Despite in situ analyses by several landers, the composition of the Martian soil remains unclear. Recently, constituent minerals in the soil have been proposed from thermal emissivity and M¿ssbauer spectroscopic data from Mars. These data were interpreted to show spectral evidence for carbonate and an unspecified mineral containing bound water, as well as olivine. Here we present an alternative interpretation, suggesting that these spectral features can be satisfied by a single type of mineral, namely hydrous iron sulfates. Acidic waters necessary for production of hydrous iron sulfates would preclude carbonate formation and indicate a dramatically different chemical environment on Mars, in which olivine would be highly unstable and unlikely to survive soil formation, regardless of its presence in the rocks. Hydrous iron sulfates are efflorescent salts and may be crusting agents in the Martian soils.