MER OPPORTUNITY FIELD GEOLOGIC MAPPING RESULTS AT ENDEAVOUR CRATER, MARS

Between sols 2681 (August, 2011) and 3315 (June, 2013) Mars Exploration Rover Opportunity completed a 2.6-kilometer circumnavigation and traverse into the interior of "Cape York", a rim segment of the eroded 22-kilometer diameter Martian complex crater Endeavour. The textures, structures, min­eralogy, chemistry, and lithologies of outcrops were systematically examined in situ at a dozen outcrops during Opportunity’s traverse. We are able to define, correlate from site to site, and map (1) at least four distinct bedrock units related to pre-Endeavour and post Endeavour materials, (2) impactites and crater rim structures associated with the formation of Endeavour crater, and (3) at least two significant unconformities. Both local and regional impactites are coarsely clastic lithologies (informally named the “Shoemaker Formation”) bearing fragments of several generations of basaltic substrate. Significant changes are present in rock characteristics across the unconformities, such as gypsum veins in rocks pre-dating the later Meridiani sulfate-rich sandstones, and the absence of veins thereafter. Together with other observations of stratigraphically lower rocks, these results are ground truth for the environmental changes on early Mars identified from photogeologic mapping, such as the reduction in water availability and overall change in climate between the Noachian valley network time and later Meridiani Planum sandstone. The methods used were basic terrestrial field geologic mapping techniques: Opportunity determined the lithology and relative stratigraphic positions of widely separated outcrops, compared rocks and outcrops from site to site, correlated, and mapped on a regional base map. The actual geologic mapping was restricted to within the nominal limit of reliable rover-based terrain characterization, which we have found is within approximately 20 m of the traverse. The resulting narrow strip map is a compromise of mission goals, restrictions on the number of geologic tests that can be done, and limitations on the number samples and sites that can be visited. The final result is a reconnaissance level geologic map recording the occurrences of each lithology and information regarding the relative sequence of events, and, in some cases, subsequent deformation or alteration events.