Illumination, thermal conditions, and distribution of volatiles at Mercury ’s south pole based on new stereo topographic models

Radar observations have revealed extensive radar-bright deposits at Mercury’s poles, generally interpreted as evidence of water ice within permanently shadowed regions (PSRs). Previous thermal modeling and illumination studies based on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Mercury Laser Altimeter (MLA) data have demonstrated a strong correlation between radar-bright features and cold-trap conditions in the north polar region. In contrast, gaps in Digital Terrain Models (DTMs) derived from images have limited the completeness of such analyses at the south pole, where no MLA data is available. This study is based on an improved Mercury Dual Imaging System (MDIS) stereo-photogrammetric DTM of the Hermean south polar region covering 65–90 ∘ S at a resolution of 220 m/pixel . We present a novel gap-filling approach for crater floors in darkness to reconstruct realistic crater morphologies based on depth-to-diameter scaling relations. Using this DTM, we conduct synthetic illumination modeling and one-dimensional thermal simulations of the surface and shallow subsurface up to 2 m depth, while explicitly accounting for indirect radiation and light scattering contributions. Our results indicate that both, patterns of low surface temperatures and low mean sublimation rates, such that water ice might be expected, show little agreement with the distribution of radar-bright deposits observed by the Arecibo radar. Specifically, only 30 − 40 % of those areas with appropriate temperatures and mean sublimation rates show radar signatures indicative for water ice at the surface. When extending the analysis to the upper decimeters of regolith however, the overlap with radar detections increases to 75 % , implying that most radar-bright features can be explained by ice deposits within the upper 25 cm of regolith. We find residual discrepancies between thermal models and radar observations, primarily in unresolved micro-cold traps and within unmodeled DTM gaps, highlighting the limitations of the current datasets. We conclude that Mercury’s south polar water ice reservoirs are consistent with shallowly buried deposits and emphasize that forthcoming high-resolution topography from BepiColombo’s Laser Altimeter (BELA) will be critical to refine their spatial extent and depth distribution.