Arecibo S-band Radar Characterization of Local-scale Heterogeneities within Mercury’s North Polar Deposits

Ground-based planetary radar observations first revealed deposits of potentially nearly pure water ice in some permanently shadowed regions (PSRs) on Mercury’s poles. Later, the MESSENGER spacecraft confirmed the icy nature of the deposits, as well as their location within PSRs. Considering the geologic context provided by MESSENGER, we further characterized the north polar deposits by pairing spacecraft data with new Arecibo S-band radar observations. Here we show that some ice deposits within PSRs have a gradational pattern in their radar properties that is likely associated with differences in ice purity. Radar-bright features with a circular polarization ratio μ_c > 1 can be characterized by water ice with >̰ 3% impurities by volume while those with μ_c < 1 by >̰20% impurities. Furthermore, areas in PSRs with μ_c < 1 typically surround locations of stronger radar backscatter with μ_c > 1. Therefore, deposits of nearly pure water ice are likely surrounded by lower-purity material, such as water-ice-rich regolith, which could be the result of impact gardening or the crater’s thermal environment. However, such deposits are not always colocated within large polar craters where ice should be the most stable, even at the surface. In fact, we found that there is no significant difference between the radar backscattering properties of deposits thought to have surficial ice and those with buried ice. Our results also help improve the identification of icy reservoirs elsewhere, such as the Moon. Indeed, we found that μ_c is not an adequate diagnostic, but rather the radar backscatter in each circular polarization independently provides information to identify water-ice deposits.