The Alpha-Particle X-ray Spectrometer (APXS) is part of the scientific payload of all four Mars rovers to date. It determines the chemical composition of rocks and soils using X-ray spectroscopy during irradiation with alpha particles and X-rays from 244 cm. All elements heavier than fluorine can be detected by their characteristic X-ray lines. Typically, 16 elements are quantified for each martian sample. An additional 10 trace elements can be quantified for unusual high abundances. The APXS has provided compositional data at 4 landing sites, analyzing more than 1000 samples along a combined traverse of ~70 km. The diverse composition of soils and rocks has provided insights about martian geology and environmental conditions. Soils at all landing sites are similar and basaltic, but enriched in S, Cl, and Zn, likely from volcanic exhalations. A variety of igneous rocks have been documented. High sulfur concentrations in Ca sulfate veins, ferric sulfate subsurface soil deposits, and the extensive Burns formation with ~30% sulfate indicate extensive interactions with acidic fluids in the past. APXS bulk geochemistry complements mineralogy data and images and delivers crucial constraints for the interpretation of other investigations, like ground truth for orbital remote sensing instruments or comparison with martian meteorites.

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.