A Thermal Origin to the Asymmetry of the Permanent Dust Cloud at the Moon

Abstract The Moon's surface, lacking an atmosphere, is continually bombarded by high‐speed micro‐meteoroids, creating a highly porous regolith composed of very fine grains. This regolith's porosity decreases with depth due to compression. Besides creating vapor and melt, micro‐meteoroid impacts eject lunar dust, redistributing regolith grains, which can travel ballistically around the Moon. The Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft, which orbited the Moon from 2014 to 2015, discovered a permanent asymmetric dust cloud with a higher dust density at dawn compared to that at dusk. In addition, the dust cloud at dawn exhibits an asymmetry with a higher density on the dayside, which is attributed to different populations of impactors with varying orbits. Moreover, the inferred ejecta yield was found to be much smaller than expected. Numerical modeling is useful for studying these phenomena, as current experimental capabilities cannot reproduce the mass and velocity combinations typical of actual micro‐meteoroid impacts. This study utilizes the iSALE‐2D shock physics code to investigate the ejecta mass yield and velocity distribution caused by typical micro‐meteoroid impacts as a function of the lunar regolith's porosity and temperature. Findings suggest that (a) the dust cloud asymmetry at the Moon may have a thermal contribution, (b) the predicted ejecta velocity distribution differs from what is assumed to interpret LADEE measurements, and (c) the ejecta yield could serve as an indirect measurement of the regolith structure.