The Mössbauer (MB) spectrometer on Opportunity measured the Fe oxidation state, identified Fe‐bearing phases, and measured relative abundances of Fe among those phases at Meridiani Planum, Mars. Eight Fe‐bearing phases were identified: jarosite (K,Na,H3O)(Fe,Al)(OH)6(SO4)2, hematite, olivine, pyroxene, magnetite, nanophase ferric oxides (npOx), an unassigned ferric phase, and metallic Fe (kamacite). Burns Formation outcrop rocks consist of hematite‐rich spherules dispersed throughout S‐rich rock that has nearly constant proportions of Fe3+ from jarosite, hematite, and npOx (29%, 36%, and 20% of total Fe). The high oxidation state of the S‐rich rock (Fe3+/FeT ∼ 0.9) implies that S is present as the sulfate anion. Jarosite is mineralogical evidence for aqueous processes under acid‐sulfate conditions because it has structural hydroxide and sulfate and it forms at low pH. Hematite‐rich spherules, eroded from the outcrop, and their fragments are concentrated as hematite‐rich soils (lag deposits) on ripple crests (up to 68% of total Fe from hematite). Olivine, pyroxene, and magnetite are primarily associated with basaltic soils and are present as thin and locally discontinuous cover over outcrop rocks, commonly forming aeolian bedforms. Basaltic soils are more reduced (Fe3+/FeT ∼ 0.2–0.4), with the fine‐grained and bright aeolian deposits being the most oxidized. Average proportions of total Fe from olivine, pyroxene, npOx, magnetite, and hematite are ∼33%, 38%, 18%, 6%, and 4%, respectively. The MB parameters of outcrop npOx and basaltic‐soil npOx are different, but it is not possible to infer mineralogical information beyond octahedrally coordinated Fe3+. Basaltic soils at Meridiani Planum and Gusev crater have similar Fe‐mineralogical compositions.

The Alpha Particle X-ray Spectrometer on the Opportunity rover determined major and minor elements of soils and rocks in Meridiani Planum. Chemical compositions differentiate between basaltic rocks, evaporite-rich rocks, basaltic soils, and hematite-rich soils. Although soils are compositionally similar to those at previous landing sites, differences in iron and some minor element concentrations signify the addition of local components. Rocky outcrops are rich in sulfur and variably enriched in bromine relative to chlorine. The interaction with water in the past is indicated by the chemical features in rocks and soils at this site.