NOACHIAN-AGED PRE-IMPACT LITHOLOGY EXPOSED IN ENDEAVOUR CRATER RIM: MARS EXPLORATION ROVER OPPORTUNITY OBSERVATIONS

Mars Exploration Rover Opportunity is exploring the western rim of 22 km diameter, Noachian-aged Endeavour Crater. The region is equivalent to the tectonic rim of complex craters such as the Ries Crater. Degradation by multiple processes has modified the original post-impact topography of Endeavour Crater [1]. Marathon and Perseverance Valleys cut the rim providing windows into the lower stratigraphic record. Endeavour’s rocks are polymict breccias of basaltic composition composed of 1-10 cm-sized darker clasts in a brighter matrix (Shoemaker formation). Two texturally distinct breccias are evident: a lower, clast-poor unit with generally smaller clasts; an upper, clast-rich unit with generally coarser clasts [2]. At the Ries Crater, polymict breccias outside the tectonic rim show no systematic variation in grain size with height in the deposit, but mean clast size and abundance decrease with radial distance outward [3]. The stratigraphy in the Marathon and Perseverance Valley regions is thus consistent with a lower unit of more distal crater ejecta overlain by a unit of more proximal ejecta. The lower breccia unit on the floor of Marathon Valley hosts alteration styles not observed in the upper breccia unit: (i) areally extensive (tens of m) occurrences of Fe-Mg smectite observed from orbit [4], and (ii) enrichments in Si, Al, Ti and Cr in rock within curvilinear fractures. The only other known occurrence of smectites and Si-Al enrichments on the rim are hosted by the pre-impact Matijevic formation [5], roughly 5 km north of Marathon Valley. We conclude that the upper unit that forms the topographic rim is Endeavour Crater ejecta while the lower unit is older eroded ejecta from an unidentified crater. An unconformity separates them. The lower unit is consistent with an origin within the Early to Middle Noachian highlands subdued crater unit that underlies the aeolian Meridiani etched plains units [6].

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[6] Hynek B. M., & Di Achille G. (2017). U.S. Geol. Survey Sci. Invest. Map 3356, doi:10.3133/sim3356