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A new class of Martian impact craters have been discovered by Northern Arizona University scientist Prof Nadine Barlow and Dr Joseph Boyce from the University of Hawaii in Oct 2013. They have termed it as ‘low-aspect-ratio layered ejecta (LARLE) craters’. Prof Nadine Barlow, a scientist Northern Arizona University described this class of craters with “thin-layered outer deposit” surpassing “the typical range of ejecta”. “The combination helps vaporize the materials and create a base flow surge. The low aspect ratio refers to how thin the deposits are relative to the area they cover,” Prof Barlow said. The scientists used data from continuing reconnaissance of Mars using the old Mars Odyssey Orbiter and the Mars Reconnaissance Orbiter, discovered 139 LARLE craters ranging in diameter from 1.0 to 12.2 km, with 97 per cent of the LARLE craters are found poleward of 35N and 40S, while remaining mainly traced in the equatorial Medusae Fossae Formation. LARLE craters are characterized by a crater and normal layered ejecta pattern surrounded by an extensive but thin outer deposit which ends in a flame-like shape.〔Barlowa, N., J. Boyce, C. Cornwalc. Martian Low-Aspect-Ratio Layered Ejecta (LARLE) craters: Distribution, characteristics, and relationship to pedestal craters. Icarus:239, 186-200.〕 The ejecta layers of LARLE craters have higher aspect ratios compared with base surge deposits from explosion craters. This difference is probably caused by large amounts of small particles of dust and ice in the areas where LARLE craters form. This ice and dust came from mantles of snow and dust that were deposited during the many climate changes in Martian history. After the impact, deposits are quickly stabilized (order of a few days to a few years) from eolian erosion by formation of a crust formed from diffusion of water vapor out of the deposits.〔Boycea, J., L. Wilsona, N. Barlow. Origin of the outer layer of martian low-aspect ratio layered ejecta craters. Icarus:245, 263-272. 〕 LARLE craters may be useful as a marker of ice under the surface. == Background == An impact crater is an approximately circular depression in the surface of a planet, moon or other solid body in the Solar System, formed by the hypervelocity impact of a smaller body with the surface. In contrast to volcanic craters, which result from explosion or internal collapse,〔Basaltic Volcanism Study Project. (1981). Basaltic Volcanism on the Terrestrial Planets; Pergamon Press, Inc: New York, p. 746. http://articles.adsabs.harvard.edu//full/book/bvtp./1981//0000746.000.html.〕 impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain.〔Consolmagno, G.J.; Schaefer, M.W. (1994). ''Worlds Apart: A Textbook in Planetary Sciences;'' Prentice Hall: Englewood Cliffs, NJ, p.56.〕 Impact craters range from small, simple, bowl-shaped depressions to large, complex, multi-ringed impact basins. Meteor Crater is perhaps the best-known example of a small impact crater on the Earth. Impact craters are not to be confused with landforms that in some cases appear similar, including calderas and ring dikes. Impact craters are the dominant geographic features on many solid Solar System objects including the Moon, Mercury, Callisto, Ganymede and most small moons and asteroids. On other planets and moons that experience more active surface geological processes, such as Earth, Venus, Mars, Europa, Io and Titan, visible impact craters are less common because they become eroded, buried or transformed by tectonics over time.〔French, B.M. (1998). ''Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures;'' Simthsonian Institution: Washington DC, p. 97. http://www.lpi.usra.edu/publications/books/CB-954/CB-954.intro.html.〕 The cratering records of very old surfaces, such as Mercury, the Moon, and the southern highlands of Mars, record a period of intense early bombardment in the inner Solar System around 3.9 billion years ago.〔Carr, M.H. (2006) ''The surface of Mars;'' Cambridge University Press: Cambridge, UK, p. 23.〕 The cratering rate in the inner solar system fluctuates as a consequence of collisions in the asteroid belt that create a family of fragments that are often sent cascading into the inner solar system. 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「LARLE crater」の詳細全文を読む スポンサード リンク
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