Geology and Geochemistry of Noachian Bedrock and Alteration Events, Meridiani Planum, Mars: MER Opportunity Observations

Abstract

We have used Mars Exploration Rover Opportunity data to investigate the origin and alteration of lithic types along the western rim of Noachian-aged Endeavour crater on Meridiani Planum. Two geologic units are identified along the rim: the Shoemaker and Matijevic formations. The Shoemaker formation consists of two types of polymict impact breccia: clast rich with coarser clasts in upper units; clast-poor with smaller clasts in lower units. Comparisons terrestrial craters show that the lower units represent more distal ejecta from at least two earlier impacts, and the upper units are proximal ejecta from Endeavour crater. Both are mixtures of target rocks of basaltic composition with subtle compositional variations caused by differences in post-impact alteration. The Matijevic formation and lower Shoemaker units represent pre-Endeavour geology, which we equate with the regional Noachian subdued cratered unit. An alteration style unique to these rocks is formation of smectite and Si- and Al-rich vein-like structures crosscutting outcrops. Post-Endeavour alteration is dominated by sulfate formation. Rim-crossing fracture zones include regions of alteration that produced Mg-sulfates as a dominant phase, plausibly closely associated in time with the Endeavour impact. Calcium-sulfate vein formation occurred over extended time, including before the Endeavour impact and after the Endeavour rim had been substantially degraded, likely after deposition of the Burns formation that surrounds and embays the rim. Differences in Mg, Ca and Cl concentrations on rock surfaces and interiors indicate mobilization of salts by transient water that has occurred recently and may be ongoing.