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Terby (crater)
Terby is a crater on the northern edge of Hellas Planitia, Mars. It is in the Iapygia quadrangle.The 174 km diameter crater is centered at 28°S, 73°E with an elevation of −5 km. It is named after François J. Terby. It is the site of an ancient lakebed and has clay deposits.〔Murchie, S. et al. 2009. A synthesis of Martian aqueous mineralogy after 1 Mars year of observations from the Mars Reconnaissance Orbiter. Journal of Geophysical Research: 114.〕 Using data from Mars Global Surveyor, Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter missions researchers believe Terby's layers were formed from sediments settling under water. Crater counts show this happened during the Noachian period. It used to be thought that Terby Crater contained a large delta.〔Ansan, V. et al. 2005. Analysis of layered deposits in Terby crater Hellas region, Mars) using multiple datasets MOC, THEMIS, and OMEGA/MEX date. Lunar Planet. Sci., XXXVI (CD-ROM). Abstract 1324.〕 However, newer observations have led researchers to think of the layered sequence as part of a group of layers that may have extended all the across Hellas. There is no valley large enough at the northern rim of Terby to have carried the large amount of sediments necessary to produce the layers. Other details in the layers argue against Terby containing a delta.〔Cabrol, N. and E. Grin (eds.). 2010. Lakes on Mars. Elsevier. NY. 〕 Fan deposits are some of the thickest on Mars. Hydrated minerals, including Fe/Mg phyllosilicates, have been detected in several layers.〔Ansan, V., D. Loizeau, M. Mangold, S., Le Mouélic, J. Carter, F. Poulet, G. Dromart, A. Lucas, J.-P. Bibring, A. Gendrin, B. Gondet, Y. Langevin, Ph. Massond, S. Murchie, J. Mustard, G. Neukum. 2011. Stratigraphy, mineralogy, and origin of layered deposits inside Terby crater, Mars. Icarus: 211, 273–304.〕
==Impact craters in general==
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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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