New Insights Into Composition Variation of Mars South Polar Layered Deposits From SHARAD Radar Sounder

The south polar layered deposits (SPLD) of Mars record its Amazonian climate history. Detailed stratigraphic analyses of the SPLD will improve our understanding of the Martian depositional system. Radar reflection patterns correlate with the varying composition of the internal layers, thus providing a key to unlocking the depositional history of the SPLD and Martian paleoclimate. Using data from SHAllow RADar (SHARAD) onboard the Mars Reconnaissance Orbiter, we studied a new type of weak reflection pattern (shadow zone) across the SPLD. These shadow zones lack identifiable bases and are generally deeper than previously reported low-reflection zones interpreted as sealed CO2 ice. The discrepancy between the waveforms of shadow zones in focused and unfocused radargrams was used to identify whether the shadow zones were associated with fog-related data-processing artifacts. Numerical simulations utilizing various dielectric models demonstrate that lossy materials are most likely the cause of the shadow zone. These materials are expected to exhibit a gradual increase in concentration with depth, ranging from pure water ice to lossy materials at a scale of >50 m. The presence of such lossy materials is consistent with the compositional heterogeneity of SPLD, but their origins remain obscure.