Topographical analysis of a candidate subglacial water region in Ultimi Scopuli, Mars

Radar signals of the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument at the base of the South Polar Layered Deposits (SPLD) in the region known as Ultimi Scopuli, on Mars, have been interpreted as reflections from putative subglacial standing bodies of liquid water. The interpretation of the MARSIS radar signals is not unique and different explanations have been proposed for the anomalous bright radar reflection observed in Ultimi Scopuli, including the presence of CO2 deposits within the ice, or clays and metal-bearing minerals. We analysed the surface topography of Ultimi Scopuli with Digital Elevations Models (DEMs) derived from MOLA (Mars Orbiter Laser Altimeter), photogrammetric processing of the High-Resolution Stereo Camera (HRSC) images and SHARAD radargrams. In each case, we found an anomalous flat and smooth area in correspondence to the potential subglacial water zone, indicating a condition of hydrostatic equilibrium of the ice, similar to the surface observations found above large terrestrial subglacial lakes. We found that the surface topography, in addition to MARSIS radar sounder signals, are compatible with the presence of subglacial standing bodies of liquid water at the base of Ultimi Scopuli.