| Summary: | Introduction: The southern residual ice cap on Mars has been observed for more than a century. Thermal data from Viking indicate a dominantly CO2 composition. Infrared spectral mapping from Mars Express Orbiter has detected CO2-ice in bright areas and H2O ice darker areas. Mars Global Surveyor Mars Orbiter Camera images show that there are two distinct layered units, an older unit ( 10 m thick) and a younger unit (few meters). Those CO2 layers (8-15 meter total thickness) overlap H2O-ice rich layers. From reflectivity maps at 3, 4 and 5 MHz (i.e. 100 - 75 meters wavelength) obtained with the MARSIS radar [5], it appears clearly that the reflectivity is weaker in the residual cap region than the rest of south polar layered deposits. To understand this phenomenon, we use a simple model of reflectivity with three layers (atmosphere, CO2 ice and H2O ice). Next, we statistically estimate the MARSIS reflectivity decrease between a reference region (only H2O layer+atm ) and the southern residual cap. This comparison gives us an estimation of southern residual cap thickness and dielectric constant. Finally we discuss the uncertainties and validity of our approach. Reflectivity model of a layered medium: As the thickness of residual cap is of the order of 10 meters and as MARSIS wavelength is between 60 to 160 meters (in vacuum medium) is unresolved, but its has an impact surface reflectivity. Our reflectivity model is composed by three plane overlapping layers. We consider a medium without losses. The two free parameters of our model are the thickness and the dielectric constant of southern residual cap. The first layer is atmosphere (semi-infinite, dielectric constant equals to 1), the second layer is the residual cap considered as a pure CO2-ice layer (height : h, dielectric constant between 1.4 and 2.4) and the third layer is a pure H2O-ice layer (semi-infinite, dielectric constant equals to 3.15). As the porosity and dust content of CO2 ice is unknown, the dielectric constant can vary from 1.4 (CO2 powder) to 2.4 ( ...
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