Reflectance and Mössbauer spectroscopy of ferrihydrite-montmorillonite assemblages as Mars soil analog materials
Spectroscopic analyses show that Fe 3+-doped smectites prepared in the laboratory exhibit important similarities to the soils on Mars. Ferrihydrite has been identified as the interlayer ferric component in Fe 3+-doped smectites by a low quadrupole splitting and magnetic field strength of ~ 48 tesla in Mössbauer spectra measured at 4.2 K, as well as a crystal field transition at 0.92 μm. Ferrihydrite in these smectites explains features in the visible-near infrared region that resemble the energies and band strengths of features in reflectance spectra observed for several bright regions on Mars. Clay silicates have met resistance in the past as Mars soil analogs because terrestrial clay silicates exhibit prominent hydrous spectral features at 1.4, 1.9, and 2.2 μm; and these are observed weakly, if at all, in reflectance spectra of Mars. However, several mechanisms can weaken or compress these features, including desiccation under low-humidity conditions. The hydration properties of the interlayer cations also effect band strengths, such that a ferrihydrite-bearing smectite in the Martian environment would exhibit a 1.9 μm H 2O absorption that is even weaker than the 2.2 μm structural OH absorption. Mixing experiments demonstrate that infrared spectral features of clays can be significantly suppressed and that the reflectance can be significantly darkened by mixing with only a few percent of a strongly absorbing opaque material. Therefore, the absolute reflectance of a soil on Mars may be disproportionately sensitive to a minor component. For this reason, the shape and position of spectral features and the chemical composition of potential analogs are of utmost importance in assessing the composition of the soil on Mars. Given the remarkable similarity between visible-infrared reflectance spectra of soils in bright regions on Mars and Fe 3+-doped montmorillonites, coupled with recent observations of smectites in SNC meteorites and a weak 2.2 μm absorption in some Mars soils, ferrihydrite-bearing smectites warrant serious consideration as a Mars soil analog.