Dark slope streaks

Dark slope streaks are narrow, avalanche-like features common on dust-covered slopes in the equatorial regions of Mars. They form in relatively steep terrain, such as along escarpments and crater walls. Although first recognized in Viking Orbiter images from the late 1970s, 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.

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). The avalanching disturbs and removes a bright surface layer of dust to expose a darker substrate. 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.

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).

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). 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.

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