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The solar wind has been implicated as a source of water on airless bodies such as the Moon, asteroids, and possibly Mercury, yet a kinetic and mechanistic chemical model consistent with present-day observational data is still lacking. Utilizing available data sets on temperature-driven water formation and desorption from metal oxides (e.g., SiO2, TiO2, and Al2O3) with surface hydroxyl defects (-OH) and experimental data from a lunar mare regolith Apollo sample (10084), the 2.8-mu m optical signal on the Moon is modeled. Specifically, the presence and persistence of this band result from the balance of formation and loss mechanisms associated with solar wind production and thermal transformation of hydroxyls on and within the regolith. This cycle involves formation and release of molecular water via recombinative desorption of the chemically bound -OH. Though this mechanism forms gas-phase H2O on the sunlit side, photodissociation and dissociative adsorption lead to rehydroxylation and very limited exospheric water over a lunation.

Plain Language Summary The idea that water exists on the Moon has been around for many years, and its presence would provide a useful resource for human exploration. Lunar water is often observed by examining the 2.8-3 micron optical absorption feature seen in the reflecting sunlight. This feature is mainly associated with bound -OH groups made from solar wind implantation and/or from molecular water dissociating upon adsorption onto the regolith. Molecular water can form when the Moon's surface reaches 50 K above room temperature. In this process, neighboring -OH groups combine and react producing molecular water. This has been documented to occur at these relatively low temperatures for some metal oxides that are known constituents of the lunar regolith. The water will then leave following a ballistic trajectory and either molecularly adsorb or dissociate. We have modeled this process and show that the recent observations of the Moon's water may be mostly related to the presence of -OH and only a small amount of exospheric water. This process can also happen on asteroids and Mercury or any other surface that is bombarded by the solar wind and can heat up above 350 K.