A Novel Theoretical Approach to Predict the Inter-Annual Variability of Sulfur in Mercury's Exosphere and Subsurface

Sébastien Verkercke^1,2*Jean-Yves Chaufray^1,2François Leblanc^2,3Anastasis Paul Georgiou⁴Michael Steven Phillips⁵Giovanni Munaretto⁶Jesse Lewis⁴Amanda Ricketts⁴Liam Morrissey⁴

• ¹Université de Versailles Saint-Quentin-en-Yvelines, Versailles, France
• ²UMR8190 Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Guyancourt, Île-de-France, France
• ³Université Paris-Sorbonne, Paris, France
• ⁴Memorial University of Newfoundland, St. John's, Newfoundland and Labrador, Canada
• ⁵Lunar and Planetary Laboratory, College of Science, University of Arizona, Tuscon, Arizona, United States
• ⁶Osservatorio Astronomico di Padova (INAF), Padua, Veneto, Italy

The surfaces of airless bodies are constantly weathered by ions, meteoroids and radiation, leading to the ejection of surface atoms to form a tenuous, collisionless atmosphere around the body. In the case of Mercury, its high surface temperatures can also lead to thermal desorption of atoms. Since its discovery about 50 years ago, Mercury's exosphere has been extensively observed by both onground and space-borne telescopes. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft operated four years in orbit around Mercury and allowed for the surface composition species to be inferred, notably including sulfur. Sulfur was however never observed in Mercury's exosphere. Here, we use a unique theoretical approach that combines modelling methods across different dimensional scales to understand the presence of sulfur on Mercury. 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. Our model predicts the formation of subsurface reservoirs at different depths according to the planetary longitude, with an equatorial reservoir peak location at ~ 21 cm and ~ 8 cm below the surface at the hot and cold poles respectively. Cold longitudes are also predicted to accumulate 6.7 times more sulfur than the hot ones. Regarding the exosphere, the larger abundance of sulfur at the cold longitudes induces a local enhancement of the exospheric density around aphelion. The calcium surface abundance is predicted to influence the sulfur adsorption location leading to a sulfur content enhancement in the vicinity of the -90°E longitude. Our results could be beneficial for optimizing the planning and aid the analysis and interpretation of future observations of Mercury's exosphere by BepiColombo.