Spectral characterization of weathering products of elemental iron in a Martian atmosphere: Implications for Mars hyperspectral studies

Visible and near-infrared spectroscopic properties have been measured on elemental iron experimentally weathered in simulated Martian atmosphere and correlated to mineralogical compositions determined by X-ray diffraction. Two main features are observed in the reflectance spectra, corresponding to two deep bands located at 0.9 and 3.1 μm, respectively the iron band and the hydration band. In early weathering stages both Fe²⁺ and Fe³⁺ bands are identified. In addition, whereas the water band position does not change with time, the Fe²⁺ band disappears, and the Fe³⁺ band shifts towards longer wavelength (from 0.88 to 0.92 μm) because of transition from Fe²⁺ phases (siderite) to Fe³⁺ phases (ferrihydrite and goethite). Apart from these spectral signatures, other bands more specific of each phase are not clearly evidenced, especially for siderite. This is due to relatively low abundance of siderite (<20 wt%), but also to the very small grain size of secondary phase as well as surface coatings of iron (oxy)hydroxides. Therefore, our results suggest that carbonates, even if not detected, could be present in the form of very small grains in the surface of Mars.