Mass movement slope streaks imaged by the Mars Orbiter Camera

Narrow, fan-shaped dark streaks on steep Martian slopes were originally obs erved in Viking Orbiter images, but a definitive explanation was not possib le because of resolution limitations. Pictures acquired by the Mars Orbiter Camera (MOC) aboard the Mars Global Surveyor (MGS) spacecraft show innumer able examples of dark slope streaks distributed widely, but not uniformly, across the brighter equatorial regions, as well as individual details of th ese features that were not visible in Viking Orbiter data. Dark slope strea ks (as well as much rarer bright slope streaks) represent one of the most w idespread and easily recognized styles of mass movement currently affecting the Martian surface. New dark streaks have formed since Viking and even du ring the MGS mission, confirming earlier suppositions that higher contrast dark streaks are younger, and fade (brighten) with time. The darkest slope streaks represent similar to 10%) contrast with surrounding slope materials . No small outcrops supplying dark: material (or bright material, for brigh t streaks) have been found at streak apexes. Digitate downslope ends indica te slope streak formation involves a ground-hugging flow subject to deflect ion by minor topographic obstacles. The model we favor explains most dark s lope streaks as scars from dust avalanches following oversteepening of air fall deposits. This process is analogous to terrestrial avalanches of overs teepened dry, loose snow which produce shallow avalanche scars with similar morphologies. Low angles of internal friction typically 10-30(i) for terre strial loess and clay materials suggest that mass movement of (low-cohesion ) Martian dusty air fall is possible on a wide range of gradients. Martian gravity, presumed low density of the air fall deposits, and thin (unresolve d by MOC) failed layer depths imply extremely low cohesive strength at time of failure, consistent with expectations for an air fall deposit of dust p articles. As speed increases during a dust avalanche, a growing fraction of the avalanching dust particles acquires sufficient kinetic energy to be lo st to the atmosphere in suspension, limiting the momentum of the descending avalanche front. The equilibrium speed, where rate of mass lost to the atm osphere is balanced by mass continually entrained as the avalanche front de scends, decreases with decreasing gradient. This mechanism explains observa tions from MOC images indicating slope streaks formed with little reserve k inetic energy for run-outs on to valley floors and explains why large dista l deposits of displaced material are not found at downslope streak ends. Th e mass movement process of dark (and bright) slope streak formation through dust avalanches involves renewable sources of dust only, leaving underlyin g slope materials unaffected. Areas where dark and bright slope streaks cur rently form and fade in cycles are closely correlated with low thermal iner tia and probably represent regions where dust currently is accumulating, no t just residing.