Ferric sulfates on Mars

Evidence is presented for the possible formation and existence of ferric sulfato complexes and hydroxo ferric sulfate minerals in the permafrost on Mars. Acidic groundwater, derived from atmospheric oxidation of volcanogenic H 2 S to H 2 SO 4 aerosols, promoted chemical weathering of fayalitic olivines, iron‐rich pyroxenes, plagioclase feldspar, and pyrrhotite‐pentlandite mineral assemblages in crustal ultramafic and basic igneous rocks. The acidic groundwater entered into electrochemical reactions with the iron sulfides, yielding dissolved FeSO 4 + , Fe(SO 4 ) 2 − , and FeOH 2 + complex ions, and the precipitation of basic ferric sulfate minerals such as those belonging to the roemerite, copiapite, botryogen, and jarosite‐alunite groups. These phases are stabilized at low temperatures and pH in Martian permafrost. The occurrence of jarosites in terrestrial arid regions suggests that they could also survive on the surface of Mars. Melting of the permafrost and raising of the pH may have initiated the hydrolysis of dissolved ferric sulfato complex ions and led to the precipitation of FeOOH, which reacted with precipitated silica to form phyllosilicates. Alternatively, degradation of the hydrolysate FeOOH to Fe 2 O 3 during sublimation of permafrost exposed on Mars' surface may account for the presence of eolian maghemite suspected to be the magnetic mineral observed on the Viking Landers.