Assessing the role of clay and salts on the origin of MARSIS basal bright reflections

• Bright basal reflections have been detected by MARSIS at Ultimi Scopuli, Mars. • High permittivity contrast between the ice and the basal material is required. • Wet sediments or ponded water have been suggested as the cause of such reflections. • Hydrous minerals and salts are considered as possible alternative to basal water. • Only brines can generate such high dielectric contrast at low basal temperature. Anomalously bright basal reflections detected by MARSIS at Ultimi Scopuli have been interpreted to indicate the presence of water-saturated materials or ponded liquid water at the base of the South Polar Layered Deposits (SPLD). Because conventional models assume basal temperatures (≤200 K) much lower than the melting point of water, this interpretation has been questioned and other explanations for the source of the bright basal reflections have been proposed, involving clay, hydrated salts, and saline ices. Combining previous published data, simulations, and new laboratory measurements, we demonstrate that the dielectric properties of these materials do not generate strong basal reflections at MARSIS frequencies and Martian temperatures. Plausible candidates remain perchlorates and chlorides brines that exhibit a strong dielectric response at much lower temperatures than other materials. This explanation might require that metastability could be maintained for a long period of time on a geological scale.