GEOLOGIC CONTEXT OF THE BRIGHT MARSIS REFLECTOR IN ULTIMI SCOPULI, SOUTH POLAR LAYERED DEPOSITS, MARS

Radar bright basal reflectors have been detected below the South Polar Layered Deposits (SPLD), Mars using Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) data and have an exciting but controversial interpretation: liquid water from a subglacial lake.

One way to evaluate this hypothesis is to examine the surface geomorphology of the region. A liquid water lake would result in a rapid change in the basal stress of the SPLD ice. While current SPLD flow is not observed, the contrast in basal stress may be enough to generate observable features at the surface.

We mapped the surface of the SPLD immediately above and surrounding the putative lake at 1:1M map scale, in order to provide geologic context for the MARSIS bright basal reflector. We use THEMIS daytime IR data (100 m/pixel) and HiRISE (25 cm/pixel) image data to characterize map units and typical surface roughness. The largest feature mapped was the scarps towards the northern portion of the map area.

We quantified the typical wavelength of roughness of several map units. The spacing of the ubiquitous surface troughs was typically ~100-200 m, which is too broadly spaced to strongly interfere significantly with MARSIS. We can eliminate interference from surface roughness as the cause of the MARSIS bright reflector in this region.

We find evidence for multiple map units with HiRISE-scale features, like araneiforms, that are due to typical SPLD processes. We do not find evidence for characteristic smooth areas that correspond to the postulated lake locations as would be expected from analogy from subglacial lakes in Antarctica on Earth, nor do we find large fractures suggesting epochs of large stress changes in the subsurface.

While we cannot directly rule out the subglacial lake interpretation of the bright MARSIS radar reflector, we do find that the surface geology of this region may be typical of the SPLD in general and do not find geomorphological evidence of a recent subglacial lake.