Dark slope streak
[18] Most albedo features on Mars are caused by winds, which clear some areas of dust, leaving behind a darker lag.
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.
[22] This relationship suggests that streaks lighten and become more diffuse with age,[6] probably because they become covered with fresh dust falling from the atmosphere.
At moderate resolutions (20–50 m/pixel), dark slope streaks appear as thin, parallel filaments aligned downslope along crater rims and escarpments.
[7] Slope streaks widen downslope from the apex in a triangular fashion, usually reaching their maximum widths short of the halfway point of their lengths.
[2][7][24] Dark slope streaks are most common in the equatorial regions of Mars, particularly in Tharsis, Arabia Terra, and Amazonis Planitia[25] (pictured left).
This relationship has led some researchers to suggest that liquid water is involved in dark slope streak formation.
[2][17] Dark slope streaks do not appear to correlate with elevation or areas of specific bedrock geology.
The most widely held view is that the streaks are the result of dust avalanches produced by dry granular flow[28] on oversteepened slopes.
[1] The process produces a very shallow trough (slough) on the surface of the snow, which from a distance appears slightly darker in tone than the rest of the slope.
Other models involve water, either in the form of spring discharges,[29] wet debris flows,[6] or seasonal percolation of chloride-rich brines.
[12] Using data from the Mars Odyssey Neutron Spectrometer, researchers found that slope streaks in the Schiaparelli basin occur in areas predicted to yield between 7.0 and 9.0 weight percent Water Equivalent Hydrogen (WEH) in contrast to typical background values of less than 4% WEH.
[13] Another model proposes that dark slope streaks are produced by ground-hugging density currents of dry dust lubricated by carbon dioxide (CO2) gas.
[34] Research, published in January 2012 in Icarus, found that dark streaks were initiated by airblasts from meteorites traveling at supersonic speeds.
The crater cluster lies near the equator 510 miles) south of Olympus Mons, on a type of terrain called the Medusae Fossae formation.
[38] A team of researchers found an increase in dark slope streaks after S1222, a marsquake that was detected by the Insight lander.
If accurate, this rate suggests that slope streaks are the most dynamic geologic features observed on the surface of Mars.
After the storm subsides, a thick layer of fresh dust is deposited to begin a new cycle of streak formation.
[41] Dark slope streaks occur in association with or superficially resemble a number of other small-scale, slope-related features on Mars.
Many of the slope features on Mars may originate through a continuum of processes with dry mass wasting and minor fluvial (water-related) activity occupying opposite endpoints.
[10] Gullies are another feature common on slopes in the mid-latitude southern hemisphere of Mars They have received much attention in the literature but are not discussed here.
[7][42] In the summer of 2011, a paper appeared in Science[43] describing a new class of slope features with characteristics that suggest formation by seasonal releases of liquid water.
Called "recurring slope lineae" (RSL),[44] the features received a considerable amount of media attention.
[45][46] RSLs are narrow (0.5 to 5 meters) dark markings that preferentially occur on steep, equator-facing slopes in the southern hemisphere between latitudes 48°S to 32°S.
[43] Unlike RSLs, dark slope streaks appear to occur sporadically throughout the Martian year, and their triggering seems unrelated to season or large regional events.
[48] Water tracks are little-studied slope features common in permafrost-dominated terrains in the arctic and Antarctic regions of Earth.
They are zones of enhanced soil moisture that route water downslope over the top of the permanently frozen ground just below the surface (ice table).
Although water tracks have not been specifically identified on Mars, several researchers have noted their morphological and spectroscopic similarity to Martian slope streaks.
[49] Like dark slope streaks, water tracks are narrow, sublinear features elongated in the downslope direction.
During peak flow conditions, they appear as damp, darkened, patches of soil that are generally less than 60 m wide and several hundred meters long.
