Hydrogen Implantation on the Moon: Effect of Plasma Sheath

The implantation of solar wind (SW) and Earth’s magnetospheric protons is recognized as a primary source of hydrogen (H) in the top 100 nm of the lunar regolith. The effective proton flux reaching the lunar surface, however, gets modified by the plasma environment. We present a framework for H implantation that considers this sheath-modified proton flux. Our simulation indicates that a plasma sheath enhances H implantation during dawn and dusk, resulting in higher nighttime retention than in previous studies. Further simulation using the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) measurements suggests that H implantation occurs continuously on the Moon throughout its orbit, either from SW or from Earth’s wind protons. A farside-nearside hemispherical asymmetry in H implantation is observed from the combined effects of the Moon’s tidal locking with Earth, periodic passage through Earth’s magnetosphere, and temperature-dependent thermal diffusion. The present analysis may have important implications for constraining the retrieval of relevant parameters from remote sensing observations.