Lunar water may have come from the Sun

New Scientist : Although the Moon has long appeared to be a desert, there is actually a lot of water contained in minerals in its soil. Three possible sources of that water have been proposed: comets and meteorites, cosmic rays, or the solar wind. To try to determine which source is most likely, Alice Stephant and François Robert of the National Museum of Natural History in Paris measured the amounts of hydrogen and deuterium in soil samples collected by two Apollo missions. Because the amount of deuterium depends on how far the source was from the Sun, the ratio of the two elements can indicate where they originated. From that information, the researchers conclude that nearly all of the water in the upper 200 nm of the Moon’s surface came from the solar wind. Furthermore, they say the amount from comets and meteorites is negligible. However, water located deeper than 200 nm in the lunar surface may come from cosmic rays, which can penetrate rock and cause chemical reactions deep below the surface.