Untangling the Origin of Molecular Hydrogen in the Lunar Exosphere

In situ experiments from the Apollo missions confirmed the presence of a tenuous exosphere on the Moon comprised of atoms and light molecular species. Of the most prominent volatiles found in the exosphere, molecular hydrogen (H₂) has drawn considerable attention because the confirmed detection of surface water has led many scientists to believe that proton bombardment of silicate minerals from the solar wind is the mechanism by which this water forms. As molecular hydrogen formation is a competing mechanism to bound OH/H₂O in the regolith, experimental studies are needed to determine the efficiency of molecular hydrogen formation from the solar wind. Here we show that, under simulated lunar conditions, the formation, storage, and release of molecular deuterium - as a proxy of molecular hydrogen - from deuteron implanted olivine is facile. Secondary ion mass spectrometry results reveal that diffusion processes also enrich grains with deuterium at depths beyond the maximum penetration depth of the incident ions close to 100 nm. In addition, the maximum yield of molecular deuterium escaping the amorphous rims under simulated lunar conditions strongly supports previous studies, which claim that the solar wind represents the dominant source of exospheric molecular hydrogen.