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Pettit Crater is a crater in the Amazonis quadrangle of Mars, located at 12.39° north latitude and 173.87° west longitude. It is 92.49 km in diameter and was named after Edison Pettit, an American astronomer (1890–1962). Image:Pettit Crater Rim.JPG|Pettit Crater Rim, as seen by HiRISE. Image:Map of Amazonis.JPG|Map of Amazonis. Nicholson crater sits right on equator. Wikipettit.jpg|CTX camera (on Mars Reconnaissance Orbiter) image of central peak of Pettit Crater. Some dark slope streaks are visible. Impact craters generally have a rim with ejecta around them, in contrast volcanic craters usually do not have a rim or ejecta deposits. As craters get larger (greater than 10 km in diameter) they usually have a central peak.〔http://www.lpi.usra.edu/publications/slidesets/stones/〕 The peak is caused by a rebound of the crater floor following the impact. If one measures the diameter of a crater, the original depth can be estimated with various ratios. Because of this relationship, researchers have found that many Martian craters contain a great deal of material; much of it is believed to be ice deposited when the climate was different.〔Garvin, J., et al. 2002. Global geometric properities of martian impact craters. Lunar Planet Sci. 33. Abstract @1255.〕 Sometimes craters expose layers that were buried. Rocks from deep underground are tossed onto the surface. Hence, craters can show us what lies deep under the surface. A pedestal crater is a crater with its ejecta sitting above the surrounding terrain and thereby forming a raised platform. This type of crater forms when an impact crater ejects material which forms an erosion resistant layer, thus protecting the immediate area from erosion. As a result of this hard covering, the crater and its ejecta become elevated when erosion removes the softer material that lies beyond the ejecta. Some pedestals have been accurately measured to be hundreds of meters above the surrounding area. This means that hundreds of meters of material were eroded away. Pedestal craters were first observed during the Mariner missions.〔http://hirise.lpl.eduPSP_008508_1870〕〔Bleacher, J. and S. Sakimoto. ''Pedestal Craters, A Tool For Interpreting Geological Histories and Estimating Erosion Rates''. LPSC〕〔http://themis.asu.edu/feature/8〕 ==Why are Craters important?== The density of impact craters is used to determine the surface ages of Mars and other solar system bodies.〔 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.〕 Studies on the earth have documented that cracks are produced and that secondary minerals veins are deposited in the cracks.〔Osinski, G, J. Spray, and P. Lee. 2001. Impact-induced hydrothermal activity within the Haughton impact structure, arctic Canada: Generation of a transient, warm, wet oasis. Meteoritics & Planetary Science: 36. 731-745〕〔http://www.ingentaconnect.com/content/arizona/maps/2005/00000040/00000012/art00007〕〔Pirajno, F. 2000. Ore Deposits and Mantle Plumes. Kluwer Academic Publishers. Dordrecht, The Netherlands〕 Images from satellites orbiting Mars have detected cracks near impact craters.〔Head, J. and J. Mustard. 2006. Breccia Dikes and Crater-Related Faults in Impact Craters on Mars: Erosion and Exposure on the Floor of a 75-km Diameter Crater at the Dichotomy Boundary. Special Issue on Role of Volatiles and Atmospheres on Martian Impact Craters Meteoritics & Planetary Science〕 Great amounts of heat are produced during impacts. The area around a large impact may take hundreds of thousands of years to cool.〔name="news.discovery.com"〕〔Segura, T, O. Toon, A. Colaprete, K. Zahnle. 2001. Effects of Large Impacts on Mars: Implications for River Formation. American Astronomical Society, DPS meeting#33, #19.08〕〔Segura, T, O. Toon, A. Colaprete, K. Zahnle. 2002. Environmental Effects of Large Impacts on Mars. Science: 298, 1977-1980.〕 Many craters once contained lakes.〔Cabrol, N. and E. Grin. 2001. The Evolution of Lacustrine Environments on Mars: Is Mars Only Hydrologically Dormant? Icarus: 149, 291-328.〕〔Fassett, C. and J. Head. 2008. Open-basin lakes on Mars: Distribution and implications for Noachian surface and subsurface hydrology. Icarus: 198, 37-56.〕〔Fassett, C. and J. Head. 2008. Open-basin lakes on Mars: Implications of valley network lakes for the nature of Noachian hydrology.〕 Because some crater floors show deltas, we know that water had to be present for some time. Dozens of deltas have been spotted on Mars.〔Wilson, J. A. Grant and A. Howard. 2013. INVENTORY OF EQUATORIAL ALLUVIAL FANS AND DELTAS ON MARS. 44th Lunar and Planetary Science Conference.〕 Deltas form when sediment is washed in from a stream entering a quiet body of water. It takes a bit of time to form a delta, so the presence of a delta is exciting; it means water was there for a time, maybe for many years. Primitive organisms may have developed in such lakes; hence, some craters may be prime targets for the search for evidence of life on the Red Planet.〔Newsom H. , Hagerty J., Thorsos I. 2001. Location and sampling of aqueous and hydrothermal deposits in martian impact craters. Astrobiology: 1, 71-88.〕 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「Pettit (Martian crater)」の詳細全文を読む スポンサード リンク
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