Polar hydrogen deposits on the Moon

Neutron and gamma-ray data measured using the Lunar Prospector spectrometer s were analyzed to define the enhanced hydrogen deposits near both poles of the Moon. Combining the new low-altitude neutron data (30 +/- 15 km) with previous high-altitude (100 +/- 20 km) neutron data and the results of seve ral recent radar investigations sharply constrains the characteristics of e ach of the polar deposits. The deposits at the north appear to be in the fo rm of many small pockets or of generally distributed hydrogen that average to a 100 ppm weight fraction enhancement over that which exists in regolith at more equatorial latitudes. Those deposits in the permanently shaded cra ters near the south pole are consistent with a thick ferroan anorthosite re golith containing an enhancement of 1670 +/- 890 ppm hydrogen, which, if in the form of water ice, amounts to 1.5 +/- 0.8% weight fraction of H2O. Neu tron data alone cannot discriminate between hydrogen implanted in lunar soi l from the solar wind, hydrated minerals, or H2O. These craters appear to b e surrounded by regolith that either contains small pockets of enhanced hyd rogen or is soil that is uniformly impregnated with hydrogen enhanced on av erage by about 100 ppm above that contained in soils at more equatorial lat itudes.