Updated Estimates of the Sputtering Contributions to the Exosphere of Mercury From Magnetospheric Ion Precipitation

Abstract

Ion impacts on airless bodies such as Mercury alter their surfaces and contribute to their exospheres via sputtering. Their exact contribution in comparison to other effects is still uncertain, but observations by the MESSENGER spacecraft largely indicated influences from micrometeoroids. In this paper, we present an updated modeling of sputtering at Mercury to help estimate the role of sputtering at average solar wind conditions. To achieve this, we account for ion precipitation due to the planet's magnetosphere and for the presence of a porous regolith: We combine H⁺ and He⁺⁺ fluxes to the surface from the Amitis hybrid model with sputter yields derived from a regolith simulation in SDTrimSP-3D. We find that H⁺ and He⁺⁺ show similar precipitation patterns, but H⁺ energies are much more reduced and variable than those of He⁺⁺. Globally, H⁺ and He⁺⁺ contribute about equal amounts of sputtering. Our laboratory-calibrated sputter yields are significantly lower than estimates used in previous studies, resulting in a global sputtering source of around 10²³ atoms s⁻¹. Specifically for Ca and Mg exospheres we find source rates from sputtering that are largely unaffected by Mercury's seasonal orientation and too small by up to around two orders of magnitudes to explain MESSENGER observations. This supports a micrometeoroid-impact-dominated source of refractory elements. We find, however, that this is an effect of the reduced magnetospheric precipitation at Mercury. At other bodies such as the Moon, a different regime should be prevalent and sputtering should contribute at least similarly to the exospheres of refractory elements.