On the Effect of Magnetospheric Shielding on the Lunar Hydrogen Cycle

The global distribution of surficial hydroxyl on the Moon is hypothesized to be derived from the implantation of solar wind protons. As the Moon traverses the geomagnetic tail, it is generally shielded from the solar wind; therefore, the concentration of hydrogen is expected to decrease during full Moon. A Monte Carlo approach is used to model the diffusion of implanted hydrogen atoms in the regolith as they form metastable bonds with O atoms, and the subsequent degassing of H2 into the exosphere. We quantify the expected change in the surface OH and the H2 exosphere using averaged SW proton flux obtained from the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) measurements. At lunar local noon, there is a small difference <∼10 ppm between the surface concentrations in the tail compared to out. However, the cumulative effect of traversing the magnetotail is a surface concentration depleted by ∼60 ppm for a region of the surface that spends most of its time in the lunar wake (waning gibbous to waxing crescent) on the nightside during time periods when Moon is exposed to unperturbed solar wind. The H2 exosphere decreased by approximately an order of magnitude while in the magnetotail due to the decreased proton flux. The model results are consistent with preliminary exospheric observations obtained by both the (Lyman Alpha Mapping Project on the Lunar Reconnaissance Orbiter) LAMP observations and mass spectrometer measurements by the Chandrayaan‐I Altitudinal Composition Explorer (CHACE).