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Lineated valley fill (LVF), also called lineated floor deposit, is a feature of the floors of some channels on Mars, exhibiting ridges and grooves that seem to flow around obstacles. Shadow measurements show that at least some of the ridges are several metres high. LVF is believed to be ice-rich.〔〔Head, J., et al. 2006. Modification if the dichotomy boundary on Mars by Amazonian mid-latitude regional glaciation. Geophys. Res Lett. 33〕 Hundreds of metres of ice probably lie protected in LVF under a thin layer of debris.〔Head, J. and D. Marchant. 2006. Evidence for global-scale northern mid-latitude glaciation in the Amazonian period of Mars: Debris-covered glacial and valley glacial deposits in the 30 - 50 N latitude band. Lunar. Planet. Sci. 37. Abstract 1127〕〔 The debris consists of wind-borne dust, material from alcove walls, and lag material remaining after ice sublimated (changed from a solid directly to a gas) from a rock-ice mixture. Some glaciers on Earth show similar ridges. High resolution pictures taken with HiRISE reveal that some of the surfaces of lineated valley 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.〕 This same type of surface is present on Lobate debris aprons and Concentric crater fill so all three are believed to be related. Wikifrettedctxpo5.jpg|Wide CTX view of mesa showing Lineated Valley Fill and Lobate Debris Apron (LDA). Both are believed to be debris covered glaciers. Location is Ismenius Lacus quadrangle. Wikifretesp 027639 2210lda.jpg|Close-up of Lobate Debris Apron from the previous CTX image of a mesa. Image shows open-cell brain terrain and closed-cell brain terrain which is more common. Open-cell brain terrain is thought to hold a core of ice. Image is from HiRISE under HiWish program. 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. Wikifrettedctxp22.jpg|Wide CTX view showing mesa and buttes with Lobate debris aprons and lineated valley fill around them. Location is Ismenius Lacus quadrangle. WikiESP 020769 2225fretted.jpg|Close-up of Lineated Valley Fill (LVF), as seen by HiRISE under HiWish program Note: this is an enlargement of the previous CTX image. Lineated floor deposits began as lobate debris aprons (LDAs), which form as material leaves narrow mountain valleys and spreads out as an apron.〔Souness, C. and B. Hubbard. 2013. An alternative interpretation of late Amazonian ice flow: Protonilus Mensae, Mars. Icarus 225, 495-505.〕 By tracing the paths of the ridges on LDAs, researchers have come to believe that the curved ridges characteristic of lobate debris aprons straighten out to form the more or less straight ridges of LVF. In the regions where LVF and LDAs occur, many craters have concentric crater fill: large ridges and other surfaces nicknamed brain terrain, after the surface corrugations of the human brain. The study of lineated valley fill and other features related to debris-covered ice has been greatly aided by the abundance of data received from Mars orbiting instruments. Excellent images have been obtained from THEMIS, MOC, CTX, and HiRISE. Detailed altimetry was collected by MOLA. The Mars Reconnaissance Orbiter's Shallow Radar gave a strong reflection from the top and base of LDAs, meaning that pure water ice made up the bulk of the formation (between the two reflections), strong evidence that the LDAs in Hellas Planitia are glaciers covered with a thin layer of rocks. Since lineated valley terrain is derived from lobate debris aprons, it probably contains buried ice—at least in places.〔〔Plaut, J. et al. 2008. Radar Evidence for Ice in Lobate Debris Aprons in the Mid-Northern Latitudes of Mars. Lunar and Planetary Science XXXIX. 2290.pdf〕 ==Connection to past climate== Studies of LDAs and LVF give evidence that there have been multiple episodes of glaciation on Mars, including ones that produced glaciers nearing a kilometre in thickness. These ice ages are related to major climate shifts caused by major variations in axial tilt.〔Madeleine, J. et al. 2007. Exploring the northern mid-latitude glaciation with a general circulation model. In: Seventh International Conference on Mars. Abstract 3096.〕〔Barlow, N. 2008. Mars: An Introduction to its Interior, Surface and Atmosphere. Cambridge University Press. ISBN 978-0-521-85226-5〕 Earth's rather large moon prevents large changes in its tilt. The two moons of Mars are tiny. So Mars undergoes large periods when its ice cap receives more direct sunlight.〔http://hirise.lpl.arizona.edu/PSP_002917_2175〕〔Forget, F., et al. 2006. Planet Mars Story of Another World. Praxis Publishing, Chichester, UK. ISBN 978-0-387-48925-4〕 During this time ice in the cap sublimates, and thick snow falls in mid-latitudes — the zones where concentric crater fill, lineated valley fill and lobate debris aprons are common.〔Carr, M. 2006. The Surface of Mars. Cambridge University Press. ISBN 978-0-521-87201-0〕 The distribution of craters on LVF indicates a late Amazonian age for at least some areas. 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「lineated valley fill」の詳細全文を読む スポンサード リンク
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