Evidence for water ice near the lunar poles

Improved versions of Lunar Prospector thermal and epithermal neutron data w ere studied to help discriminate between potential delivery and retention m echanisms for hydrogen on the Moon. Improved spatial resolution at both pol es shows that the largest concentrations of hydrogen overlay regions in per manent shade. In the north these regions consist of a heavily cratered terr ain containing many,small (less than similar to 10-km diameter), isolated c raters. These border circular areas of hydrogen abundance ([H]) that is onl y modestly enhanced above the average equatorial value but that falls withi n large, flat-bottomed, and sunlit polar craters. Near the south pole, [H] is enhanced within several 30-km-scale craters that are in permanent shade but is only modestly enhanced within their sunlit neighbors. We show that d elivery by the solar wind cannot account for these observations because the diffusivity of hydrogen at the temperatures within both sunlit and permane ntly shaded craters near both poles is sufficiently low that a solar wind o rigin cannot explain their differences. We conclude that a significant port ion of the enhanced hydrogen near both poles is most likely in the form of water molecules.