Investigating Azimuthal Anisotropy in the Angular Distribution of Solar Wind–sputtered Atoms

Abstract The solar wind (SW) bombards the surfaces of airless bodies like the Moon and Mercury, inducing sputtering that erodes regolith and sustains exospheres. While the sputtering yield has been given significant attention, the angular distribution of sputtered atoms remains underexplored for SW-like impacts, with prior studies confined to incidence plane measurements. This work employs binary collision approximation simulations to quantify azimuthal anisotropy for SW-like impacts (H and He at 1 keV amu −1 ) on a flat amorphous silica surface, contrasted with a heavier, higher-energy laboratory-like projectile (20 keV Kr). At incidence angles >30°, ejecta angular distributions show stronger forward bias for H than for He and Kr. With H being the main component in the SW, experiments using heavier mass impactors may be overlooking behavior occurring for SW sputtering. Additionally, we find that incidence plane measurements likely exaggerate (1) the degree of forward bias in ejecta angular distributions and (2) the discrepancy in forward–backward anisotropy for H compared to He and Kr. From these insights, we advocate that granular and exosphere models consider ejecta angular distributions for SW-like (light, low-energy) projectiles and full azimuthal coverage. We also investigate the atomic-scale behavior underlying the observed ejecta angular distributions, remarking on distinct differences in the types of ejecta (defined according to how they are displaced in the target) and their emission tendencies depending on the projectile and incidence angle. From this, we connect physical behavior such as atom emission depth and momentum retention to understanding the observed forward–backward anisotropies.