Aeolian Processes at Meridiani Planum

The Opportunity Rover has observed a variety of aeolian features at Meridiani Planum. These features imply wind-related processes that operate on a range of time-scales, so that at least a relative time-scale of formation and modification can be assigned to many. (a) Features forming and changing in the shortest time include probable impact ripples (cm-scale) that occur in dark basalt sand on the floors of local depressions throughout the plains. Also in this category are deposits of bright airfall dust (in the form of streaks) that are not removed downwind of topographic features such as crater rims. Analysis of MOC imagery indicates that streaks change orientation after intense dust storms. The similar orientations of impact ripples and bright streaks are thought to indicate the prevailing direction of the most-recent vigorous wind regime. (b) Forming and changing on a longer, intermediate time-scale (and hence older) are deflationary ripples armored with well-sorted mm-sized hematitic grains that likely propagate by creep (i.e., pushed by the impacts of smaller saltating grains). The removal of dust from the plains during storms implies saltation of sand ( ∼ 100 μ m) which indicates shear velocities approaching what is required to roll the ∼ 1 mm hematitic grains. The ``plains ripples'' rarely occupy local depressions and cover the plains at Meridiani Planum ( ∼ 1 cm tall, ∼ 10 cm wide, up to ∼ 1 m long). Plains ripples indicate two prominent orientations: the orientation of individual bedforms as well as that of en-echelon ripple trains. As deflationary structures that form by the winnowing of small grains, these features require relatively long periods to form. The presence of multiple orientations indicates that, once formed, multiple episodes of vigorous winds may be required to change markedly the orientations of these features. (c) Requiring probably the longest formation times are tails of protected rock downwind of hematite spherules embedded in outcrop. The spherule-tails are thought to develop during periods of intense sand-blasting of the rock surface, and may indicate the consequence of wind abrasion over very long periods (i.e., many intense storms). These tails possibly indicate mean wind directions over long periods, and therefore the directions to sources of sediments deposited at Meridiani, or deposition-zones for sediments removed from Meridiani. In addition to these features, Opportunity has observed a collection of dunes at the bottom of Endurance crater. These basalt-sand dunes occur in a cellular pattern with star-shaped junctions and solitary arms that extend several meters from the main sand body. The cellular arrangement implies a complex pattern of winds in multiple directions at the crater floor. High cohesion of the sands is implied by lineations in dune faces that suggest cross-bedding. Red-filter observations indicate the presence of bright dust along crest-lines, implying that these are not currently active. Slip faces are steep, indicating minimal degradation. Well-defined high-frequency ripples that occur on or adjacent to most dune faces are interpreted as potentially active impact ripples.