HAND-LENS SCALE TEXTURAL FEATURES PRODUCED BY AOELIAN EROSION OF MUDSTONES

On Earth, mudstones are among the most easily eroded sedimentary rocks because of their lack of cementation and their softness. In the presence of water, hydration of clays and chemical reactions of unstable minerals hasten deterioration to such a degree that aeolian erosion features have no chance to survive past the next wetting of the surface. On Mars, in contrast, mudstones should show extensive sculpting by aeolian processes. In order to better understand how mudstones might be modified by aeolian erosion, we conducted experiments on two types of mudstones. Using commercial sandblasting equipment and medium size quartz sand, we eroded shale samples to a depth of up to 10 mm. Observed features were recorded with a digital camera at a resolution of ~30 microns per pixel, a resolution comparable to the microscopic imager (MI) of the MER rovers.

Features include erosional fluting and pedestal concretions that have direct analogs to pictures taken by the Opportunity rover. In general, aeolian erosion of mudstones produces smooth flowing surface contours, scalloped surfaces, etching out of fractures, and enhances lamina contrast. A shallow light incident angle is very helpful for enhancing features of subtle relief.

Frosting of the abraded surface by the impact of windblown grains is a universal feature of aeolian erosion and well known from the surface of quartz grains and hard rock surfaces. In mudstones, the impact of sand grains leads to a dimpling of the surface at the grain size scale of the sandblasting medium. As a result, an aolian eroded mudstone surface may show close textural resemblance to the surface of a sandstone. At the hand-lens scale, however, and at a shallow incident angle, a dimpled mudstone surface can be differentiated from the bumpy surface of a sandstone. In the latter case shadows form next to projecting sand grains, whereas in case of a dimpled mudstone surface the shadows are found in the interior of the dimples. Although to date there has been no definite report of Martian mudstones from the MER rovers, re-examination of MER MI images for above features may reveal likely mudstones among the rocks they examined.