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Mössbauer spectrometers provide quantitative information about the distribution of iron among its oxidation states, the identification of iron-bearing phases, and the distribution of iron among those phases. Characterizing the speciation and distribution of iron in martian rock and soil [1] constrains primary rock types; the conditions under which primary minerals crystallize; and the mineralogical composition, process, and extent of alteration and weathering. Hematite (α-Fe[sub 2]O[sub 3]) was detected in Meridiani Planum from orbital observations before the Mars Exploration Rover (MER) missions [2, 3], and the region was selected as a MER landing site on a scientific basis, because the mineral is a beacon for aqueous processes, and on an engineering basis, because the terrain was considered safe for landing [4]. The first Mössbauer spectrum from Meridiani Planum was obtained by the Opportunity rover on 4 February 2004, on soil near the lander at Eagle crater. [ABSTRACT FROM AUTHOR]

Mössbauer spectra measured on Mars by the Spirit rover during the primary mission are characterized by two ferrous iron doublets (olivine and probably pyroxene) and a ferric iron doublet (tentatively associated to nanophase ferric iron oxide). Two sextets resulting from nonstoichiometric magnetite are also present, except for a coating on the rock Mazatzal, where a hematite-like sextet is present. Greater proportions of ferric-bearing phases are associated with undisturbed soils and rock surfaces as compared to fresh rock surfaces exposed by grinding. The ubiquitous presence of olivine in soil suggests that physical rather than chemical weathering processes currently dominate at Gusev crater. [ABSTRACT FROM AUTHOR]