Physical characterization of the south seasonal cap of Mars during its recession from OMEGA observations
Résumé
Introduction The high latitudes of Mars are covered by a seasonal CO 2 ice deposit The time and space evolution of the CO 2 distribution as well as its physical state and texture constrains many atmospheric and surface processes involving the latter compound but also H 2 O and dust The imaging spectrometer OMEGA on board Mars Express has been scrutinizing the seasonal changes in the south polar region from mid winter Ls 127o to the end of recession Ls 305o extending to the pole only after spring equinox In the spectral range of OMEGA operation 0 37-5 10 mu m CO 2 H 2 O and to a lesser extent dust display numerous and distinctive absorption bands that allow us to recognize and characterize different classes of icy terrains with unprecedented accuracy Methods Automatic detection of CO 2 H 2 O and dust coupled with statistical techniques PCA and ICA applied to the OMEGA images differentiate several types of icy terrains according to their composition and their physical properties Then we extract the purest spectra of these terrains and model them to obtain quantitative values for the physical parameters e g abundances granularity layer thicknesses For that purpose a radiative transfer algorithm calculates the spectral reflectance of layered icy and dense materials taking into account shadowing effects due to macroscopic roughness Each layer can have a granular or a compact icy matrix with inclusions texture Results Before equinox the seasonal cap is mostly