On the Origins of Mercury's Sodium Exospheric Cold Poles Enhancement: A Multiscale Exosphere Global Model

S Verkercke and L Morrissey and JY Chaufray and A Georgiou and A Ricketts and F Leblanc, JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS, 130, e2025JE009309 (2025).

DOI: 10.1029/2025JE009309

Mercury's exosphere is sustained by the continuous ejection of atoms from its surface, driven by solar wind, micro-meteoroid impacts, and surface heating. Due to its 3:2 spin-orbit resonance, some longitudes experience greater solar exposure, creating temperature variations from similar to 90 to 700 K. This resonance also creates least exposed longitudes, called cold longitudes. These variations, combined with surface-solar interactions, lead to complex exospheric dynamics. Observations from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft revealed localized enhancements in sodium column density at cold longitudes near the aphelion, where these regions rotate into the day-side. Existing models do not explain this cold-pole enhancement but assume a smooth, impermeable surface, neglecting the highly porous nature of Mercury's regolith. This porosity allows for subsurface diffusion of volatiles through adsorption and desorption across regolith grains, influenced by temperature and species-specific surface binding energies. We couple a subsurface transport model with a 3D Monte Carlo-based Exosphere Global Model to investigate whether sodium accumulated in Mercury's regolith could explain the cold pole enhancement. Results suggest that at cold longitudes, low temperatures favor sodium retention near the surface. The gradual heating induces a release of sodium from the subsurface, producing the observed localized enhancements. This mechanism reconciles MESSENGER's findings with physical processes and highlights the significance of subsurface reservoirs in volatile dynamics. We highlight the need to consider regolith structure and subsurface processes in exosphere modeling. These results improve our understanding of Mercury's volatile cycle but also offer broader insights into the behavior of surface-bound species on airless planetary bodies.

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