The Topography of Polar Ice Deposits on Mercury from the Mercury Laser Altimeter: Implications for ice thickness.

The permanently shadowed regions of Mercury's poles contain anomalous radar-bright regions, which are thought to be water ice. To constrain the age and delivery mechanism of the water ice, it is necessary to understand both its physical properties and the deposition process. The aerial extent of these deposits is known from radar maps and laser altimeter albedo variations; however, the thickness of the deposits has been harder to constrain. A minimum thickness of around 1 m is required to match current datasets, but estimates of the maximum thickness has ranged from nearly a kilometer down to 15 m from a recent study of individual Mercury Laser Altimeter (MLA) tracks. None of the studies to-date have been able to say anything about either the deposition process or any resulting regional variations in thickness. Here, we investigate how variations in the topography of inferred water ice deposits on crater floors compare to variations in the topography of portions of the same crater that are not covered in water ice, and to control craters that do not contain such deposits.

Surface roughness provides a statistical measure of the change in topography over a specific horizontal scale. For the ice deposits on Mercury, the ice is proposed to have migrated to cold-traps within craters and then been emplaced on pre-existing topography. By comparing the surface roughness of regions of the crater floor that contain ice to both regions of the same crater floor that do not contain ice and to crater floors from control craters that contain no ice deposits, we can explore how the presence of ice affects the surface roughness at different scales. Initial measurements have shown that at a horizontal scale of 1 km the surface roughness of the radar-bright deposits is lower than the surface roughness of regions that do not contain radar-bright deposits, but substantial surface roughness on the scale of tens of meters is still present on the radar-bright deposits. This result implies that the underlying crater floor topography is reflected in the radar-bright deposits and is consistent with the inference that the deposits are quite thin. Future analysis of the surface roughness spectrum could provide insight into the overall ice thickness and lateral variations in ice thickness. Current information on the topography of the ice deposits is limited by MLA's coverage and density, but the BepiColombo Laser Altimeter, BELA, will provide higher resolution topography covering both poles allowing the origin and deposition of these deposits to be addressed.