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Nirgal Vallis is a long river channel bordering the Coprates quadrangle and Margaritifer Sinus quadrangle of Mars at 28.4° south latitude and 42° west longitude. It is 496 km long and is named after the word for ''"Mars"'' the god of war in Babylonian.〔http://planetarynames.wr.usgs.gov/Feature/4307〕 Nirgal Vallis had a discharge of 4800 cubic meters/second.〔Irwin, J., R. Craddock, R. Howard. 2005. Interior channels in Martian valley networks: Discharge and runoff production. Geology: 33,489-492.〕 The western half of Nirgal Valles is a branched system, but the eastern half is a tightly sinuous, deeply entrenched valley. Nirgal Valles ends at Uzboi Vallis. Tributaries are very short and end in steep-walled valley heads, often called "amphitheater-headed valleys." The shape of these valley heads is like cirques on the Earth.〔Baker, V. 1982. The Channels of Mars. University of Texas Press. Austin〕 Water from Nirgal Vallis contributed to a great flood that went though the rim of Holden Crater and helped form a lake in the crater. It's estimated that Nirgal Vallis had a discharge of 4800 cubic meters/second.〔Irwin, J., R. Craddock, R. Howard. 2005. Interior channels in Martian valley networks: Discharge and runoff production. Geology: 33,489-492.〕 Water from Nirgal Vallis was inbounded in Uzboi Vallis because the rim of Holden Crater blocked the flow. At a certain point the stored water broke through the rim of Holden and created a lake 200–250 m deep.〔Grant, J., R. Irwin, S. Wilson. 2010. Aqueous depositional settings in Holden crater, Mars In Cabrol, N. and E. Grin (eds.). 2010. Lakes on Mars. Elsevier. NY.〕 Water with a depth of at least 50 m entered Holden at a rate that 5-10 times the discharge of the Mississippi River.〔Grant, J., T. Parker. 2002. Drainage evolution of the Margaritifer Sinus region, Mars. J. Geophysic. Res. 107, doi:10.1029/2001JE001678.〕〔Komar, P. 1979. Comparisons of the hydraulics of water flows in Martian outflow channels with flows of similar scale on Earth. Icarus: 37, 156-181.〕〔Grant, J. et al. 2008. HiRISE imaging of impact megabreccia and sub-meter aqueous strata in Holden Crater, Mars. Geology: 36, 195-198.〕〔Irwin, et al. 2005. An intense terminal epoch of widespread fluvial activity on early Mars: 2. Increased runoff and paleolake development. J. Geophysical. Res. 110, E12S14, doi: 10.1029/2005JE002460.〕 Terraces and the presence of large rocks (tens of meters across) support these high discharge rates.〔〔〔Boothroyd, J. 1983. Fluvial drainage systems in the Ladon Basin area: Margaritifer Sinus area, Mars. Geol. Soc. Am. Abstr. Programs 15, 530〕〔Grant, J. 1987. The geomorphic evolution of Eastern Margaritifer Sinus, Mars. Adv. Planet. Geol. NASA Tech memo. 89871, 1-268.〕〔Parker, T. 1985. Geomorphology and geology of the southwestern Margaritifer Sinus-northern Argyre region of Mars, California State University, M. S. Thesis, Los Angeles, California〕 == Nirgal Vallis and sapping== Nirgal Valles is one of the longest valley networks on Mars. It is so large that it is found on two quadrangles. Scientists are not sure about how all the ancient river valleys were formed. There is evidence that instead of rain or snow, the water that formed the valleys originated under ground. One mechanism that has been advanced is sapping.〔(【引用サイトリンク】url=http://themis.asu.edu/zoom-20030916a )〕 In this case, the ground just gives away as water comes out. Sapping is common in some desert areas in America's Southwest. It forms alcoves and stubby tributaries; these features are visible in the pictures below of Nirgal Vallis taken with Mars Odyssey's THEMIS. Image:Nirgal Vallis.jpg|Nirgal Vallis, as seen by THEMIS. Image:Nirgal Vallis Close-up.JPG|Nirgal Vallis Close-up, as seen by THEMIS. 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「Nirgal Vallis」の詳細全文を読む スポンサード リンク
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