on floor are dunes. Wikiorsondunes.jpg|Dunes on floor of Orson Welles Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Small curved channels are also visible. Note: this is an enlargement of previous photo. Many places on Mars show rocks arranged in layers. Rock can form layers in a variety of ways. Volcanoes, wind, or water can produce layers. Sometimes the layers are of different colors. Light-toned rocks on Mars have been associated with hydrated minerals like sulfates. The Mars Rover Opportunity examined such layers close-up with several instruments. Some layers are probably made up of fine particles because they seem to break up into find dust. Other layers break up into large boulders so they are probably much harder. Basalt, a volcanic rock, is thought to in the layers that form boulders. Basalt has been identified on Mars in many places. Instruments on orbiting spacecraft have detected clay (also called phyllosilicate) in some layers. Recent research with an orbiting near-infrared spectrometer, which reveals the types of minerals present based on the wavelengths of light they absorb, found evidence of layers of both clay and sulfates in many places, especially craters.〔Cabrol, N. and E. Grin (eds.). 2010. Lakes on Mars. Elsevier.NY.〕 This is exactly what would appear if a large lake had slowly evaporated.〔Wray, J. et al. 2009. Columbus Crater and other possible plaelakes in Terra Sirenum, Mars. Lunar and Planetary Science Conference. 40: 1896.〕 Moreover, since some layers contain gypsum, a sulfate which forms in relatively fresh water, life could have formed in some craters. Scientists are excited about finding hydrated minerals such as sulfates and clays on Mars because they are usually formed in the presence of water. Places that contain clays and/or other hydrated minerals would be good places to look for evidence of life. ==Why are Craters important?== The density of impact craters is used to determine the surface ages of Mars and other solar system bodies.〔http://www.lpi.usra.edu/publications/slidesets/stones/〕 The older the surface, the more craters present. Crater shapes can reveal the presence of ground ice. The area around craters may be rich in minerals. On Mars, heat from the impact melts ice in the ground. Water from the melting ice dissolves minerals, and then deposits them in cracks or faults that were produced with the impact. This process, called hydrothermal alteration, is a major way in which ore deposits are produced. The area around Martian craters may be rich in useful ores for the future colonization of Mars.〔http://www.indiana.edu/~sierra/papers/2003/Patterson.html.〕 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「Orson Welles (crater)」の詳細全文を読む スポンサード リンク
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