Mars surface materials from MARSIS radar reflectivity

The MARSIS radar has now achieved a global Martian survey and sub-surface interfaces have been locally observed in polar and non-polar regions. Here, we have developed a method to extract the surface reflectivity and the dielectric constant of surface materials. The method involves the extraction of the intensity of the first radar echo, corrections from ionospheric distortion, surface slope and roughness. For both hemispheres, the highest values of dielectric constant (6 - 10) are observed at middle latitudes. These values are in agreement with laboratory measurements on dry igneous rocks and are consistent with igneous basaltic to granitic rocks. Poleward of around 40-50° in both hemispheres, terrains show a steep decrease of dielectric constant down to values of 3 - 4. This dielectric transition is not associated with a systematic change in surface albedo or thermal inertia, which implies that the surfacial geological material does not change much upon crossing this transition. There is a general agreement between the ground-ice limit detected by GRS and the dielectric decrease observed by MARSIS, which suggests that this limit correspond to the onset of ground ice. Because our technique is sensitive to the properties of the near-surface layer typically between 50 and 80 m thick, our observations suggest the presence of a thick ground-ice layer. Examination of the surface dielectric map in the equatorial regions reveals the occurrence of a nearly continuous low reflectivity belt between -30° and +30° latitude. Extremely low values are reached in the case of the Medussae Fossae area (e’=3). The spatial agreement between the low-reflectivity values on the MARSIS map and the H-enriched areas on the GRS map strongly points toward a water-related explanation.