PRESENT-DAY AEOLIAN SEDIMENTATION ON MARS FROM AN ORBITAL PERSPECTIVE

Modern sedimentation on Mars is a story of dust erosion, transport, and deposition. Monitoring of sand dunes shows little evidence for movement, and many are indurated. However, a small amount of sand must be active, as dust-raising events are common and impingement of saltating sand lowers the threshold at which clay- and silt-sized particles are set into motion. The narrow angle (0.5–15 m/pixel) and wide angle (0.23–7.5 km/pixel) cameras of the Mars Global Surveyor Mars Orbiter Camera have been monitoring Mars since September 1997, a period spanning portions of 5 Mars years. The observations provide synoptic and detailed views of meteorological events (local wind gusts, dust devils, dust storms, and dust hazes) which can alter surface albedo patterns. Dust devils are 10s of meters to > 6 km high. They can create bright and dark streaks and occur at nearly all latitudes (80°N-80°S) and at all elevations, from the summit of Olympus Mons (> 20 km), to the depths of Hellas (< -5 km). The tracks are seasonal, change over periods as short as 1 month, and appear to control some local, seasonal albedo changes. At a larger-scale, dust storms (sizes from 100s to millions of square km) occur nearly every sol, somewhere, on Mars. The largest storms, some of which generate planet-encircling dust hazes and alter regional albedo patterns, occur between Ls 135° and 340°. Dust storms can remove dust and darken surfaces; an example occurred in Gusev Crater in March 2005 when a series of storms passed through the crater at 15-25 m/s, clearing dust off the crater floor and revealing a darker (6-14%) substrate. The 2001 planet-encircling dust event, consisting of a series of local and regional dust storms, generated a haze that encircled all of Mars between 55°S and 55°N for several months. During this time, at one location near the Claritas Fossae, dust was raised every sol for 90 sols. The low albedo feature, Syrtis Major, was almost completely obscured by dust deposited during the 2001 event. A small region in Syrtis was monitored on a weekly basis thereafter. The region remained bright for several weeks and then very quickly darkened over a 2 week period, returning to its previous albedo. The removal of dust in this case was not the result of dust devil or dust storm activity, but caused by the northern winter regional surface circulation on the slopes of Syrtis Major.