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Dark slope streaks are narrow, avalanche-like features common on dust-covered slopes in the equatorial regions of Mars.〔Chuang, F.C.; Beyer, R.A.; Bridges, N.T. (2010). Modification of Martian Slope Streaks by Eolian Processes. ''Icarus,'' 205 154–164.〕 They form in relatively steep terrain, such as along escarpments and crater walls.〔Schorghofer, N.; Aharonson, O.; Khatiwala, S. (2002). Slope Streaks on Mars: Correlations with Surface Properties and the Potential Role of Water. ''Geophys. Res. Lett.,'' 29(23), 2126, .〕 Although first recognized in Viking Orbiter images from the late 1970s,〔Morris, E.C. (1982). Aureole Deposits of the Martian Volcano Olympus Mons. ''J. Geophys. Res.,'' 87(B2), 1164–1178.〕〔Ferguson,H.M.; Lucchitta, B.K. (1984). Dark Streaks on Talus Slopes, Mars in ''Reports of the Planetary Geology Program 1983, NASA Tech. Memo., TM-86246,'' pp. 188–190. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19840015363_1984015363.pdf.〕 dark slope streaks were not studied in detail until higher-resolution images from the Mars Global Surveyor (MGS) and Mars Reconnaissance Orbiter (MRO) spacecraft became available in the late 1990s and 2000s.〔〔Chuang, F.C. ''et al.'' (2007). HiRISE Observations of Slope Streaks on Mars. ''Geophys. Res. Lett.,'' 34 L20204, .〕 The physical process that produces dark slope streaks is still uncertain. They are most likely caused by the mass movement of loose, fine-grained material on oversteepened slopes (i.e., dust avalanches).〔〔Sullivan, R.; Daubar, I.; Fenton, L.; Malin, M.; Veverka, J. (1999). Mass-Movement Considerations for Dark Slope Streaks Imaged by the Mars Orbiter Camera. 30th Lunar and Planetary Science Conference, Abstract #1809. http://www.lpi.usra.edu/meetings/LPSC99/pdf/1809.pdf.〕〔Barlow, 2008, p. 141.〕 The avalanching disturbs and removes a bright surface layer of dust to expose a darker substrate.〔Ferris, J. C.; Dohm, J.M.; Baker, V.R.; Maddock III, T. (2002). Dark Slope Streaks on Mars: Are Aqueous Processes Involved? ''Geophys. Res. Lett.,'' 29(10), 1490, . http://www.agu.org/journals/ABS/2002/2002GL014936.shtml.〕 The role that water and other volatiles plays, if any, in streak formation is still debated. Slope streaks are particularly intriguing because they are one of the few geological phenomena that can be observed occurring on Mars in the present day.〔Kreslavsky, M.A.; Head, J.W. (2009). Slope Streaks on Mars: A New "Wet" Mechanism. ''Icarus,'' 201 517–527.〕〔Aharonson, O.; Schorghofer, N.; Gerstell, M.F. (2003). Slope Streak Formation and Dust Deposition Rates on Mars. ''J. Geophys. Res.,'' 108(E12), 5138, .〕 ==Nature of streaks on Mars== Dark slope streaks are albedo features. They appear to the eye as a brightness difference between the streak and the lighter-toned background slope. Usually no topographic relief is visible to distinguish the streak from its surroundings, except in the very highest resolution (<1 m/pixel) images.〔 In many cases, the original surface texture of the slope is preserved and continuous across the streak, as though unaffected by events involved in dark streak formation (pictured left). The overall effect is equivalent in appearance to a partial shadow cast down the sloping surface.〔 These observations indicate that whatever process forms the streaks, it affects only the very thinnest layer at the surface. Slope streaks are only about 10% darker than their surroundings but often appear black in images because the contrast has been enhanced (stretched).〔Baratoux, D. et al. (2006). The Role of the Wind-Transported Dust in Slope Streaks Activity: Evidence from the HRSC Data. ''Icarus,'' 183 30–45.〕 Albedo features cover the Martian surface at a wide variety of scales. They make up the classical light and dark marking seen on Mars through telescopes. (See Classical albedo features on Mars.) The markings are caused by differing proportions of dust covering the surface. Martian dust is bright reddish ochre in color, while the bedrock and soil (regolith) is dark gray (the color of unaltered basalt). Thus, dusty areas on Mars appear bright (high albedo), and surfaces with a high percentage of rocks and rock fragments are generally dark (low albedo).〔Barlow, 2008, p. 73.〕 Most albedo features on Mars are caused by winds, which clear some areas of dust, leaving behind a darker lag. In other areas, dust is deposited to produce a bright surface. The selective removal and deposition of dust is most conspicuous around impact craters and other obstacles where a variety of streaks (wind tails) and blotches are formed.〔Hartmann, 2003, pp. 36–41.〕 Dark slope streaks are relatively small features. (See A in Photo Gallery.) They differ from larger albedo features in being produced by gravity rather than wind, although wind may contribute to their initial formation.〔〔〔Schorghofer, Aharonson, O.; Gerstell, M.F.; Tatsumi, L. (2007). Three Decades of Slope Streak Activity on Mars. ''Icarus,'' 191 132–140, .〕 (See B in Photo Gallery.) The cause of the darkening is uncertain. The particle sizes involved are believed to be very small (sand, silt, and clay-sized particles). No clasts large enough to be imaged are present, and the underlying bedrock slope is never exposed ( i.e., dust is avalanching on a surface of dust).〔Malin, M.C.; Edgett, K.S. (2001). Mars Global Surveyor Mars Orbiter Camera: Interplanetary cruise through primary mission. ''J. Geophys. Res.,'' 106(E10), 23,429–23,570.〕 Apparently, other optical, mechanical, or chemical properties are involved in producing the darker tone. Dark slope streaks commonly share the same slope with other slope streaks of varying tones. The darkest streaks are presumed to be youngest; they have margins that are more sharply defined than streaks that are not as dark.〔Williams, S.H. (1991). Dark Talus Streaks on Mars are Similar to Aeolian Dark Streaks. 22nd Lunar and Planetary Science Conference, Abstract #1750. http://www.lpi.usra.edu/meetings/lpsc1991/pdf/1750.pdf.〕 This relationship suggests that streaks lighten and become more diffuse with age,〔 probably because they become covered with fresh dust falling from the atmosphere.〔〔 Faded dark slope streaks should not be confused with bright slope streaks (discussed below). 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「Dark slope streak」の詳細全文を読む スポンサード リンク
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