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Porter is a large-scale impact crater in the Thaumasia quadrangle on the planet Mars, situated in Aonia Terra at 50.8° south and 113.9º west. The impact caused a bowl across. The name was chosen in 1973 by the International Astronomical Union in honour of the US astronomer and explorer, Russell W. Porter (1871-1949).〔http://www.flag.wr.usgs.gov〕 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. Wikiporter.jpg|Porter Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Gullies are visible in the upper left. Wikiportergullies.jpg|Gullies in Porter Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of the previous photo. Martian gullies are small, incised networks of narrow channels and their associated downslope sediment deposits, found on the planet of Mars. They are named for their resemblance to terrestrial gullies. First discovered on images from Mars Global Surveyor, they occur on steep slopes, especially on the walls of craters. Usually, each gully has a dendritic ''alcove'' at its head, a fan-shaped ''apron'' at its base, and a single thread of incised ''channel'' linking the two, giving the whole gully an hourglass shape.〔Malin, M., Edgett, K. 2000. Evidence for recent groundwater seepage and surface runoff on Mars. Science 288, 2330–2335.〕 They are believed to be relatively young because they have few, if any craters. A subclass of gullies is also found cut into the faces of sand dunes which themselves considered to be quite young. 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,〔 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. ==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「Porter (Martian crater)」の詳細全文を読む スポンサード リンク
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