Contributions of Solar Wind and Micrometeoroids to Molecular Hydrogen in the Lunar Exosphere

Abstract We investigate the density and spatial distribution of the H 2 exosphere of the Moon assuming various source mechanisms. Owing to its low mass, escape is non-negligible for H 2 . For high-energy source mechanisms, a high percentage of the released molecules escape lunar gravity. Thus, the H 2 spatial distribution for high-energy release processes reflects the spatial distribution of the source. For low energy release mechanisms, the escape rate decreases and the H 2 redistributes itself predominantly to reflect a thermally accommodated exosphere. However, a small dependence on the spatial distribution of the source is superimposed on the thermally accommodated distribution in model simulations, where density is locally enhanced near regions of higher source rate. For an exosphere accommodated to the local surface temperature, a source rate of 2.2 g s −1 is required to produce a steady state density at high latitude of 1200 cm −3 . Greater source rates are required to produce the same density for more energetic release mechanisms. Physical sputtering by solar wind and direct delivery of H 2 through micrometeoroid bombardment can be ruled out as mechanisms for producing and liberating H 2 into the lunar exosphere. Chemical sputtering by the solar wind is the most plausible as a source mechanism and would require 10–50% of the solar wind H + inventory to be converted to H 2 to account for the observations.