Theoretical Calculations on the Effect of Adsorbed Atom Coverage on the Sodium Exospheres of Airless Bodies

Our ability to understand the formation of the exospheres of airless bodies such as the Moon and Mercury has been hindered by uncertainties in how surface processes influence exospheric sources. Ejection processes important for exosphere formation rely on the notion that an emitted atom must first overcome an attractive energy with the surface to be ejected into the exosphere (the surface binding energy, SBE). Recent studies have shown that atoms from minerals are more tightly bound than commonly assumed, making it difficult to reconcile how such high volatile concentrations are being observed in the exospheres of airless bodies. Here, we used molecular dynamics modeling to explain the physics underlying the interaction of low-energy returning atoms, initially ejected below the escape energy of the body, with mineral surfaces. Global exosphere models make ill-informed assumptions for these interactions due to a lack of SBEs for adsorbed atoms. Results provide first-of-their-kind SBE distributions for adsorbed atoms and can be used by global models to better understand exosphere formation on airless bodies. We highlight the importance of adsorbate coverage and the atomic arrangement of a surface on the SBE. At low absorbate coverage sodium forms ionic bonds with oxygen, leading to tightly bound adsorbates (SBE ∼6 eV). At 1 ML of coverage the free O is terminated and Na is unable to form strong ionic bonds, leading to loosely bound adsorbates (SBE 1–3 eV). Emission processes from covered surfaces will be far more efficient than those without adsorbates. These improvements will allow for better interpretation of mission data such as from MESSENGER, BepiColombo, LADEE, Europa Clipper, and Artemis.