Chapter 10 - Mars Exploration Rover Opportunity: Water and Other Volatiles on Ancient Mars

Abstract

The Opportunity rover has explored the surface of Mars in the area of southwestern Meridiani Planum since 2004. This site was specifically chosen because of orbital evidence for a watery past, evidenced by the occurrence of abundant hematite. Opportunity demonstrated that the primary source of hematite inferred from orbit is 1–6-mm concretions, observed both within regionally exposed sandstones and as residual erosional products distributed across the plains. Sandstones of the Late Noachian to Early Hesperian Burns formation also contain high concentrations of sulfate minerals, some of which are hydrated, including jarosite. The mineralogy and chemical compositions of ancient Meridiani sandstones bear witness to the alteration of basaltic protoliths, regional precipitation of evaporites, sedimentary reworking, diagenetic alteration involving groundwater interactions, oxidation, and perhaps more recent formation of alteration rinds and veneers on exposed rock surfaces. Traverses of the interiors and ejecta of successively larger impact craters have permitted Opportunity to investigate materials from different stratigraphic levels and thus different times. In particular, exploration of materials along the rim of the 22-km-diameter Endeavour Crater has revealed older Noachian rocks that bear evidence of phyllosilicate alteration. Aqueous alteration and cation/anion mobilization along fractures related to the Endeavour impact reflect ancient aqueous processes. Its fortuitous landing site and its longevity have allowed Opportunity to explore terrain and rocks spanning the Late Noachian to Early Hesperian transition on Mars, a time when an early, relatively wet Martian surface was essentially drying out.