A new view of dark Martian regions from geomorphic and spectroscopic analysis of Syrtis Major

New analysis of Mars Observer Camera (MOC) images and Thermal Emission Imaging System (THEMIS) data focussed on West Syrtis Major show that this region, usually interpreted as typical of in-situ bedrocks, is covered by a dark mantle which could result from a transient deposition of suspended fine particles still active until a few million years ago. To help address the questions of the nature of these low albedo regions covered by blankets, we revisited the surface composition by modelling Imaging Spectrometer (ISM) spectra with a radiative transfer model, the strength of such a modelling method being to aid in determining appropriate endmembers as well as their relative abundance, their grain size, and the type of mixtures (sand/dust). Two distinct surface compositions ( basaltic sand vs. sand/dust mixture of pyroxenes and oxides) are possible. The solution more consistent with the geomorphic analysis is the sand / dust mixture with a large proportion of dusty grains of oxides and / or pyroxenes ( about 50 - 60%). This implies that Type 1 plagioclase-rich lithology derived from Thermal Emission Spectrometer (TES) is not required by NIR data. This work gives new evidence of the presence of dark dust on the surface of Mars.