Understanding the Physical Evolution and Chemical Differentiation of the Northern Seasonal Condensates on Mars from the Analysis and Modelling of OMEGA Spectra

The determination of the physical state of the ices composing the seasonal condensations is of prime importance for the understanding of the microphysics of the sublimation/condensation/deposition processes of volatiles. 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. Several tens of observations of the northern seasonal condensates at different longitudes and times have been recorded by the OMEGA imaging spectrometer during northern spring. Statistical analyses (PCA) performed on the observations clearly shows a continuum evolution of the seasonal ice deposits from CO2- rich ice to dusty water ice then to bare hydrated soil when going to lower latitudes. The initial sublimation of CO2 ice leads to the progressive enrichment in water ice and dust of the CO2-rich seasonal deposits. Then a segregation of dusty water ice occurs to finally form a CO2-free ice layer. Finally the progressive sublimation of water ice of the top surface layers lead to an ice-free soil in the optical layer of dust. Spectra extracted at the different stages of the ice sublimation sequence have been modelled 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 and their evolution with latitude. Their relative abundances as well as their grain size are also determined. In addition we try to constrain the temperature of the H2O ice in the area where it is freely exposed at the surface by using a set of temperature dependent laboratory experiments on H2O ice performed at LPG (Grundy and Schmitt 1998). A sketch of the sublimation sequence can be drawn using longitude profiles of the condensate structure and composition at different times (Ls). From these results we discuss the sublimation processes of volatiles and the segregation of water ice and dust during spring