Solar-wind hydrogen at the lunar poles

lunar poles as potentially indicating the presence of large quantities of water ice. Water ice might be deposited as a consequence of cometary impacts on the Moon as has been predicted theoretically. The interpretation of the neutron spectra as indicating water ice at the poles appears premature, however, in the face of the proven presence of over 100 wppm solar-wind hydrogen in many soil and regolith breccia samples. Solar-wind hydrogen also would be concentrated and preserved as a distributed regolith component in permanently shadowed areas. No significant amount of implanted solar wind hydrogen in such areas would be lost to thermal cycling and both primary and pickup ions would be continuously deposited. Further, a continuous blanket of cometary water ice, precipitated on rare occasions in permanent shadow, would remain subject to solar-wind sputtering and micrometeoroid erosion comparable to the complete reworking of the upper few centimeters of regolith approximately every ten million years. Solar-wind hydrogen, therefore, probably accounts for most of the epithermal neutron signal. Other solar-wind volatiles of interest, such as He-3, are probably also concentrated in permanent shadow; however, their relative value will be reduced relative to titanium-rich mare sources due to the higher cost of retrieval from the poles.