Water on the Moon from a Surface Science Perspective

Understanding solar wind attributed sources of water and how water interacts with the lunar regolith requires knowledge of first order binding and second order desorption energies. These quantities will depend on the regolith composition (chemical and mineralogical) in addition to particle size and space weathering history. First order (chemisorbed water) desorption activation energies of chemisorbed water on Apollo lunar samples 14163 and 10084 were determined by temperature program desorption (TPD) experiments conducted under ultra-high vacuum conditions. Results from these experiments showed that highland sample 14163 exhibited a rather broad distribution of binding site energies peaking at 0.6 eV extending up to 1.8 eV at zero coverage limit while mare sample 10084 displayed a tighter distribution of binding site energies peaking at 0.65 eV ranging only to 1.4 eV at the zero coverage limit. In addition, second order (water formation) recombinative desorption activation energies of mare sample 10084 were measured as well. Water production via recombinative desorption begins around 350 K, corresponding to an activation energy of 0.8 eV assuming a hydroxyl (-OH) saturated surface. Utilizing the above measured experimental temperature driven water formation and desorption data from a lunar mare regolith Apollo sample (10084), the development and progression of the 2.8 μm optical signal on the Moon was simulated. Specifically, this infrared absorption band results from the balance of solar wind production and the subsequent thermal conversion of surficial hydroxyls to water. Overall, the model is consistent with latitude trends in the observed 2.8 μm feature on the Moon.