Schwertmannite on Mars? Spectroscopic analyses of schwertmannite, its relationship to other ferric minerals, and its possible presence in the surface material on Mars

Abstract

Schwertmannite, a hydroxylated ferric sulfate mineral, is presented as a possible component of the surface material on Mars. Schwertmannite could have formed on Mars through chemical weathering processes as acidic groundwater came in contact with the atmosphere. Visible and infrared spectra of schwertmannite are presented here and analyzed in context of spectra of other ferric minerals and Mars soil analogs. Infrared transmittance spectra and Moessbauer spectra of schwertmannite are studied as well to facilitate interpretation of the reflectance spectra. The visible region spectra of schwertmannite are similar to those of jarosite andakaganeite in that they are dominated by a broad Fe3+ transition near0.91 muem and a local reflectance maximum near 0.73 muem. Spectra of schwertmannite are relatively bright in the near infrared and exhibit broad bound and absorbed water features near 1.45, 1.95 and 3 muem. The mid-infrared region spectra are significantly darker and include a water feature near 6.1 muem and several weak sulfate and OH features at longer wavelengths. The formation conditions and spectral features of schwertmannite are consistent with those of the ferric sulfate phase present in chemically-treated ferric sulfate- bearing montmorillonites, which have been examined as Mars soil analogs. Methods suitable for detecting the possible presence of schwertmannite in the Martian surface soils are discussed, as well as implications for the surface geochemistry of Mars should schwertmannite occur there.