South Pole of Mars: Nature and composition of the icy terrains from Mars Express OMEGA observations

Monitoring the exchange of CO2, H2O and dust between the atmosphere, regolith, seasonal deposits and the permanent polar caps of Mars is required to study the climate of the planet and its evolution. The imaging spectrometer Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) of Mars Express contributes to such a task by regularly acquiring hyperspectral images of the polar regions in the 0.35-5.10μm range. We analyze five OMEGA observations that were acquired over the high southern latitudes in 2004 at the end of summer (LS=335-348∘). We conduct statistical analyses of data as well as radiative transfer modelings of selected spectra in order to investigate the nature, distribution and properties of various icy terrains: the bright permanent polar cap (BPPC), the layered deposits (SPLD), and the seasonal accumulations of water frost (SFA). Annealed CO2 ice, potentially polycrystalline, dominates the BPPC and contains trace amounts of dust and water probably trapped in the cap during southern winter. Vast areas of the BPPC are quite homogeneous with dust and water contents of ≈0.03-0.06 and ≈0.02-0.06 wt%, respectively, but we observe larger variations for specific regions. The BPPC lays on a water-rich (≈75wt%) basement that emerges at the edges of the cap. In the circumpolar region, we observe the superficial part of the SPLD that shows a mixture of perennial H2O ice (up to ≈70wt%) and dust. The SPLD are generally buried under an optically thick layer of sand or of seasonal frost accumulations. The SFA are composed of water ice (≈65-70 wt%) and dust and cover large areas at the outskirts of the BPPC or at distance from it. We also found intermediate situations (≈20 water wt%) between the SFA and the dust cover.