In 2009, the LCROSS satellite determined that there is a surprising amount of water ice located on earth's moon (or at least within one particular crater that LCROSS was designed to investigate).
So the question is: where did this water come from? First of all this crater is permanently shielded from the Sun's light, so any liquid water that might find its way to the the crater would freeze and remain in a frozen state. Liquid water finding its way to other locations would probably be lost as liquid water is not stable at those locations.
Of course that only explains why there is ice in the crater, we still need to know how water got to the moon in the first place. A well known theory is that this water came from comets. A less well known theory is that hydrogen ions from the solar wind combined with oxygen to form water as well as other related compounds. (There is oxygen bound up in compounds within the moon's regolith; Regolith is the material on and near the surface of the moon roughly equivalent to soil on the earth, though regolith has a very different composition than soil).
A recent article published in the online version of Nature Geoscience support the theory that water on the moon was formed from the solar wind plus oxygen in the regolith. The article also suggests that water ice could be located on mercury and other solar system objects.
For more details, see "Solar wind particles likely source of water locked inside lunar soils" Published on Oct 15, 2012.
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