COMPARISON OF IRON METEORITES FOUND AT THREE MARS LANDING SITES — SIMILARITIES, DIFFERENCES, AND IMPLICATIONS FOR MARTIAN WEATHERING PROCESSES

As non-indigenous materials containing varying amounts of ultramafic and reduced (metallic) iron phases, with analog specimens available to Earth-based laboratories, meteorites found on Mars provide unique opportunities to study a variety of surface weathering mechanisms. The effects of both aeolian abrasion and aqueous alteration are evident within a suite of 17 known and candidate meteorites found by the Mars Exploration Rover (MER) science teams at Meridiani Planum and Gusev Crater. Crosscutting relationships between oxide coatings and abraded surfaces provide relative timing of climate-related events near the martian equator.

Recently released images collected by the Mars Science Laboratory (MSL) Mast Camera (Mastcam) and Remote Micro-Imager of the Chemistry and Camera (ChemCam) instrument targeted three meteorite candidates in Gale crater. The rocks share the color, luster and general morphology of previous iron finds, and so are accepted as meteoritic. The Gale crater samples appear similar in most respects to previously identified meteorites, but present several noteworthy distinctions:

Portions of the meteorite surfaces appear polished or varnished, exhibiting near-specular reflectance. In contrast to several Meridiani Planum irons, no obvious iron oxide coatings or Widmanstätten patterns are evident within the areas imaged by ChemCam. Other portions of the surface have undergone differential mass removal that penetrates to rock interiors. Weathering appears to attack structurally or mineralogically weak zones, excavating deeply to form overhanging cornices, circumferential voids, scallops (some within other scallops), scours, facets, and enlarged regmaglypts. Significant pitting is present across meteorite surfaces, with some pits crosscutting scallop crests. Cavernous weathering appears restricted to localized occurrences, perhaps relying more on subtle enlargement of pre-existing hollows than upon wholesale corrosive excavation, as with many Meridiani irons. All but the deepest cavities above a few centimeters appear to be sand- and dust-free, attesting to wind-removal efficiency. Assuming aeolian scouring to be a prevailing process in the Gale crater case, further study of these features could help constrain wind direction and velocity during the epochs of sculpting.