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Asimov Crater is an impact crater in the Noachis quadrangle of Mars, located at 47.0° S and 355.05° W. It is 84.0 km in diameter and was named after Isaac Asimov (1920–1992), an American biochemist and writer.〔http://planetarynames.wr.usgs.gov/Feature/14567〕 Wikiasimov.jpg|Western side of Asimov crater, as seen by CTX camera on Mars Reconnaissance Orbiter. Wikiasimovlayers.jpg|Layers and gullies in Asimov crater, as seen by CTX camera on Mars Reconnaissance Orbiter. Note: this is an enlargement of the previous image. Image:Close-up of Asimov Crater.JPG|Gullies on mound in Asimov Crater, as seen by HiRISE. Image:Asimov Crater Layers.jpg|Layers in west slope of Asimov Crater, as seen by HiRISE. Image:Asimov Layers Close-up.JPG|Close-up of layers in west slope of Asimov Crater. Shadows show the overhang. Some of the layers are much more resistant to erosion, so they stick out. Image from HiRISE. Image:Asimov Crater Central Pit.jpg|East Slope of Central Pit in Asimov Crater, as seen by HiRISE. Click on image to see more details of the many gullies. ==Gullies in Asimov== Asimov contains many gullies. 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〕 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「Asimov (crater)」の詳細全文を読む スポンサード リンク
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