Spectral Characterization of Weathering Products of Elemental Iron in a Martian Atmosphere: Implications for Mars Hyperspectral Studies

Introduction: Imaging spectroscopy is one of the most powerful methods to characterize the surface of Mars, either from orbit or in situ using rovers. These observations have shown that iron chemistry and mineralogy dominates most of the surface processes. We have described in [1] a laboratory experiment in which elemental iron particles have been exposed to weathering in a simulated (CO 2 + H 2 O) Martian atmosphere, using conditions close to those originally present on Mars. We present here the visible and near-infrared spectra of the alteration products obtained during these iron weathering experiments. Our main objective is to study the evolution of the iron spectral features during weathering, with a particular interest in the evolution of the oxidation state of the iron from 0 to III. Moreover , as the alteration products are fully characterized in terms of mineralogy, grain size, and composition, we can examine the effects of these parameters on the resulting spectral features. Protocol: 10 g of iron powder was put in a dessic-cator previously filled with 1L of pure water. The des-iccator was then equilibrated with gaseous CO 2 at the initial pressure of 0.8 bar and the temperature was controlled to remain in the range 15-20°C. and then analyzed with X-ray diffraction and Rietveld refinement to determine the mineralogical composition at each step (Fig. 1). Reflectance spectra of the corresponding samples have been acquired in the 0.7 to 4.7 µm spectral domain (Fig. 2). Results: Elemental iron α-Fe undergoes a strong evolution with about 75% of the initial mass converted into secondary products after 259 days and almost no further evolution until 805 days (Fig. 1). The first phase to appear was siderite (FeCO 3), which was already observed after ~40 days. Siderite is unstable in the experimental conditions and transforms progressively into goethite (α-FeOOH). In addition, the transformation of siderite into goethite involves ferrihydrite as an intermediate phase. This ferrihydrite can become itself a dominant product, mainly depending on the transformation kinetics of each phase. Two main features are observed in the reflectance spectra, corresponding to two deep absorption bands located at 0.9 and 3.1 µm, respectively the iron band and the hydration band (Fig. 2). The spectral evolution of the 0.9-1 µm band is shown in Fig. 3. A global increase of the reflectance with time is observed (fig. 3a), probably resulting from a general decrease of the