Radar Observations of Outlier Polar Ice Deposits on Mars

Martian ice likely holds the key to interpreting Mars’ past climate, but much is still unknown regarding the distribution and properties of Mars’ ice deposits. It is well known that Mars has extensive polar ice caps the size of Greenland. Included in these large polar caps are the north and south polar layered deposits (NPLD and SPLD, respectively), that are composed of kilometers-thick deposits of water ice. In addition, surveys by Conway et al. (2012) and Sori et al. (2019) identified craters in the surrounding terrains that contain “outlying” deposits of ice, which may or may not have formed concurrently with the polar caps. There are many differences between the NPLD and SPLD, including higher dust content and sequestered CO2 within the SPLD, a younger surface age of the NPLD, and properties in some locations of the SPLD that causes a ‘fog-like’ scattering in radar observations. These differences between the NPLD and SPLD may or may not be shared in these outlying deposits, and may provide clues to the climate conditions under which the PLDs and outlier deposits formed. We have analyzed a total of 517 SHARAD radar tracks across 24 ice deposits within craters. For the northern population, we detected subsurface layers in 3 out of 4 crater deposits. In the southern population, we detected subsurface layers in 4 of the 20 crater deposits. Of the 4 southern crater deposits that exhibited subsurface layers, 3 were contiguous with the SPLD. We also found deposits with radargrams that contain fog, and one that contains a low reflectance zone. After examining the subsurface radar observations, we determined that the northern outlier deposits share many common characteristics with the NPLD, and thus may have a shared depositional history or at least were emplaced under similar environmental conditions. The southern outlying crater deposits exhibit a variety of subsurface characteristics, and likely represent 2 or more populations that may have differing depositional histories.