Quantities of Ballistically Hopping Water Molecules on the Moon: Consistent With Exospheric Hydration Observations

Measurements of the lunar surface have revealed a variable presence of hydration, which has contributions from both hydroxyl (OH) and molecular water (H₂O). Recent observations of the lunar hydration suggest that a component of this signature is comprised of molecules that are readily mobile and actively migrate across the lunar surface over the course of a lunar day due to surface temperature variations. However, exospheric measurements of H₂O suggest very low abundances above the dayside surface which previous work has argued is in conflict with the surface abundances and the putative occurance of ballistic migration. Here, we use a ballistic transport model to quantify the amounts of OH and H₂O in the lunar exosphere and to characterize patterns in the transportation and retention of hydration across the lunar surface. We find that ∼0.5% of a monolayer of hydration on the surface, with 99% OH and 1% H₂O contribution to hydration signatures, matches observational upper limits for the presence of hydration in the exosphere. We conclude that there is no discrepancy between the low exospheric measurements and ballistic migration. However, the previously observed day-time recovery of the hydration signal cannot be explained by this ballistic migration, suggesting that OH/H₂O production is also occurring on timescales less than a lunar day. Additionally, we find that ballistic transport results in the transportation of ∼2% of the hydration sourced from surface desorption to the polar regions of the Moon.