The Mössbauer spectrometer on Spirit measured the oxidation state of Fe, identified Fe‐bearing phases, and measured relative abundances of Fe among those phases for surface materials on the plains and in the Columbia Hills of Gusev crater. Eight Fe‐bearing phases were identified: olivine, pyroxene, ilmenite, magnetite, nanophase ferric oxide (npOx), hematite, goethite, and a Fe3+‐sulfate. Adirondack basaltic rocks on the plains are nearly unaltered (Fe3+/FeT < 0.2) with Fe from olivine, pyroxene (Ol > Px), and minor npOx and magnetite. Columbia Hills basaltic rocks are nearly unaltered (Peace and Backstay), moderately altered (WoolyPatch, Wishstone, and Keystone), and pervasively altered (e.g., Clovis, Uchben, Watchtower, Keel, and Paros with Fe3+/FeT ∼ 0.6–0.9). Fe from pyroxene is greater than Fe from olivine (Ol sometimes absent), and Fe2+ from Ol + Px is 40–49% and 9–24% for moderately and pervasively altered materials, respectively. Ilmenite (Fe from Ilm ∼3–6%) is present in Backstay, Wishstone, Keystone, and related rocks along with magnetite (Fe from Mt ∼10–15%). Remaining Fe is present as npOx, hematite, and goethite in variable proportions. Clovis has the highest goethite content (Fe from Gt = 40%). Goethite (α‐FeOOH) is mineralogical evidence for aqueous processes because it has structural hydroxide and is formed under aqueous conditions. Relatively unaltered basaltic soils (Fe3+/FeT ∼ 0.3) occur throughout Gusev crater (∼60–80% Fe from Ol + Px, ∼10–30% from npOx, and ∼10% from Mt). PasoRobles soil in the Columbia Hills has a unique occurrence of high concentrations of Fe3+‐sulfate (∼65% of Fe). Magnetite is identified as a strongly magnetic phase in Martian soil and dust.

Mossbauer spectra measured by the Opportunity rover revealed four mineralogical components in Meridiani Planum at Eagle crater: jarosite- and hematite-rich outcrop, hematite-rich soil, olivine-bearing basaltic soil, and a pyroxene-bearing basaltic rock (Bounce rock). Spherules, interpreted to be concretions, are hematite-rich and dispersed throughout the outcrop. Hematitic soils both within and outside Eagle crater are dominated by spherules and their fragments. Olivine-bearing basaltic soil is present throughout the region. Bounce rock is probably an impact erratic. Because jarosite is a hydroxide sulfate mineral, its presence at Meridiani Planum is mineralogical evidence for aqueous processes on Mars, probably under acid-sulfate conditions.