On the origin of excess hydrogen at the lunar poles

The Lunar Prospector results pointing to the presence of water on the Moon are analyzed. Since water ice is very volatile, these results are difficult to interpret as evidence for its presence on the Moon. It was demonstrated that hydrogen detected with a neutron spectrometer can be of radiative ori gin, i.e., hydrogen can accumulate in the lunar regolith as a result of imp lantation of solar-wind protons and formation of hydroxyl groups. A compari son of the neutron-spectrometer data with the results of laboratory experim ents on proton implantation confirms this presumption. In the context of th is hypothesis, a nonuniform distribution of excess hydrogen over the lunar surface is explained by the difference in hydrogen diffusivity in materials of various lunar regions, and high hydrogen concentration at the lunar pol es, as compared to equatorial zones, is attributed to a sharp decrease in h ydrogen diffusivity in solids with decreasing temperature. Since mobility o f atoms is low at the temperatures of the lunar poles, hydrogen concentrati on may reach its limiting value in the polar regions.