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Terra Cimmeria is a large Martian region, centered at and covering at its broadest extent. It covers latitudes 15 N to 75 S and longitudes 170 to 260 W.〔http://planetarynames.wr.usgs.gov/Features/5930〕 It lies in the Eridania quadrangle. Terra Cimmeria is one part of the heavily cratered, southern highland region of the planet. The Spirit rover landed near the area. Wikiterracimmeriaboundaries.jpg|MOLA map showing boundaries of Terra Cimmeria and other nearby regions Wikimolasouthpole.jpg|MOLA map showing boundaries of Terra Cimmeria near the south pole and other regions A high altitude visual phenomena, probably a condensation cloud,〔(Alan Boyle - Mars' mystery cloud explained (2012) - MSNBC )〕 was seen above this region in late March 2012.〔(Alan Boyle -Mysterious cloud spotted on Mars (2012) - MSNBC )〕 NASA tried to observe it with some of its Mars orbiters, including the THEMIS instrument on the 2001 Mars Odyssey spacecraft and MARCI on the Mars Reconnaissance Orbiter.〔〔 ==Martian Gullies== Terra Cimmeria is the location of gullies that may be due to recent flowing water.〔http://hirise.lpl.arizona.edu/PSP_004071_1425〕〔http://hirise.lpl.arizona.edu/PSP_001948_1425〕 Gullies occur on steep slopes, especially on the walls of craters. Gullies are believed to be relatively young because they have few, if any craters. Moreover, they lie on top of sand dunes which themselves are considered to be quite young. Usually, each gully has an alcove, channel, and apron. Some studies have found that gullies occur on slopes that face all directions,〔Edgett, K. et al. 2003. Polar-and middle-latitude martian gullies: A view from MGS MOC after 2 Mars years in the mapping orbit. Lunar Planet. Sci. 34. Abstract 1038.〕 others have found that the greater number of gullies are found on poleward facing slopes, especially from 30-44 S.〔http://www.planetary.brown.edu/pdfs/3138.pdf〕〔Dickson, J. et al. 2007. Martian gullies in the southern mid-latitudes of Mars Evidence for climate-controlled formation of young fluvial features based upon local and global topography. Icarus: 188. 315-323〕 Although many ideas have been put forward to explain them,〔http://www.psrd.hawaii.edu/Aug03/MartianGullies.html〕 the most popular involve liquid water coming from an aquifer, from melting at the base of old glaciers, or from the melting of ice in the ground when the climate was warmer.〔Heldmann, J. and M. Mellon. Observations of martian gullies and constraints on potential formation mechanisms. 2004. Icarus. 168: 285-304.〕〔Forget, F. et al. 2006. Planet Mars Story of Another World. Praxis Publishing. Chichester, UK.〕 Because of the good possibility that liquid water was involved with their formation and that they could be very young, scientists are excited. Maybe the gullies are where we should go to find life. There is evidence for all three theories. Most of the gully alcove heads occur at the same level, just as one would expect of an aquifer. Various measurements and calculations show that liquid water could exist in aquifers at the usual depths where gullies begin.〔Heldmann, J. and M. Mellon. 2004. Observations of martian gullies and constraints on potential formation mechanisms. Icarus. 168:285-304〕 One variation of this model is that rising hot magma could have melted ice in the ground and caused water to flow in aquifers. Aquifers are layer that allow water to flow. They may consist of porous sandstone. The aquifer layer would be perched on top of another layer that prevents water from going down (in geological terms it would be called impermeable). Because water in an aquifer is prevented from going down, the only direction the trapped water can flow is horizontally. Eventually, water could flow out onto the surface when the aquifer reaches a break—like a crater wall. The resulting flow of water could erode the wall to create gullies.〔http://www.space.com/scienceastronomy/mars_aquifer_041112.html〕 Aquifers are quite common on Earth. A good example is "Weeping Rock" in Zion National Park Utah.〔Harris, A and E. Tuttle. 1990. Geology of National Parks. Kendall/Hunt Publishing Company. Dubuque, Iowa〕 As for the next theory, much of the surface of Mars is covered by a thick smooth mantle that is thought to be a mixture of ice and dust.〔Malin, M. and K. Edgett. 2001. Mars Global Surveyor Mars Orbiter Camera: Interplanetary cruise through primary mission. J. Geophys. Res: 106> 23429-23570〕〔Mustard, J. et al. 2001. Evidence for recent climate change on Mars from the identification of youthful near-surface ground ice. Nature: 412. 411-414.〕〔Carr, M. 2001. Mars Global Surveyor observations of fretted terrain. J. Geophys. Res: 106. 23571-23595.〕 This ice-rich mantle, a few yards thick, smoothes the land, but in places it has a bumpy texture, resembling the surface of a basketball. The mantle may be like a glacier and under certain conditions the ice that is mixed in the mantle could melt and flow down the slopes and make gullies.〔http://www.msnbc.msn.com/id/15702457?〕〔http://www.pnas.org/content/105/36/13258.full〕〔Head, J. et al. 2008. Formation of gullies on Mars: Link to recent climate history and insolation microenvironments implicate surface water flow origin. PNAS: 105. 13258-13263.〕 Because there are few craters on this mantle, the mantle is relatively young. An excellent view of this mantle is shown below in the picture of the Ptolemaeus Crater Rim, as seen by HiRISE.〔Christensen, P. 2003. Formation of recent martian gullies through melting of extensive water-rich snow deposits. Nature: 422. 45-48.〕 The ice-rich mantle may be the result of climate changes.〔http://news.nationalgeographic.com/news/2008/03/080319-mars-gullies_2.html〕 Changes in Mars's orbit and tilt cause significant changes in the distribution of water ice from polar regions down to latitudes equivalent to Texas. During certain climate periods water vapor leaves polar ice and enters the atmosphere. The water comes back to ground at lower latitudes as deposits of frost or snow mixed generously with dust. The atmosphere of Mars contains a great deal of fine dust particles. Water vapor will condense on the particles, then fall down to the ground due to the additional weight of the water coating. When Mars is at its greatest tilt or obliquity, up to of ice could be removed from the summer ice cap and deposited at midlatitudes. This movement of water could last for several thousand years and create a snow layer of up to around thick.〔Jakosky B. and M. Carr. 1985. Possible precipitation of ice at low latitudes of Mars during periods of high obliquity. Nature: 315. 559-561.〕〔Jakosky, B. et al. 1995. Chaotic obliquity and the nature of the Martian climate. J. Geophys. Res: 100. 1579-1584.〕 When ice at the top of the mantling layer goes back into the atmosphere, it leaves behind dust, which insulating the remaining ice.〔MLA NASA/Jet Propulsion Laboratory (2003, December 18). Mars May Be Emerging From An Ice Age. ScienceDaily. Retrieved February 19, 2009, from http://www.sciencedaily.com /releases/2003/12/031218075443.htmAds by GoogleAdvertise〕 Measurements of altitudes and slopes of gullies support the idea that snowpacks or glaciers are associated with gullies. Steeper slopes have more shade which would preserve snow.〔〔Dickson, J. et al. 2007. Martian gullies in the southern mid-latitudes of Mars Evidence for climate-controlled formation of young fluvial features based upon local and global topography. Icarus: 188. 315-323.〕 Higher elevations have far fewer gullies because ice would tend to sublimate more in the thin air of the higher altitude.〔Hecht, M. 2002. Metastability of liquid water on Mars. Icarus: 156. 373-386.〕 The third theory might be possible since climate changes may be enough to simply allow ice in the ground to melt and thus form the gullies. During a warmer climate, the first few meters of ground could thaw and produce a "debris flow" similar to those on the dry and cold Greenland east coast.〔Peulvast, J. Physio-Geo. 18. 87-105.〕 Since the gullies occur on steep slopes only a small decrease of the shear strength of the soil particles is needed to begin the flow. Small amounts of liquid water from melted ground ice could be enough.〔Costard, F. et al. 2001. Debris Flows on Mars: Analogy with Terrestrial Periglacial Environment and Climatic Implications. Lunar and Planetary Science XXXII (2001). 1534.pdf〕〔http://www.spaceref.com:16090/news/viewpr.html?pid=7124,〕 Calculations show that a third of a mm of runoff can be produced each day for 50 days of each Martian year, even under current conditions.〔Clow, G. 1987. Generation of liquid water on Mars through the melting of a dusty snowpack. Icarus: 72. 93-127.〕 Image:Gully in Phaethontis.jpg|Group of gullies near Newton crater () (Mars Global Surveyor). Image:25090gullies.jpg|Gullies (HiRISE). Image:2509gulliesclosenew.jpg|Gullies - Close-up (HiRISE). Image:2509gullyfan.jpg|Gullies apron - Close-up (HiRISE). ESP 040993 1450gullies.jpg|Gullies on two different levels in crater, as seen by HiRISE under HiWish program 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「Terra Cimmeria」の詳細全文を読む スポンサード リンク
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