Mössbauer instruments were included on the Mars Exploration Rover (MER) Mission to determine the mineralogic composition, diversity, and oxidation state of Fe-bearing igneous materials and alteration products. A total of 16 Fe-bearing phases (consistent with bulk-sample chemistry) were identified, including Fe associated with rock-forming minerals (olivine, pyroxene, magnetite, ilmenite, and chromite), Fe3+-bearing oxyhydroxides (nanophase ferric oxide, hematite, and goethite), sulfates (jarosite and an unassigned Fe3+ sulfate phase), and Fe2+ carbonate. Igneous rock types ranged from basalts to ultramafic rocks at Gusev crater. Jarosite-hematite bedrock was pervasive at Meridiani Planum, and concretions winnowed from the outcrop were mineralogically hematite. Because their structures contain hydroxyl, goethite, and jarosite provide mineralogic evidence for aqueous processes on Mars, and jarosite and Fe3+ sulfate are evidence for acid-sulfate processes at both Gusev crater and Meridiani Planum. A population of rocks on the Meridiani Planum outcrop was identified as iron and stony meteorites by the presence of Fe metal (kamacite) and the sulfide troilite. The MER mission demonstrates that Mössbauer spectrometers landed on any Fe-bearing planetary surface provide first-order information on igneous provinces, alteration state, and alteration style and provide well-constrained criteria for sample selection on planetary sample-return missions including planets, moons, and asteroids.