Water on the Moon

The Moon was long thought to be completely dry. With only a few exceptions, lunar rocks brought back by the Apollo astronauts were dry. In contrast, almost all Earth rocks contain hydrous minerals that incorporate water as building blocks of their crystal structures. Moreover, almost all lunar samples include iron metal, whose presence indicates a lack of available oxygen, which would oxidize the iron as it does on Earth. And not just water, but all volatile elements—those that condense at temperatures less than about 700 K—are depleted in lunar samples. Together, the above observations led scientists to estimate that the Moon formed with something like 1 part per billion by mass (ppbm) of water.