New views of Mars eolian activity, materials, and surface properties: Three vignettes from the Mars Global Surveyor Mars Orbiter Camera

Prior to the Mars Global Surveyor (MGS) mission, a very general view had em erged in which Martian surface materials were seen as consisting of a mixtu re of bright dust, dark sand, and rocks. The configuration of these materia ls and the winds that transport and deposit sand and dust have been thought to be directly linked to the albedo patterns that have been observed on Ma rs for centuries. High spatial resolution images (1.4-20 m/pixel) obtained by the MGS Mars Orbiter Camera (MOC) between September 15, 1997, and July 4 , 1999, provide new information about the physical nature of the windblown material on the Martian surface and show that the pro-MGS view was much too simple. In addition to bright dust and dark sand, MOC images show evidence of bright sediment that can be transported by saltation (e.g., sand) and d ark material that can be transported in suspension (e.g., silt). New views of eolian wind streaks in Daedalia Planum show that part of this region, th ought to be mantled by bright dust based upon Viking and Mariner observatio ns, is instead covered by a thin veneer of bright, windblown sand. MOC imag es of Sinus Sabaeus and parts of Syrtis Major, two regions thought to be sa ndy based upon Viking era observations, exhibit thick mantles that are infe rred to consist of fine-grained sediment deposited from eolian suspension. Low albedo wind streaks in western Arabia Terra are also dark mantles, and their association with eroded crater floors and megaripples/dunes on these floors suggest that these particular wind streaks are deposits of silt-size d material that was only briefly suspended before settling to the surface. MOC images also show evidence that some eolian dunes are active on Mars tod ay (i.e., in 1998 and 1999); the evidence for activity is largely based upo n wind- and avalanche-induced streaks superposed on or eroded into seasonal frost on high-latitude dune fields. MOC images shaw that some other dunes are inactive, but the albedo of dunes relative to surrounding terrain is no t a good indicator of dune activity because some inactive dunes are not man tled by dust. Inactive dunes are best identified by superposed features suc h as impact craters, landslide deposits, and yardangs.