Presence of water
Main article: Lunar water
Twenty degrees of latitude of the Moon's disk, completely covered in the overlapping circles of craters. The illumination angles are from all directions, keeping almost all the crater floors in sunlight, but a set of merged crater floors right at the south pole are completely shadowed.
Mosaic image of the lunar south pole as taken by Clementine: note permanent polar shadow.
Liquid water cannot persist at the Moon's surface, and water vapour quickly evaporates, breaks up through photodissociation due to sunlight, and is lost to space. However, scientists have thought since the 1960s that water ice, deposited by impacting comets or produced by the reaction of oxygen-rich lunar rocks and hydrogen in the solar wind, could survive in the cold, permanently shadowed craters at the Moon's poles. These craters have been in shadow for the past two billion years, and computer simulations suggest that up to 14,000 km2 might be in permanent shadow. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation cost-effective, since transporting it from Earth would be prohibitively expensive.
Many different signatures of lunar water have since been found. In 1994, Clementine's bistatic radar experiment found indications of small, frozen pockets of water close to the surface (though later Arecibo radar observations suggested these might be rocks ejected from young impact craters); Lunar Prospector's neutron spectrometer indicated in 1998 that high concentrations of hydrogen are present in the upper metre of the regolith near the polar regions; in 2008, new analysis found small amounts of water in the interior of volcanic lava beads brought to Earth by Apollo 15. Chandrayaan-1's imaging spectrometer detected water and hydroxyl absorption lines in reflected sunlight, evidence of large quantities of water on the Moon's surface, possibly as high as 1,000 ppm. Weeks later, the LCROSS mission flew its 2300 kg impactor into a permanently shadowed polar crater, and detected at least 100 kg of water in the plume of ejected material.
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