An Overview of Moessbauer Mineralogy at Gusev Crater, Mars

The Mars Exploration Rover (MER) Spirit landed on the plains of Gusev Crater on 4 January 2004 [1]. The scientific objective of the Moessbauer (MB) spectrometer on Spirit is to provide quantitative information about the distribution of Fe among its oxidation and coordination states, identification of Fe-bearing phases, and relative distribution of Fe among those phases. The speciation and distribution of Fe in Martian rock and soil constrains the primary rock types, redox conditions under which primary minerals crystallized, the extent of alteration and weathering, the type of alteration and weathering products, and the processes and environmental conditions for alteration and weathering.We discuss the Fe-bearing phases detected by Spirit s MB instrument during its first 540 sols of exploration [2,3]. Spirit roved eastward across the plains from its landing site to the Columbia Hills during the first approx.150 sols. Rocks are unweathered to weakly weathered olivine basalt, with olivine, pyroxene (Ol > Px), magnetite (Mt), and minor hematite (Hm) and nanophase ferric oxide (npOx) as their primary Fe-bearing minerals. Soils are generally similar basaltic materials, except that the proportion of npOx is much higher (up to approx.40%). NpOx is an oct-Fe3+ alteration product whose concentration is highest in fine-grained soils and lowest in rock interiors exposed by grinding with the Rock Abrasion Tool (RAT). Spirit explored the lower slopes of the Columbia Hills (West Spur) during sols approx.150-320. West Spur rocks are highly altered, even for interior surfaces exposed by grinding (Fe3+/FeT approx.0.56-0.84). High concentrations of npOx, Hm, and Mt are present. One rock (Clovis) contains significant quantities of goethite (alpha-FeOOH; approx.40% of total Fe). The detection of goethite is very significant because it is a mineralogical marker for aqueous alteration.