Post by Frank Chuang.
The High Resolution Imaging Science Experiment (HiRISE) onboard Mars Reconnaissance Orbiter (MRO) has imaged the Martian surface for two full Earth years at spatial scales up to 25 centimeters/pixel. These images allow very detailed studies of small surface features, such as slope streaks, commonly seen in the high albedo, low thermal inertia, and dust-rich equatorial regions of Mars. Slope streaks have been observed in all spacecraft images from the early Mariner missions to the most recent Mars Reconnaissance Orbiter (MRO) mission to Mars [1-8]. Recently formed slope streaks are typically darker than their surroundings and appear to fade (i.e., brighten) over time (Image 1). Typical characteristics of slope streaks are initiation at a point source, one streak splitting into two, deflection around or over obstacles such as small boulders or crater rims, widening below the source area up to a few hundred meters, and lengths of up to a few kilometers. Numerous models have been proposed for their formation, both dry- and wet-based, and these are described briefly in .
Although there is not one universally accepted model, one key observation up to now was that streaks do not appear to have topographic relief. However HiRISE images from the first seven months of MRO showed that slope streaks do indeed have topographic relief . The relief is typically observed under conditions where there is low sun illumination, sufficient dust coverage, slopes facing away from the sun, and high spatial scale. Image 1 shows an example of relief where the streak interior is lower than their surroundings. This is particularly noticeable along the streak margins. However, there are also many streaks that do not have relief in HiRISE images. This does not necessarily mean they lack relief, but rather, are below the capability of HiRISE to detect. Other interesting observations include triggering of slope streaks by an impact crater, blast from an impact explosion, or boulders rolling or bouncing downslope, and formation of long linear ridges within the streak that are parallel to its margins. Streaks tend to initiate below localized features such as rock outcrops, crater rims, or boulders.
The discovery of topographic relief provides an important clue in that sediment had to be removed for the relief to be present. There are several processes, both wet and dry, that can remove material. A small stream of liquid water running over the surface could carve out the streak area, but liquid water exposed at the surface is not stable under current Martian conditions. Water flowing below the surface, covered by an insulating layer of materials is also possible. HiRISE and other spectral cameras have yet to detect liquid water on the surface. Other evidence that would favor a wet process are detection of salts, alcoves or channels above the point source, or streaks forming at the intersection of two different geologic layers (i.e., permeable and non-permeable layers). However, there are no documented cases of these thus far. Thus, a dry process is appealing and one of these are dry dust avalanches [2,5,6].
Further analyses of the slope streaks have documented other features related to modification of the streaks . These include ridge-like structures and mantle deposits . In particular, the presence of ridge-like structures (Image 2), interpreted to be eolian ripples, indicates that eolian modification (i.e., saltation of grains) within streak beds are occurring on Mars today, and have likely occurred in the recent past.
 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. [Abstract]
 Sullivan, R., Thomas, P., Veverka, J., Malin, M., Edgett, K.S., 2001. Mass movement slope streaks imaged by the Mars Orbiter Camera, J. Geophys. Res. 106, (E10), 23,607-23,633. [Abstract]
 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, 2126, doi:10.1029/2002GL015889. [Abstract]
 Aharonson, O., Schorghofer, N., Gerstell, M.F., 2003. Slope streak formation and dust deposition rates on Mars, J. Geophys Res. 108 (E12), doi:10.1029/2003JE002123. [Abstract]
 Baratoux, D., Mangold, N., Forget, F., Cord, A., Pinet, P., Daydou, Y., Jehl, A., Masson, P., Neukum, G., and The HRSC Co-Investigator Team, 2006. The role of the wind-transported dust in slope streaks activity: Evidence from the HRSC data, Icarus 183, 30-45. [Abstract]
 Chuang, F.C., Beyer, R.A., McEwen, A.S., and Thomson, B.J., 2007. HiRISE observations of slope streaks on Mars, Geophys. Res. Lett. 34, L20204, doi:10.1029/2007GL031111. [Abstract]
 Phillips, C.B., Burr, D.M., Beyer, R.A., 2007. Mass movement within a slope streak on Mars, Geophys. Res. Lett. 34, L21202, doi:10.1029/2007GL031577. [Abstract]
 Schorghofer, N., Aharonson, O., Gerstell, M.F., Tatsumi, L., 2007. Three decades of slope streak activity on Mars, Icarus 191, 132-140. [Abstract]
 Chuang, F.C., Beyer, R.A., and Bridges, N.T., 2009. Modification of Martian slope streaks by eolian processes, Icarus, in press. [Abstract]