A MARINE ORIGIN FOR THE OPPORTUNITY LANDING SITE IN MERIDIANI PLANUM

Opportunity has driven over 40km across the plans at Meridiani Planum. Stratigraphic exposures up to 10m thick were documented in Endurance and Victoria Craters. The bulk of these exposures consist of thick cross-bedded sulfate-rich sandstones overlain by eolian sulfate sand sheets that were altered by rising and falling water tables. In Endurance, these appear to transition vertically into aqueous ripples that have been interpreted to indicate interdune playa settings associated with rises in the water table above ground locally. The rover traverse crosses the modern eolian sand sheets and ripple fields and the subjacent surface of the Burns Formation where it has been exposed via “windows” in the eolian materials. The 40km traverse includes ~ 80m of elevation change, but the consistent morphology of the outcrop surface and gradual changes in elevation suggest the last process to act on the Burns Fm did so over the entire region – there are no mappable separate dune and interdune playa settings. The surface of the Burns Fm exhibits volume-loss cracking (desiccation?) at centimeter-, meter-, decameter-, and hundreds of meter-scales across the region, implying a lithologic homogeneity over much of the several hundred meter thick deposit.

The Opportunity landing site includes a late Noachian to Early Hesperian population of degraded craters that must have formed within the deposit (at a few hundred meters to a few kilometers in diameter, they’re too small to be protruding from beneath it), as well as a number of larger craters (10-20km) that either clearly protrude from beneath the Meridiani deposits (Endeavour) or are superposed on all but the uppermost surface of the deposits (Bopolu, Iazu). We propose that the Meridiani deposits accumulated in a shallow marginal marine environment during a transgression of a northern plains ocean, associated with the Late Heavy Bombardment. After the late heavy bombardment ended, the ocean rapidly regressed.