SMECTITE CLAY DEPOSITS IN MARATHON VALLEY, ENDEAVOUR CRATER, MARS

Smectites have been identified on Mars using orbital hyperspectral data throughout Noachian-aged deposits, although until recently these deposits were not accessible by landed missions. The Mars Exploration Rover Opportunity has been exploring the rim of Endeavour Crater, a 22-km wide Noachian impact crater in Meridiani Planum, providing the first in-situ observations of altered ancient terrains. Five overlapping MRO/CRISM observations, including both Full Resolution Targeted (FRT) mode observations and Along-Track Oversampled observations (ATO), indicate the presence of Fe and Mg smectites in the upper portion of Marathon valley, an erosional feature that cross-cuts the Cape Tribulation segment of Endeavour’s rim. Smectite minerals were identified and mapped based on Fe-OH and Mg-OH combination absorptions centered at 2.3 and 2.4 µm. The exposures are relatively dehydrated, based on the fact that the 1.4 µm OH overtone absorption is missing and the 1.9 µm OH and H₂O combination absorption, typical of hydrated minerals, is significantly reduced relative to other detections using CRISM data. The five data sets were radiometrically modeled to remove atmospheric aerosol and gas scattering and absorption, and the Hapke model was used to retrieve surface single scattering albedo from 0.45 – 2.65 µm. Each scene was regularized (18 m/pixel for FRTs and 12 m/pixel for ATOs) and smoothed using a maximum log likelihood algorithm to retrieve signal in the presence of Poisson noise and carefully inspected to detect and map the 2.3 and 2.4 µm absorptions. Based on Opportunity images as of early August 2015 the upper floor of Marathon valley is dominated by bright, fractured bedrock exposures free of soil and sand cover. Opportunity is in Marathon valley beginning an extensive campaign to understand the geologic and geochemical setting of the smectite exposures, which extend over ~1000 m². The combination of orbital and landed data thus provides an opportunity to understand in detail the nature and extent of aqueous alteration during the Noachian Period.