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Arandas Crater is a crater in the Mare Acidalium quadrangle of Mars, located 42.77° North and 15.17° West. It is 24.76 km in diameter and is named after the town of Arandas in Mexico. Gullies are visible in the pictures below. On the basis of their form, aspects, positions, and location amongst and apparent interaction with features thought to be rich in water ice, many researchers believed that the processes carving the gullies involve liquid water. However, this remains a topic of active research. As soon as gullies were discovered,〔Malin, M., Edgett, K. 2000. Evidence for recent groundwater seepage and surface runoff on Mars. Science 288, 2330–2335.〕 researchers began to image many gullies over and over, looking for possible changes. By 2006, some changes were found.〔Malin, M., K. Edgett, L. Posiolova, S. McColley, E. Dobrea. 2006. Present-day impact cratering rate and contemporary gully activity on Mars. Science 314, 1573_1577.〕 Later, with further analysis it was determined that the changes could have occurred by dry granular flows rather than being driven by flowing water.〔Kolb, et al. 2010. Investigating gully flow emplacement mechanisms using apex slopes. Icarus 2008, 132-142.〕〔McEwen, A. et al. 2007. A closer look at water-related geological activity on Mars. Science 317, 1706-1708.〕〔Pelletier, J., et al. 2008. Recent bright gully deposits on Mars wet or dry flow? Geology 36, 211-214.〕 With continued observations many more changes were found in Gasa Crater and others.〔NASA/Jet Propulsion Laboratory. "NASA orbiter finds new gully channel on Mars." ScienceDaily. ScienceDaily, 22 March 2014. www.sciencedaily.com/releases/2014/03/140322094409.htm〕 With more repeated observations, more and more changes have been found; since the changes occur in the winter and spring, experts are tending to believe that gullies were formed from dry ice. Before-and-after images demonstrated the timing of this activity coincided with seasonal carbon-dioxide frost and temperatures that would not have allowed for liquid water. When dry ice frost changes to a gas, it may lubricate dry material to flow especially on steep slopes.〔http://www.jpl.nasa.gov/news/news.php?release=2014-226〕〔http://hirise.lpl.arizona.edu/ESP_032078_1420〕〔http://www.space.com/26534-mars-gullies-dry-ice.html?cmpid=557882〕 In some years frost, perhaps as thick as 1 meter, Wikisklodowskamola.jpg|MOLA map showing Arandas Crater and other nearby craters. Color indicates elevation. Wikiarandas.jpg|Arandas Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Wikiarandasgullies.jpg|Gullies in Arandas Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of the previous photo. WikiarandasESP 034907 2230.jpg|Gullies in Arandas Crater, as seen by HiRISE under HiWish program. 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. 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. ==Scientific value of craters== 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. 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「Arandas (crater)」の詳細全文を読む スポンサード リンク
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