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The Upper Plains Unit is the remnants of a 50-100 meter thick mantling that has been discovered in the mid-latitudes of the planet Mars. It was first investigated in the Deuteronilus Mensae (Ismenius Lacus quadrangle) region, but it occurs in other places as well. The remnants consist of sets of dipping layers in impact craters, in depressions, and along mesas.〔Carr, M. 2001. Mars Global Surveyor observations of martian fretted terrain. J. Geophys. Res. 106, 23571-23593.〕 Sets of dipping layers may be of various sizes and shapes—some look like Aztec pyramids from Central America. ESP 019778 1385pyramid.jpg|Layered structure in crater that is probably what is left of a layered unit that once covered a much larger area. Material for this unit fell from the sky as ice-coated dust. The picture was taken by HiRISE, under the HiWish program. Picture is from Hellas quadrangle. ESP 035684 2160pyramidsbrains.jpg|Dipping layers, as seen by HiRISE under HiWish program ESP 036790 2200pyramids.jpg|Dipping layers in a crater, as seen by HiRISE under HiWish program ESP 037146 2195pyramidismenius.jpg|Layered features in crater, as seen by HiRISE under HiWish program P1010377redrocksfall.jpg|Layered feature in Red Rocks Park, Colorado. This has a different origin than ones on Mars, but it has a similar shape. Features in Red Rocks region were caused by uplift of mountains. ESP 036660 2130pyramidismenius.jpg|Dipping layers, as seen by HiRISE under HiWish program ESP 035801 2210pyramidsismenius.jpg|Layered structures, as seen by HiRISE under HiWish program This unit also degrades into brain terrain. Brain terrain is a region of maze-like ridges 3–5 meters high. Some ridges may consist of an ice core, so they may be sources of water for future colonists. ESP 028336 1395pyramidhellas.jpg|Small, layered structure, as seen by HiRISE under the HiWish program Picture also shows brain terrain forming. Brain terrain seems to start with pits that get longer and more complicated. Picture is from Hellas quadrangle. ESP 028273 2240pyramidbrain.jpg|Layered features and brain terrain, as seen by HiRISE under HiWish program The upper plains unit often changes into brain terrain. ESP 042105 2235brainsforming.jpg|Brain terrain being formed from a thicker layer, as seen by HiRISE under HiWish program. Arrows show the thicker unit breaking up into small cells. Some regions of the upper plains unit display large fractures and troughs with raised rims; such regions are called ribbed upper plains. Fractures are believed to have started with small cracks from stresses. Stress is suggested to initiate the fracture process since ribbed upper plains are common when debris aprons come together or near the edge of debris aprons—such sites would generate compressional stresses. Cracks exposed more surfaces, and consequently more ice in the material sublimates into the planet’s thin atmosphere. Eventually, small cracks become large canyons or troughs. ESP 028339 2245headarticle.jpg|Well developed ribbed upper plains material. These start with small cracks that expand as ice sublimates from the surfaces of the crack. Picture was taken with HiRISE under HiWish program ESP 042765 2245cracks.jpg|Small and large cracks, as seen by HiRISE under HiWish program The small cracks to the left will enlarge to become much larger dues to sublimation of ground ice. A crack exposes more surface area, hence greatly increases sublimation in the thin Martian air. ESP 035011 2240pyramidshead.jpg|Dipping layers, as seen by HiRISE under HiWish program Also, Ribbed Upper plains material is visible in the upper right of the picture. It is forming from the upper plains unit, and in turn is being eroded into brain terrain. ESP 042198 2235pyramid.jpg|View of stress cracks and larger cracks that have been enlarged by sublimation (ice changing directly into gas) This may be the start of ribbed terrain. ESP 042554 2245ribbedterrain.jpg|Evolution of ribbed terrain from stress cracks--cracks to the left eventually will enlarge and become ribbed terrain toward the right side of picture, as seen by HiRISE under HiWish program Small cracks often contain small pits and chains of pits; these are thought to be from sublimation of ice in the ground.〔Morgenstern, A., et al. 2007〕〔Baker, D., J. Head. 2015. Extensive Middle Amazonian mantling of debris aprons and plains in Deuteronilus Mensae, Mars: Implication for the record of mid-latitude glaciation. Icarus: 260, 269-288.〕 Large areas of the Martian surface are loaded with ice that is protected by a meters thick layer of dust and other material. However, if cracks appear, a fresh surface will expose ice to the thin atmosphere.〔Mangold, N. 2003. Geomorphic analysis of lobate debris aprons on Mars at Mars Orbiter Camera scale: Evidence for ice sublimation initiated by fractures. J. Geophys. Res. 108, 8021.〕〔Levy, J. et al. 2009. Concentric〕 In a short time, the ice will disappear into the cold, thin atmosphere in a process called sublimation. Dry ice behaves in a similar fashion on the Earth. On Mars sublimation has been observed when the Phoenix lander uncovered chunks of ice that disappeared in a few days.〔(Bright Chunks at ''Phoenix'' Lander's Mars Site Must Have Been Ice ) – Official NASA press release (19.06.2008)〕〔http://www.nasa.gov/mission_pages/phoenix/news/phoenix-20080619.html〕 In addition, HiRISE has seen fresh craters with ice at the bottom. After a time, HiRISE saw the ice deposit disappear.〔Byrne, S. et al. 2009. Distribution of Mid-Latitude Ground Ice on Mars from New Impact Craters: 329.1674-1676〕 Image:Ice sublimating in the Dodo-Goldilocks trench.gif|Die-sized clumps of bright material in the enlarged "Dodo-Goldilocks" trench vanished over the course of four days, implying that they were composed of ice which sublimated following exposure.〔 Image:Evaporating ice on Mars Phoenix lander image.jpg|Color versions of the photos showing ice sublimation, with the lower left corner of the trench enlarged in the insets in the upper right of the images. The upper plains unit is thought to have fallen from the sky. It drapes various surfaces, as if it fell evenly. As is the case for other mantle deposits, the upper plains unit has layers, is fine-grained, and is ice-rich. It is widespread; it does not seem to have a point source. The surface appearance of some regions of Mars is due to how this unit has degraded. It is a major cause of the surface appearance of lobate debris aprons.〔 The layering of the upper plains mantling unit and other mantling units are believed to be caused by major changes in the planet’s climate. Models predict that the obliquity or tilt of the rotational axis has varied from its present 25 degrees to maybe over 80 degrees over geological time. Periods of high tilt will cause the ice in the polar caps to be redistributed and change the amount of dust in the atmosphere.〔Head, J. et al. 2003.〕〔Madeleine, et al. 2014.〕〔Schon, et al. 2009. A recent ice age on Mars: Evidence for climate oscillations from regional layering in mid-latitude mantling deposits. Geophys. Res. Lett. 36, L15202.〕 ==See also== * Brain terrain * Climate of Mars * Geology of Mars * Ismenius Lacus quadrangle * Hellas quadrangle * Lobate Debris Apron * Pedestal crater * Phoenix (spacecraft) 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「Upper Plains Unit」の詳細全文を読む スポンサード リンク
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