Modeling the thermal environment of Mercury’s north pole using MLA. Implications for locations of water ice

Abstract

Radar observations made by the Arecibo Observatory have revealed radar-bright deposits in the north polar region of Mercury which are consistent with water ice. These deposits are generally associated with floors of deep craters in permanent shadow. In this paper, we revisit topographic data, illumination, and associated thermal conditions of the north polar region of Mercury to investigate these radar-bright deposits. We used Mercury Laser Altimeter (MLA) data from the MESSENGER mission to derive a dedicated, adjusted high-resolution Digital Terrain Model (DTM) of the north polar region at 250 m/pixel. Based on this DTM, illumination conditions and temperature were derived for the first time on the fully available MLA dataset. Generally, craters equatorward of 86 ∘N are found to be too hot to contain surface water ice due to effective self-heating. For those warm craters that do contain radar-bright deposits, temperatures within the upper 10–20 cm of regolith are cold enough for water ice to be stable. Using our improved DTM, we confirm previously MLA-modeled illumination conditions in Prokofiev crater and argue that radar-bright patches outside the PSR in the area may be explained by icy deposits in cold and shallow (within 5 cm) sub-surface layers. In the radar observations we find a strong, unphysical correlation between radar-bright pixels and the number of radar observations per pixel. Consequently, several PSRs which were classified as free of radar-bright material (mainly at longitudes 120–340 ∘E) are most likely subject to a bias in the averaging process in deriving the composite radar image and might contain water ice after all.