Solar Wind Proton Implantation and Hydrogen Diffusion: Model Comparisons to M 3 Observations

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Abstract The Moon Mineralogy Mapper (M 3 ) observed a widespread 3 μ m absorption feature on the lunar surface, commonly attributed to surficial OH and/or H 2 O. The feature varies with latitude, local time, and lunar phase, suggesting that at least part of the optically active reservoir is dynamic. We compare phase- and local-time-resolved M 3 abundance estimates with a global solar wind proton implantation, hydrogen diffusion, and H 2 exosphere model. The model tracks implanted H in the upper grain rim, thermally activated retention and loss, recombinative H 2 release, and phase-driven source variations associated with the solar wind, magnetosheath, and magnetotail. We compute the surface abundance using a weighted activation-energy-bin representation, in which the activation-energy distribution is discretized into fixed probability-weighted bins rather than sampled stochastically by Monte Carlo particles. This implementation reduces particle sampling noise in low-flux regions. A refined grid of Gaussian effective activation-energy distributions shows that the low- and midlatitude M 3 trends are best matched by distributions centered near E c = 0.520 eV with width E w = 0.090 eV. The preferred case gives a combined low- and midlatitude root mean square error of approximately 27 ppm and a small mean bias of only a few parts per million. These fitted parameters should be interpreted as an effective response of the M 3 -sensitive retained H/OH reservoir, not as a unique mineralogical activation energy. The model reproduces the dominant local-time and phase-dependent abundance structure, including the reduced retained abundance near local noon, while model–data differences near plasma-transition intervals and high-latitude behavior may reflect additional sampling, saturation, or surface-plasma processes.

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