Martian surface mineralogy from Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activite on board the Mars Express spacecraft (OMEGA/MEx): Global mineral maps

[1] After 2 years of operation the Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activite (OMEGA) imaging spectrometer on board Mars Express has acquired data coverage of the Martian surface with spatial resolution varying between 300 m and 4.8 km, depending on the pericenter altitude of the spacecraft's elliptical orbit. We report the global surface distributions of some minerals using the OMEGA observations in the visible and near infrared (VNIR) wavelength domains (0.35-2.5 mu m). Global maps of ferric phases, mafic minerals (pyroxenes and olivines), and hydrated minerals have been derived from spectral parameters. The limits of detection in terms of abundance for some minerals of different grain size distributions are given. The distribution of pyroxenes is in general agreement with the mineral maps of previous telescopic and space observations. The Fe3+ absorption feature in the visible wavelength region is present everywhere on the surface. The spectra of the bright regions compare with anhydrous nanophase ferric oxides. Terrains with water-bearing minerals cover a very small fraction of the Martian surface. Olivine (Mg-rich compositions) is detected in more extensive regions of the pyroxene-rich zones than previously reported. Olivine with higher iron content and/or larger grain size (> 100 mu m) is only detected in isolated areas. The mineralogy of the northern low-albedo regions is discussed in the light of these mineral maps. Chemical alteration or oxidation during extrusion producing a coating or varnish of anhydrous ferric phases over a dark basaltic surface best accounts for the VNIR spectral properties of these regions, although a glassy composition resulting from impact is also considered.