Hydroxylation and Hydrogen Diffusion in Lunar Samples: Spectral Measurements During Proton Irradiation

Widespread OH/$H_2$O has been detected across the lunar surface, but its origin remains a matter of active investigation. We present laboratory measurements of Apollo-17 samples LS 78421 and LS 73131 that demonstrate, in situ, the formation of OH/$H_2$O during irradiation by energetic hydrogen ions similar to the solar wind. Results unambiguously show the growth of the $\sim$3 $\mu$m spectral band due to hydrogen irradiation alone with no contribution from terrestrially adsorbed water. Significant variations in the $\sim$3 $\mu$m band characteristics and position are measured between LS 78421, LS 73131, and a control of crushed silica, reflecting the unique crystal structures and chemical environments within each sample. By cycling sample temperatures to 400 K (lunar dayside maximum), we highlight the role of hydrogen diffusion in the dynamics of OH/$H_2$O within the lunar soil. Diffusion activation energies of 0.66–0.75 eV and 0.72–0.81 eV are obtained for LS 78421 and LS 73131, respectively.