Physical State and Distribution of H2O and CO2 Ices and Dust on the Perennial Mars South Polar Cap from OMEGA/MEx Observations

The determination of the physical state and composition of the ices composing the surface layer of the Mars polar caps is of prime importance for the understanding of the sublimation history of volatiles, the thermal state and the extend of the south polar cap. It is also a prerequisite for an accurate mapping of the abundance of the ices and of their physical characteristics by spectral inversion of the OMEGA observations. Four observations covering about the whole bright south polar cap and part of the surrounding terrains have been recorded by the OMEGA imaging spectrometer before the southern autumn equinox The statistical analyses (PCA) of the observations shows the presence of two types of CO2-rich terrains on the perennial cap : one concentrated in the cap centre and the other on the outer parts. Two types of dusty water ice terrains are also found on the border of the cap and in isolated patches at some distance from the perennial cap. From this analysis we extracted several end-member and representative spectra of the different types of icy terrains. Then, we modelled these spectra using our radiative transfer code in layered media (Douté and Schmitt 1998) in order to determine the coexistence modes of CO2 ice, H2O ice and dust at the centimetre scale (granular mixtures, layered deposits, ...) and pixel scale (geographical mixture). The relative abundances of these three components are determined. From these results we discuss the state of volatiles in the south polar cap just before southern winter