NEW GEOLOGIC DISCOVERIES OF THE OPPORTUNITY MARS ROVER MISSION

The Opportunity rover landed in Meridiani Planum, where orbital remote sensing had previously detected hematite. The rover discovered that the surface is a lag deposit of small hematite concretions that weathered out of evaporites. The intact vertical stratigraphy, mineralogy, and chemistry have been documented in the Burns formation on the walls of Endurance Crater. The formation contains three separate dune and sand sheet facies that define a “wetting upward” succession recording progressive increase in the influence of groundwater. All these materials consist of altered, laminated or cross-bedded basaltic sand cemented by large quantities of Mg, Fe, and Ca sulfates, chlorides, and possibly silica. The unusual salt mineralogy and absence of carbonate indicate evaporation of strongly acidic solutions. Vertical variations in outcrop composition are consistent with non-uniform deposition of salts from varying fluids or with a uniform reference composition modified by movement of soluble sulfates upward and halides downward. The rocks were then diagenetically altered, producing the hematite concretions. Survival of large quantities of soluble salts suggests absence of water since the ancient deposition and diagenesis of this widespread unit. After exiting the crater, Opportunity has driven south in an attempt to reach other large impact craters that may show more of the regional stratigraphy. This direction is topographically higher and thought to be stratigraphically higher than the Burns section, allowing the possibility of finding subaerial evaporites. Discoveries along the traverse include a large iron meteorite and changes in the size distribution of hematite spherules.