Martian Surface Radar Reflectivity by Marsis

Introduction: Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on board Mars Express is a decameter sounder radar, which can work in two different modes : subsurface or ionosphere sounding. The subsurface sounding uses 4 different frequency bands, which are centered to 1.8, 3, 4 and 5 MHz. Each band is 1 MHz wide. [1,2]. In this work, we study the reflectivity of Martian surface at radar wavelength (3-5 MHz) in order to evaluate the dielectric constant of the shallow subsurface. These values enable to constraint the nature of materials that composed the few tens of meters under the surface. The geology at this scale is poorly known because the classical instruments of the Martian exploration probe few microns to one meter of the surface. MARSIS offers the new opportunity to explore more deeply the surface. Methods: For this study, it is necessary to calibrate the data and to separate the different effects that affect the reflectivity. Firstly, we present our extraction method of the surface echo power from the MARSIS radargrams. Next, we show our correction of spacecraft altitude changes and ionospheric absorption. Once the surface reflectivities extracted from the radargrams and corrected from the ionospheric absorption, we have plotted a global map of the Martian surface echo power (global radar albedo map) at 3-5 MHz (see Figure 1). For crossing tracks, we average the data from multiple measurements. MARSIS is a nadir looking radar and the Mars Express polar orbit does not allow us to sound the surface poleward of about 87°N and 87°S; this lack of data results in a gap centered at the pole. This reflectivity map combines the different frequency bands of MARSIS because we observe few differences between them and thus we obtain a better coverage of the Martian surface.