|
Trouvelot Crater is a crater on Mars, located in the Oxia Palus quadrangle at 16.2° north latitude and 13.1° west longitude. It 154.7 km in diameter and was named after Étienne Léopold Trouvelot, a French astronomer (1827–1895). 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. There is a part of Trouvelot that displays many thin, light-toned layers; these may be evidence that a lake was present in the past. 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.〕 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. Wikitrouvelot.jpg|Trouvelot Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Major features are labeled. Wikitrouvelotlayers.jpg|Layers in mound in Trouvelot Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of the previous image. Image:Trouvelot crater floor.JPG|Trouvelot Crater floor, as seen by HiRISE. ==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.〕 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「Trouvelot (Martian crater)」の詳細全文を読む スポンサード リンク
|