Molecular dynamics simulations of solar-wind induced H₂O formation and retention in vesicles of lunar soil

Several observations have shown that the lunar surface contains both [Formula: see text]O molecules and OH. One possible origin of these chemical species are implanted solar-wind hydrogen. However, the reaction pathway from hydrogen to [Formula: see text]O remains unclear. Here, we use reactive molecular dynamics simulations to investigate [Formula: see text]O formation at vesicle walls in plagioclase grains. When hydrogen atoms were implanted into plagioclase containing a spherical vesicle, 2-3 times more hydrogen became preferentially trapped at the vesicle wall as OH due to oxygen dangling bonds. The accumulation of OH and subsequent trapping of hydrogen atoms led to the synthesis of [Formula: see text]O molecules. If the trapped hydrogen does not diffuse over several lunar days, which is plausible given the strong bonding energy of the dangling bonds (>5 eV), up to a few wt% of [Formula: see text]O can form near vesicle walls. Furthermore, in vesicles lacking pathways to the outer space, their closed structure inhibits [Formula: see text]O diffusion, consistent with the detection of [Formula: see text]O in Apollo soils.