Low-Temperature Reflectivity Spectra of Ferric Mineralogies: Implications for Identification of Mineral Phases on Mars

Comparison of remote sensing data for Mars at visible and near-IR wavelengths to corresponding data for hematite and hematite-bearing materials has led to the inference that red (well-crystalline and pigmentary) hematite is a subordinate pigmenting phase on the optical surface of Mars where the hematite band minimum near 850-870 nm is present. We have recently shown that low-temperature spectra of synthetic hematite powders and hematite-bearing natural samples are consistent with the identification of hematite on Mars. In fact, the significant increase in reflectivity near 600 nm observed for red hematite with decreasing temperature is additional evidence for its presence on the planet. To rigorously demonstrate that the Martian band near 850-870 nm does not result from other ferric mineralogies, it is necessary to know their spectral properties at low-temperatures. Reported here are low-temperature reflectivity data for eight additional ferric mineralogies: maghemite, goethite, lepidocrocite, akaganeite, ferrihydrite, jarosite, schwertmannite, and nontronite.