Radar Subsurface Sounding Over the Putative Frozen Sea in Cerberus Palus, Mars

We present observations acquired by the orbiting radar sounders MARSIS and SHARAD over Eastern Elysium Planitia, Mars. This area encompasses Cerberus Palus, where the Mars Express High Resolution Stereo Camera acquired images that were interpreted as evidence for a frozen sea close to Mars' equator. MARSIS and SHARAD are synthetic-aperture, orbital sounding radars, carried respectively by ESA's Mars Express and NASA's Mars Reconnaissance Orbiter. They work by transmitting a low-frequency radar pulse that is capable of penetrating below the surface, and is reflected by any dielectric discontinuity present in the subsurface. Whereas MARSIS is optimized for deep penetration, having detected echoes down to a depth of 3.7 km over the South Polar Layered Deposits, SHARAD is capable of a tenfold-finer vertical resolution, namely 15 m or less, depending on the dielectric constant of the material being sounded. MARSIS is capable of transmitting at four different bands between 1.3 MHz and 5.5 MHz, with a 1 MHz bandwidth. SHARAD operates at a central frequency of 20 MHz transmitting a 10 MHz bandwidth. Mapping the location of subsurface echoes detected by either instrument, we find that there are different structures at different depths, with a deeper interface seen by MARSIS in the westernmost part of the area under examination, and shallower reflectors found over parts of Cerberus Palus by SHARAD. Analyzing the properties of the radar signal, we find that SHARAD echoes from the easternmost part of Cerberus Palus are stronger than those detected over other areas. One possible explanation is that the material being sounded through is a more transparent medium, such as dirty ice or very porous rock or regolith. At this stage, however, other interpretations consistent with the data exist and cannot be ruled out. We discuss the issue of the nature of the material in the context of what is known of the geologic history of Elysium.