Sputter Yields of the Lunar Surface: Experimental Validation and Numerical Modelling of Solar Wind Sputtering of Apollo 16 Soils

TL;DR

This study combines experimental measurements and numerical simulations to accurately determine solar wind sputter yields of Apollo lunar soils, revealing significant overestimations by models and emphasizing the importance of experimental validation for space weathering predictions.

Contribution

It provides the first experimental validation of sputter yields for Apollo lunar soils and highlights the limitations of existing simulation codes in predicting these yields.

Findings

Simulation codes overestimate sputter yields by at least a factor of 2 for hydrogen.

Porous regolith reduces sputter yields to below 0.1 atoms/ion for H and He.

Surface roughness has minimal impact on sputter yields across lunar latitudes.

Abstract

Sputtering by solar wind ions is a key process driving the ejection of high-energy particles into the exospheres of airless bodies like asteroids, Mercury and the Moon. In view of upcoming missions which will deliver new in-situ data on these exospheres like the Artemis program at the Moon and BepiColombo at Mercury, a deeper understanding of sputtering effects is crucial. In this work, we combine sensitive quartz crystal microbalance measurements and numerical simulations to quantify sputter yields of Apollo soil 68501 under solar wind relevant conditions. We find that none of the commonly used simulation codes can reliably predict laboratory sputter yields without experimental benchmarks. All of the employed packages significantly overestimate the sputter yields of flat samples by at least a factor of 2 for the case of hydrogen. When accounting for surface roughness and regolith-like porosity, sputter yields are decreased even further to 7.3E-3 atoms\ion and 7.6E-2 atoms\ion for H and He at solar wind energies of 1 keV\amu, respectively. The reduced yields of porous regolith structures are largely independent of the ion incidence angle, making them applicable across a wide range of lunar latitudes. This study highlights the need for experimental validation of sputtering models to ensure accurate predictions for space weathering and lunar exosphere composition.