Resolution of the lunar hydrogen enigma

Abstract

[1] The recent discovery, by instruments on the IBEX and Chandrayaan-1 spacecraft, of significant fluxes of energetic hydrogen atoms escaping from the moon suggests that there are flaws in the common wisdom regarding the interaction of solar wind with lunar regolith that have prevailed in attempts to explain the failure of the Apollo 17 far-ultraviolet spectrometer to detect neutral H in the lunar exosphere. A new theory for the interaction of solar wind with the lunar regolith surface is tested by comparing simulated spectra of reflected energetic neutral hydrogen and protons with analogous neutral spectra from Chandrayaan-1 and proton data from the Kaguya mission. Overall, the theory indicates that roughly 1% of solar wind protons incident on the lunar regolith surface exit as energetic protons, and about 98.5% exit as neutral H with super-escape speeds. The remaining 0.5%, which exit as neutral H atoms with sub-escape speeds, form a tenuous exosphere that is compatible with the levels of Lyman-α allowed by Apollo 17 observations.