Several observations have shown that the lunar surface contains both \(\hbox {H}_2\) O molecules and OH. One possible origin of these chemical species are implanted solar-wind hydrogen. However, the reaction pathway from hydrogen to \(\hbox {H}_2\) O remains unclear. Here, we use reactive molecular dynamics simulations to investigate \(\hbox {H}_2\) 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 \(\hbox {H}_2\) 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 \(\hbox {H}_2\) O can form near vesicle walls. Furthermore, in vesicles lacking pathways to the outer space, their closed structure inhibits \(\hbox {H}_2\) O diffusion, consistent with the detection of \(\hbox {H}_2\) O in Apollo soils.
