Surface Roughness Variation across Polar Ice Deposit Boundaries on Mercury

We analyze the surface roughness of five north polar craters on Mercury using 125 m/pixel Mercury Laser Altimeter data. Each crater hosts a polar deposit (PD) with a low-reflectance surface. The PDs have distinct geologic contacts, enabling an analysis of roughness across deposit boundaries onto ice-free portions of the host craters' floors. The low-reflectance surfaces in Angelou, Desprez, and Ensor collocate with radar-bright signatures consistent with the presence of several-meters-thick water-ice deposits beneath their low-reflectance surfaces. Subdued roughness in these three craters is consistent with the superposition of several meters of ice. The difference between roughness on and off their low-reflectance surfaces is within one standard deviation (SD) of the results, but is found to be statistically significant, as indicated by K-S and Mann-U Whitney tests, and meaningful, as indicated by Cohen's d tests. There is no meaningful difference in Jimenez and Josetsu, two craters that lack strong radar-bright signatures, consistent with the hypothesis that they do not host substantial water ice beneath their low-reflectance surfaces and thus not all of Mercury's available cold traps are occupied by water ice. Analyzing the roughness of polar craters may provide insight into the presence/absence of thick volatile deposits, but higher-resolution topography would be helpful given the high SD associated with roughness. Finally, we identify subdued roughness along three PDs that may be related to thick lag deposit margins or enhanced diffusive mixing along deposit edges. Roughness may provide new insight into surface characteristics of the ices and inform surface evolution models.