Evidence for water ice near the lunar poles

Improved versions of Lunar Prospector thermal and epithermal neutron data were studied to help discriminate between potential delivery and retention mechanisms for hydrogen on the Moon. Improved spatial resolution at both poles shows that the largest concentrations of hydrogen overlay regions in permanent shade. In the north these regions consist of a heavily cratered terrain containing many,small (less than similar to 10-km diameter), isolated craters. These border circular areas of hydrogen abundance ([H]) that is only modestly enhanced above the average equatorial value but that falls within 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 delivery by the solar wind cannot account for these observations because the diffusivity of hydrogen at the temperatures within both sunlit and permanently shaded craters near both poles is sufficiently low that a solar wind origin cannot explain their differences. We conclude that a significant portion of the enhanced hydrogen near both poles is most likely in the form of water molecules.