Post by Dr Norbert Schörghofer, University of Hawaii, Honolulu.
Slope streaks are a form of down-slope mass movement on the surface of Mars that frequently occurs on Mars today (Image 1 and 2). Slope streaks were first identified on high-resolution Viking Orbiter images, but their present-day activity was only discovered in Mars Orbiter Camera (MOC) images.
The term ‘slope streak’ is based on appearance. Slope streaks closely follow the topography of the terrain, and they flow around obstacles. They extend downslope and have sharp edges. They are up to a few kilometers long and often tens of meters wide, but narrow streaks are observed at the resolution limit of orbital cameras. Slope streaks are abundant in the dust-mantled equatorial regions of Mars, over a wide range of elevations (Sullivan et al. 2001).
Slope streaks have remarkable mechanical properties. Most have no discernible downhill deposits. They have been observed on slopes of 20° and less, well below the angle of repose of dry granular materials. Occasionally they show an anastomosing pattern.
The cause of slope streak formation remains unclear. They are commonly interpreted as dust avalanches. They may be triggered by various types of mechanical disturbances, such as rolling boulders, meteorite impacts, dust devils, or other events. Some scientists had suggested that liquid water plays a role. The evidence available today suggests this is not the case, in part because of a lack of seasonality (Chuang et al. 2007, Schorghofer and King 2011).
Slope streaks are commonly darker, but occasionally brighter than their surroundings. Bright (light toned) slope streaks do not presently form in sudden events as dark slope streaks do. Instead, bright streaks might form from old dark slope streaks.
New slope streaks are always dark and typically darker than old ones; they brighten with time, presumably because they are covered by bright airfall dust. Slope streaks fade gradually over time and many have disappeared in the three decades that passed since the Viking Orbiter images. The turnover time of the population is estimated to be four decades.
Terrestrial morphological analogs are found in the Dry Valleys of Antarctica, where they are caused by snowmelt (Head et al. 2007).
Bergonio, J.R., K.M. Rottas, and N. Schorghofer. Properties of Martian slope streak populations. Icarus 225, 194-199 (2013).
Chuang, F.C., R.A. Beyer, A.S. McEwen, and B.J. Thomson. HiRISE observations of slope streaks on Mars. Geophys. Res. Lett., 34, L20204 (2007).
Head J.W., Marchant D.R., Dickson J.L., Levy J.S., Morgan G.A. Slope streaks in the Antarctic Dry Valleys: characteristics, candidate formation mechanisms, and implications for slope streak formation in the martian environment. 7th Int. Mars Conf., 3114 (2007).
Phillips, C.B., D.M. Burr, and R.A. Beyer. Mass movement within a slope streak on Mars. Geophys. Res. Lett. 34, L21202 (2007).
Schorghofer, N., O. Aharonson, M.F. Gerstell, and L. Tatsumi. Three decades of slope streak activity on Mars. Icarus 191, 132-140 (2007).
Schorghofer, N. and C.M. King. Sporadic formation of slope streaks on Mars. Icarus 216, 159-168 (2011).
Sullivan, R., P. Thomas, J. Veverka, M. Malin, and K.S. Edgett. Mass movement slope streaks imaged by the Mars Orbiter Camera. J. Geophys. Res. 106, 23607 (2001).