Composition and Spatial Distribution of the Northern Seasonal Condensates during Spring from the Statistical Analysis of OMEGA NIR Spectro-images

Monitoring the exchange of CO2, H2O and dust between the atmosphere, regolith, seasonal deposits and permanent polar caps of Mars is required to study the climate of the planet and its evolution. The imaging spectrometer OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité) of Mars Express contributes to such a task by acquiring regularly spectro-images of the polar regions in the 0.35-5.09 micron range. We have analyzed the Near Infrared segment of observations acquired at Ls from 350 to 90 (spring of the northern hemisphere) focusing on the northern circumpolar seasonal deposits. Our goal is to study the modalities of their progressive demise by metamorphism and sublimation. More precisely, we seek to determine the composition, physical state and granularity of the icy deposits as well as their dust content as a function of time for different latitudinal bands. Here we present the results obtained after a first phase of analysis. We apply different statistical methods (PCA, MNF, etc.) to (1) classify the spectro-images into a small number of spectrally similar terrains and (2) extract endmembers (spectra with unique and extreme signatures). We carefully examine the position, depth, width and shape of characteristic absorption bands of CO2 and H2O ices at the surface by direct comparison with laboratory data. Such operations give the spatial distribution of the different types of terrains encountered and a first estimate of their composition as well as physical properties : mixing mode between CO2, H2O and dust, texture and granularity. Consequently the identification andclassification of the series of OMEGA spectro-images allows to produce compositional maps organized in a time sequence. The interpretation of these maps provides insights about the mechanisms and timing presiding to the spring sublimation of the seasonal northern polar cap