Presence of water
The continuous bombardment of the Moon by comets and meteoroids has most likely added small amounts of water to the lunar surface. If so, sunlight would split much of this water into its constituent elements of hydrogen and oxygen, both of which would ordinarily escape into space over time, because of the Moon’s weak gravity. However, because of the slightness of the axial tilt of the Moon’s spin axis to the ecliptic plane—only 1.5°—some deep craters near the poles never receive direct light from the Sun and are thus in permanent shadow (see Shackleton crater). Water molecules that ended up in these craters could be stable for long periods of time.
Clementine has mapped craters at the lunar south pole that are shadowed in this way, and computer simulations suggest that up to 14,000 km² might be in permanent shadow. Results from the Clementine mission bistatic radar experiment are consistent with small, frozen pockets of water close to the surface, and data from the Lunar Prospector neutron spectrometer indicate that anomalously high concentrations of hydrogen are present in the upper metre of the regolith near the polar regions.Estimates for the total quantity of water ice are close to one cubic kilometre.Water ice can be mined and then split into its constituent hydrogen and oxygen atoms by means of nuclear generators or electric power stations equipped with solar panels. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation cost-effective, since transporting water from Earth would be prohibitively expensive. However, recent observations made with the Arecibo planetary radar suggest that some of the near-polar Clementine radar data that were previously interpreted as being indicative of water ice might instead be a result of rocks ejected from young impact craters.The question of how much water there is on the Moon has not been resolved.