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Glaciers, loosely defined as patches of currently or recently flowing ice, are thought to be present across large but restricted areas of the modern Martian surface, and are inferred to have been more widely distributed at times in the past.〔"The Surface of Mars" Series: Cambridge Planetary Science (No. 6) ISBN 978-0-511-26688-1 Michael H. Carr, United States Geological Survey, Menlo Park〕 Lobate convex features on the surface known as viscous flow features and lobate debris aprons, which show the characteristics of non-Newtonian flow, are now almost unanimously regarded as true glaciers.〔〔Milliken, R. E., J. F. Mustard, and D. L. Goldsby. "Viscous flow features on the surface of Mars: Observations from high-resolution Mars Orbiter Camera (MOC) images." Journal of Geophysical Research 108.E6 (2003): 5057.〕〔S.W. Squyres, M.H. Carr Geomorphic evidence for the distribution of ground ice on Mars Science, 213 (1986), pp. 249–253. doi:10.1126/science.231.4735.249〕〔J.W. Head, D.R. Marchant, J.L. Dickson, A.M. Kress, D.M. Baker Northern mid-latitude glaciation in the Late Amazonian period of Mars: Criteria for the recognition of debris-covered glacier and valley glacier landsystem deposits Earth Planet. Sci. Lett., 294 (2010), pp. 306–320〕〔J.W. Holt et al. Radar sounding evidence for buried glaciers in the southern mid-latitudes of Mars Science, 322 (2008), pp. 1235–1238〕〔G.A. Morgan, J.W. Head, D.R. Marchant Lineated valley fill (LVF) and lobate debris aprons (LDA) in the Deuteronilus Mensae northern dichotomy boundary region, Mars: Constraints on the extent, age and episodicity of Amazonian glacial events Icarus, 202 (2009), pp. 22–38〕〔J.J. Plaut, A. Safaeinili, J.W. Holt, R.J. Phillips, J.W. Head, R. Sue, N.E. Putzig, A. Frigeri Radar evidence for ice in lobate debris aprons in the mid-northern latitudes of Mars Geophys. Res. Lett., 36 (2009), p. L02203〕〔D.M.H. Baker, J.W. Head, D.R. Marchant Flow patterns of lobate debris aprons and lineated valley fill north of Ismeniae Fossae, Mars: Evidence for extensive mid-latitude glaciation in the Late Amazonian Icarus, 207 (2010), pp. 186–209〕〔J. Arfstrom, W.K. Hartmann Martian flow features, moraine-like ridges, and gullies: Terrestrial analogs and interrelationships Icarus, 174 (2005), pp. 321–335〕 However, a variety of other features on the surface have also been interpreted as directly linked to flowing ice, such as fretted terrain,〔〔Lucchitta, Baerbel K. "Ice and debris in the fretted terrain, Mars." Journal of Geophysical Research: Solid Earth (1978–2012) 89.S02 (1984): B409-B418.〕 lineated valley fill,〔〔 concentric crater fill,〔〔Levy, Joseph S., James W. Head, and David R. Marchant. "Concentric crater fill in Utopia Planitia: History and interaction between glacial “brain terrain” and periglacial mantle processes." Icarus 202.2 (2009): 462-476.Levy, Joseph S., James W. Head, and David R. Marchant. "Concentric crater fill in Utopia Planitia: History and interaction between glacial “brain terrain” and periglacial mantle processes." Icarus 202.2 (2009): 462-476.〕 and arcuate ridges.〔 A variety of surface textures seen in imagery of the midlatitudes and polar regions are also thought to be linked to sublimation of glacial ice.〔〔Hubbard, Bryn, et al. "Geomorphological characterisation and interpretation of a mid-latitude glacier-like form: Hellas Planitia, Mars." Icarus 211.1 (2011): 330-346.〕 Today, features interpreted as glaciers are largely restricted to latitudes polewards of around 30° latitude.〔Head, J. W., et al. "Extensive valley glacier deposits in the northern mid-latitudes of Mars: Evidence for Late Amazonian obliquity-driven climate change." Earth and Planetary Science Letters 241.3 (2006): 663-671.〕 Particular concentrations are found in the Ismenius Lacus quadrangle.〔 Based on our current models of the Martian atmosphere, ice should not however be stable if exposed at the surface in the mid-Martian latitudes.〔Williams, K. E., et al. "Stability of mid-latitude snowpacks on Mars." Icarus 196.2 (2008): 565-577.〕 It is thus thought that most glaciers must be covered with a layer of rubble or dust preventing free transfer of water vapor from the subliming ice into the air.〔〔 This also suggests that at in Mars' relatively recent past, its climate must have been different in order to allow the glaciers to grow stably at these latitudes.〔 This provides good independent evidence that the obliquity of Mars has changed significantly in the past, as independently indicated by modelling of Mars' orbital solutions.〔Laskar, Jacques, et al. "Long term evolution and chaotic diffusion of the insolation quantities of Mars." Icarus 170.2 (2004): 343-364.〕 Evidence for past glaciation also appears on the peaks of several Martian volcanoes in the tropics.〔Head, J. W., et al. "Tropical to mid-latitude snow and ice accumulation, flow and glaciation on Mars." Nature 434.7031 (2005): 346-351.〕〔Head, James W., and David R. Marchant. "Cold-based mountain glaciers on Mars: western Arsia Mons." Geology 31.7 (2003): 641-644.〕 Like glaciers on Earth, glaciers on Mars are not pure water ice.〔〔 Many are thought to contain substantial proportions of debris, and a substantial number are probably better described as rock glaciers.〔〔Colaprete, Anthony, and Bruce M. Jakosky. "Ice flow and rock glaciers on Mars." Journal of Geophysical Research: Planets (1991–2012) 103.E3 (1998): 5897-5909.〕〔Haeberli, Wilfried, et al. "Permafrost creep and rock glacier dynamics." Permafrost and Periglacial Processes 17.3 (2006): 189-214.〕 For many years, largely because of the modeled instability of water ice in the midlatitudes where the putative glacial features were concentrated, it was argued that almost all glaciers were rock glaciers on Mars.〔Squyres, Steven W. "Martian fretted terrain: Flow of erosional debris." Icarus 34.3 (1978): 600-613.〕 However, recent direct observations made by the SHARAD radar instrument on the Mars Reconnaissance Orbiter satellite have confirmed that at least some features are relatively pure ice, and thus, true glaciers.〔〔 Some authors have also made claims that glaciers of solid carbon dioxide have formed on Mars under certain rare conditions.〔Kreslavsky, Mikhail A., and James W. Head. "Carbon dioxide glaciers on Mars: Products of recent low obliquity epochs (?)." Icarus 216.1 (2011): 111-115.〕 Some landscapes look just like glaciers moving out of mountain valleys on Earth. Some have a hollowed out appearance, looking like a glacier after almost all the ice has disappeared. What is left are the moraines—the dirt and debris carried by the glacier. The center is hollowed out because the ice is mostly gone.〔Milliken, R., J. Mustard, D. Goldsby. 2003. Viscous flow features on the surface of Mars: Observations from high-resolution Mars Orbiter Camera (MOC) images. J. Geophys. Res. 108. doi:10.1029/2002JE002005.〕 These supposed alpine glaciers have been called glacier-like forms (GLF) or glacier-like flows (GLF).〔Arfstrom, J and W. Hartmann. 2005. Martian flow features, moraine-like ridges, and gullies: Terrestrial analogs and interrelationships. Icarus 174, 321-335.〕 Glacier-like forms are a later and maybe more accurate term because we cannot be sure the structure is currently moving.〔Hubbard B., R. Milliken, J. Kargel , A. Limaye, C. Souness . 2011. Geomorphological characterisation and interpretation of a mid-latitude glacier-like form: Hellas Planitia, Mars Icarus 211, 330–346〕 Another, more general term sometimes seen in the literature is viscous flow features (VFF).〔 ==Radar finds ice== Radar studies with the SHAllow RADar (SHARAD) on the Mars Reconnaissance Orbiter showed that Lobate Debris Aprons (LDA) and Lineated Valley Fill (LVF) contain pure water ice covered with a thin layer of rocks that insulated the ice.〔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〕〔http://hirise.lpl.arizona.edu/PSP_009535_2240〕 Ice was found both in the southern hemisphere 〔Holt, J., A. Safaeinili, J. Plaut, J. Head, R. Phillips, R. Seu, S. Kempf, P. Choudhary, D. Young, N. Putzig, D. Biccari, Y. Gim. 2008. Radar sounding evidence for buried glaciers in the southern mid-latitudes of Mars. Science 322. doi:10.1126/science.1164246.〕 and in the northern hemisphere.〔Plaut, J., A. Safaeinili,, J. Holt, R. Phillips, J. Head, J., R. Seu, N. Putzig, A. Frigeri. 2009. Radar evidence for ice in lobate debris aprons in the midnorthern latitudes of Mars. Geophys. Res. Lett. 36. doi:10.1029/2008GL036379.〕 Researchers at the Niels Bohr Institute combined radar observations with ice flow modelling to say that ice in all of the Martian glaciers is equivalent to what could cover the entire surface of Mars with 1.1 meters of ice. The fact that the ice is still there and has not evaporated out into space suggests that a thick layer of dust is protecting the ice. The current atmospheric pressure on Mars is so low that any water ice simply evaporates.〔http://spaceref.com/mars/mars-has-belts-of-glaciers-consisting-of-frozen-water.html〕〔http://www.sciencedaily.com/releases/2015/04/150408102701.htm〕〔Karlsson, N., L. Schmidt, C. Hvidberg. Volume of Martian mid-latitude glaciers from radar observations and ice-flow modelling. Geophysical Research Letters, 2015; DOI: 10.1002/2015GL063219〕 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「Glaciers on Mars」の詳細全文を読む スポンサード リンク
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