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A concentric crater fill is a landform where the floor of a crater is mostly covered with a large number of parallel ridges.〔http://hiroc.lpl.arizona.edu/images/PSP/diafotizo.php?ID=PSP_111926_2185〕 It is common in the mid-latitudes of Mars,〔Dickson, J. et al. 2009. Kilometer-thick ice accumulation and glaciation in the northern mid-latitudes of Mars: Evidence for crater-filling events in the Late Amazonian at the Phlegra Montes. Earth and Planetary Science Letters.〕〔http://hirise.lpl.arizona.edu/PSP_001926_2185〕 and is widely believed to be caused by glacial movement.〔Head, J. et al. 2006. Extensive valley glacier deposits in the northern mid-latitudes of Mars: Evidence for late Amazonian obliquity-driven climate change. Earth Planet. Sci Lett: 241. 663-671.〕〔Levy, J. et al. 2007. Lineated valley fill and lobate debris apron stratigraphy in Nilosyrtis Mensae, Mars: Evidence for phases of glacial modification of the dichotomy boundary. J. Geophys. Res: 112.〕 Areas on Mars called Deuteronilus Mensae and Protonilus Mensae contain many examples of concentric crater fill. ==Description== Concentric crater fill, like lobate debris aprons and lineated valley fill, is believed to be ice-rich.〔Levy, J. et al. 2009. Concentric crater fill in Utopia Planitia: History and interaction between glacial "brain terrain" and periglacial processes. Icarus: 202. 462-476.〕 Based on accurate topography measures of height at different points in these craters and calculations of how deep the craters should be based on their diameters, it is thought that the craters are 80% filled with mostly ice.〔Levy, J., J. Head, D. Marchant. 2010. Concentric Crater fill in the northern mid-latitudes of Mars: Formation process and relationships to similar landforms of glacial origin. Icarus 2009, 390-404.〕〔Levy, J., J. Head, J. Dickson, C. Fassett, G. Morgan, S. Schon. 2010. Identification of gully debris flow deposits in Protonilus Mensae, Mars: Characterization of a water-bearing, energetic gully-forming process. Earth Planet. Sci. Lett. 294, 368–377.〕〔http://hirise.lpl.arizona.edu/ESP_032569_2225〕〔Garvin, J., S. Sakimoto, J. Frawley. 2003. Craters on Mars: Geometric properties from gridded MOLA topography. In: Sixth International Conference on Mars. July 20–25 2003, Pasadena, California. Abstract 3277.〕 That is, they hold hundreds of meters of material that probably consists of ice with a few tens of meters of surface debris.〔Garvin, J. et al. 2002. Global geometric properties of martian impact craters. Lunar Planet. Sci: 33. Abstract # 1255.〕〔http://photojournal.jpl.nasa.gov/catalog/PIA09662〕 The ice accumulated in the crater from snowfall in previous climates.〔Kreslavsky, M. and J. Head. 2006. Modification of impact craters in the northern planes of Mars: Implications for the Amazonian climate history. Meteorit. Planet. Sci.: 41. 1633-1646〕〔Madeleine, J. et al. 2007. Exploring the northern mid-latitude glaciation with a general circulation model. In: Seventh International Conference on Mars. Abstract 3096.〕〔http://hirise.lpl.arizona.edu/PSP_002917_2175〕 Recent modeling suggests that concentric crater fill develops over many cycles in which snow is deposited, then moves into the crater. Once inside the crater shade and dust preserve the snow. The snow changes to ice. The many concentric lines are created by the many cycles of snow accumulation. Generally snow accumulates whenever the axial tilt reaches 35 degrees.〔Fastook, J., J.Head. 2014. Concentric crater fill: Rates of glacial accumulation, infilling and deglaciation in the Amazonian and Noachian of Mars. 45th Lunar and Planetary Science Conference (2014) 1227.pdf〕 Sometimes boulders are found on concentric crater fill; it is believed they fell off the crater wall, then were transported away from the wall with the movement of the glacier.〔Marchant, D. et al. 2002. Formation of patterned ground and sublimation till over Miocene glacier ice in Beacon valley, southern Victorialand, Antarctica. Geol. Soc. Am. Bull:114. 718-730.〕〔Head, J. and D. Marchant. 2006. Modification of the walls of a Noachian crater in northern Arabia Terra (24E, 39N) during mid-latitude Amazonian glacial epochs on Mars: Nature and evolution of lobate debris aprons and their relationships to lineated valley fill and glacial systems. Lunar Planet. Sci: 37. Abstract # 1126.〕 Erratics on Earth were carried by similar means. High resolution pictures taken with HiRISE reveal that some of the surfaces of concentric crater fill are covered with strange patterns called closed-cell and open-cell brain terrain. The terrain resembles a human brain. It is believed to be caused by cracks in the surface accumulating dust and other debris, together with ice sublimating from some of the surfaces. The cracks are the result stress from gravity and seasonal heating and cooling.〔Mellon, M. 1997. Small-scale polygonal features on Mars: Seasonal thermal contraction cracks in permafrost. J. Geophysical Res: 102. 25,617-625,628.〕〔Ley, J. et al. 2009. Concentric crater fill in Utopia Planitia: History and interaction between glacial "brain terrain" and periglacial processes. Icarus: 202. 462-476.〕 File:25246brainseroding.jpg|Closed-cell brain terrain, as seen by HiRISE under the HiWish program. This type of surface is common on lobate debris aprons, concentric crater fill, and lineated valley fill. File:Htalk23815 2215lvfclose.jpg|Open and closed-cell brain terrain, as seen by HiRISE, under HiWish program. Wikiconcentric.jpg|Crater showing concentric crater fill, as seen by CTX (on Mars Reconnaissance Orbiter). Location is Phaethontis quadrangle. Wikiconcentricclose22451.jpg|Close-up view of concentric crater fill, as seen by HiRISE under HiWish program Note: this is an enlargement of previous image of a concentric crater. Location is Phaethontis quadrangle. Image:Concentric Crater Fill Wide-view.jpg|Wide-view of concentric crater fill, as seen by HiRISE. Location is the Casius quadrangle. Image:Concentric Crater Fill Close-up.JPG|Concentric Crater Fill Close-up of near the top of previous image. The surface debris covers water ice. Wikiconcentrib22.jpg|Crater with concentric crater fill, as seen by CTX (on Mars Reconnaissance Orbiter). Location is Casius quadrangle. Image:Hollows as seen by hirise under hiwish program.jpg|Well-developed hollows, as seen by HiRISE under the HiWish program. Location is the Casius quadrangle. Note: this is an enlargement of the previous image that was taken by CTX. Image:crackswithpitsconcentric.jpg|Close-up showing cracks containing pits on the floor of a crater containing concentric crater fill, as seen by HiRISE under HiWish program. Location is the Casius quadrangle. Image:pitscrater23035.jpg|Close-up showing cracks containing pits on the floor of a crater, as seen by HiRISE under HiWish program. Cracks may start as a line of pits that enlarge, then join. Location is the Casius quadrangle. File:Ccffigurecaptioned.jpg|This series of drawings illustrates why researchers believe many craters are full of ice-rich material. The depth of craters can be predicted based upon the observed diameter. Many craters are almost full, instead of having bowl shape; hence it is believed that they have gained much material since they were formed by impact. Much of the extra material is believed to be ice that fell from the sky as snow or ice-coated dust. 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「concentric crater fill」の詳細全文を読む スポンサード リンク
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