Physical Characterization of the South Seasonal Cap of Mars From OMEGA Observations
Résumé
The time and space evolution of the South Seasonal Polar Cap (SSPC) is a major annual climatic signal. The composition, physical state and texture of the SSPC give clues about the exchange of CO2, H2O and dust with the atmosphere. The imaging spectrometer OMEGA on board Mars Express has acquired the most comprehensive set of observations to date in the near-infrared (0.93-5.1 microns) on the SSPC from winter solstice to the end of the recession at Ls=325° of the martian year 27 [1]. The time resolution is 3 days to one month and the spatial resolution ranges from 700m to 10 km/pixel. The spectral range covered by OMEGA is particularly relevant for our studies since it samples numerous absorption bands distinctive of CO2 and H2O in their solid state. Here we analyze with statistical techniques and a physical model a collection of OMEGA spectral images covering the SSPC at Ls ~223°, i.e. close to the maximum development of the cryptic region. Our goals are to (i) segment the SSPC into different CO2 ice terrains based on material composition and organization (ii) map the spatial variations of some of their physical properties: dust abundance, granularity, icy layer thickness, etc. In an earlier work [2] we introduced the "snowdrop time", i.e.: at a given location, the time necessary to decrease the areal coverage of CO2 from 98% to 2% during recession. We showed that it is mainly controlled by the dispersion of its local albedo distribution ~one martian month earlier. The physical characterization of the CO2 deposits allows to identify and understand the processes governing this albedo distribution variability: sub-pixel mixing, ice thickness, grain size, dust content, etc.