A Prediction of the Inter-Annual Variability of Sulfur in Mercury’s Exosphere and Subsurface

Since the discovery of atoms ejected from Mercury’s surface and forming a tenuous atmosphere around the planet, Mercury’s surface-exosphere interface has been extensively observed by both on-ground and space-borne instruments. Between 2007 and 2011, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft performed three fly-bys of Mercury followed by four years in orbit around it. This mission derived the surface composition of Mercury which notably includes some moderately volatile species such as sodium, potassium or sulfur. While the two former were clearly identified in Mercury’s exosphere, the latter has never been observed as released from the surface in its neutral form. Moreover, recent studies suggest that hollows, i.e. bright, shallow flat-floored depressions, could potentially be formed by local sulfur accumulation This suggests that sulfur should be present in both the surface and the exosphere of Mercury, with associated migration and/or diffusion processes that could sustain such geological features. Using a 3-D Exospheric Global Model with a Monte-Carlo test-particles approach and accounting for species diffusion in the first meter of Mercury’s regolith, this study aims for the first global prediction of the inter-annual variability of neutral sulfur density in both Mercury’s exosphere and subsurface. This work is particularly relevant for the preparation of ESA/BepiColombo mission which will start its scientific mission in December 2025.