U15A-01 INVITED 18:30h
Sedimentary Rocks and Evidence for Aqueous Environment on the Surface of Mars
On January 24, 2004 the Mars Exploration Rover Opportunity landed at Meridiani Planum. The landing region lies on a broad, flat plain near the martian equator. The landing site itself is within an impact crater about 20 meters in diameter which exposes small rock outcrops along its northwestern rim. As of September 1, 2004, the Opportunity rover has explored the outcrops exposed within Eagle crater, along with much larger outcrops exposed almost continuously along the rim of Endurance crater, about 175 m in diameter and approximately 750 m away from Eagle crater. The intervening plains exposed additional outcrop within a regionally-pervasive fracture system, and within a smaller impact crater of just a few meters diameter. All outcrops studied to date from these differing localities indicate the presence of regionally extensive, lithified sedimentary materials consisting of fine-grained siliciclastic sediments derived from basaltic source rocks, admixed and cemented by abundant sulfate minerals and hematite. These include the hydrated sulphate mineral jarosite, in addition to Mg-sulphate. Cross-stratification provides evidence for both eolian and aqueous transport. Subsequent alteration of these rocks produced hematite-rich concretions and vuggy porosity that is pseudomorphic after probable sulphate evaporite minerals. These combined observations indicate episodic inundation by surface water to shallow depths, followed by evaporation, exposure and desiccation. Festoon cross-lamination provides evidence for inundation by water, the mineralogy and geochemistry indicate evaporation of water and precipitation of dissolved salts, and the planar to low-angle lamination and larger scale cross-bedding are consistent with sediment transport across a dry surface. Terrestrial analogs for such a suite of facies and surface processes include interdune depressions, playa lakes, and sabkhas adjacent to marginal seaways. The primary objective of the Mars Exploration Rover mission is to search for evidence in the martian geologic record of environmental conditions that might once have been suitable for life. The results obtained by the Athena Science Team demonstrate that liquid water, regarded as a key condition for life, was once abundant at Meridiani Planum. Because evaporite sediments can easily entomb micro-organisms, Meridiani Planum is a significant target for future landed or sample return missions aimed at life detection.