Evidence for Impact-induced Hydrothermal Clay Mineral Formation At

Introduction: Over 13 years after landing at Meridiani Planum and in her 10 extended mission, Mars Exploration Rover Opportunity continues to explore the rim of ~22 km diameter Endeavour crater [1]. The rim exposes material that is older than the S-rich, layered sedimentary rock covering the Meridiani plains and filling much of the interior of Endeavour. Clay mineral exposures at the rim have been observed from orbit [2,3] and confirmed on the ground [4-6]. Clay minerals are thought to indicate wet, generally habitable environmental conditions more suitable to life than the S-rich, acidic waters responsible for the Meridiani sedimentary rocks. However, in order to assess the habitability of the past environment recorded by the Endeavour rim rocks it is important to understand how the clay minerals formed. Do they represent the conditions prior to impact; or do they result from impactinduced hydrothermal activity; or both? Here we present evidence that at least some of the observed clay mineral assemblages formed as a result of impactinduced hydrothermal alteration. Results and Discussion Geologic setting. Endeavour is a ~22 km diameter Noachian impact crater (Fig. 1). It is mostly filled with the S-rich sedimentary rock of the Hesperian Burns formation, which covers the Meridiani plains. Burns formation rocks have buried the potential central uplift as well as large tracts of the crater rim. Only isolated segments of the rim protrude through the Burns formation. Of these, Opportunty investigated Cape York, Murray Ridge, and Cape Tribulation at the western edge of the crater (Fig. 1). The rim is mostly made up of the clast-bearing Shoemaker formation impact breccia. The oldest stratigraphic unit so far is the Matijevic formation, which has to date only been observed at Cape York and is thought to predate the formation of Endeavour. Finding more of Matjevic formation or a lower stratigraphic unit is one of Opportunity’s 10 extended mission goals. Description of the observed clay mineral assemblages. Orbital detections of clay minerals at distinct locations at the Endeavour crater rim reveal Fe and Mg smectites [2,3]. Unfortunately, Opportunity’s mineralogical instrument were no longer operating, and these smectites had to be located on the ground on the basis of APXS-derived geochemical data alone [4-6]. Yet, the ground observations revealed a more complex clay mineral story by identifying Al-rich clays not observed from orbit [4-6]. These are associated with fracture fills and zones and suggest multiple episodes of aqueous alteration at Endeavour [6].