Global Mineral Maps on Mars

Introduction: Determining the mineralogy of Mars constitutes an essential part of understanding of the present and past condition of the surface. After two years of operation, the OMEGA imaging spectrometer on-board Mars Express has achieved near-global coverage with spatial resolution varying between 300 m and 4.8 km depending on the pericentre altitude of the spacecraft's highly elliptical orbit. We report the global surface distributions of some materials based on the OMEGA observations. Global maps of mafic minerals, hydrated minerals, nanophase ferric oxide, and surface ices have been derived from spectral parameters using the Visible and C channel wavelength domains of OMEGA (0.3-2.5μm). The distribution of these materials highlight unique as well as familiar processes that have occurred during Mars’ history. These global maps will be also useful to identify scientifically interesting regions and targets for CRISM. Observations: OMEGA operates from the visible (0.3μm) to the thermal infrared (5.2μm). This analysis focuses on Visible/NIR reflectance measurements of OMEGA (0.3-2.5μm). The OMEGA radiance data is corrected for solar irradiance and atmospheric effects [1]. The global mineral maps are realized by using the first 22 months of OMEGA operations. 1445 OMEGA cubes (i.e. 160 millions spectra) have been considered, which correspond to a coverage of about 90% of the surface at a few km spatial resolution. To avoid artifacts on the global mineral maps, we have taken into account the OMEGA reflectance data recorded when 1) the spectrometer points directly towards the planet, 2) the operation mode is nadir, and 3) the lamp of calibration is switched off. To avoid false detections, the saturated data have been also removed. Spectral parameters: To identify the minerals from the OMEGA reflectance spectra, we define spectral parameters based on the spectral characteristics of each class of materials: mafic minerals, water-bearing minerals, nanophase oxide and ices. The hydrated minerals resulting from an aqueous alteration are identified with the 1.9 μm absorption band which is attributed to a combination of an O-H stretch and H-O-H bend [2]. As the water ice has a close absorpion band centred at 2 μm, we create a water ice index using the 1.5 μm absorption band. Only the pixels showing a band at 1.9 μm without band at 1.5μm are plotted on the global hydrated minerals map in order to avoid false detection. Two global pyroxene maps are derived using a band at 2.2 μm (Fig. 1), and one at 0.85μm. The Mg-rich (forsterite) and Fe-rich (fayalite) olivine are identified by calculating the slope around respectively the 1.55 μm and 1.7 μm bands (Fig. 2).