MARS UP CLOSE, AS SEEN BY THE MER MICROSCOPIC IMAGERS

The Athena science payload on the Mars Exploration Rovers (MER) includes the Microscopic Imager (MI), which has returned thousands of images of Mars with higher resolution than any previous camera system. Designed to simulate a geologist's hand lens, the MI is mounted on the instrument arm and can resolve objects 0.1 mm in size or larger. MI images have uniquely revealed some of the geologic processes that have affected the Martian surface.

Spirit MI observations of soil surfaces in Gusev crater are consistent with electrostatic cohesion or minor cementation of dust grains. Bedforms at Gusev have coarser particles at their crests and finer grains in the troughs, like aeolian ripples on Earth. MI observations of basalts on the floor of Gusev crater show evidence for secondary mineralization. MI observations of holes ground into Columbia Hills rocks using Spirit's Rock Abrasion Tool show a wide variety of granular textures, with clasts of sizes ranging down to the MI resolution limit. These images, along with observations by the other Athena instruments, suggest that these rocks are altered volcaniclastics and/or impact ejecta. The MI investigation of rocks and soils continues as Spirit explores the Columbia Hills.

Opportunity MI observations of soil-like materials within Eagle crater and on the surrounding Meridiani plains suggest that cementation of surface particles has formed a crust, perhaps caused by precipitation of various salts that bridge soil particles. MI images show that outcrop rocks have four principal components: sand-size grains, secondary cements, vugs, and mm-size spherules. MI images document spatial relationships among these constituents, recording a complex history of deposition and diagenesis. In places, fine-grained evaporite muds appear to have been ripped up, transported and redeposited. Sandy laminae were cemented, probably by sulfate minerals, during earliest diagenesis. Large vugs cut across bedding, indicating that the minerals that once filled them also formed diagenetically within the sediments. The MI images confirm two key features that lead to the interpretation of water having flowed at times across the surface at the landing site: centimeter-scale cross-stratification and festoon geometry of cross-lamination.