Hydrogen has been inferred to occur in enhanced concentrations within permanently shadowed regions and, hence, the coldest areas of the lunar poles. The Lunar Crater Observation and Sensing Satellite (LCROSS) mission was designed to detect hydrogen-bearing volatiles directly. Neutron flux measurements of the Moon's south polar region from the Lunar Exploration Neutron Detector (LEND) on the Lunar Reconnaissance Orbiter (LRO) spacecraft were used to select the optimal impact site for LCROSS. LEND data show several regions where the epithermal neutron flux from the surface is suppressed, which is indicative of enhanced hydrogen content. These regions are not spatially coincident with permanently shadowed regions of the Moon. The LCROSS impact site inside the Cabeus crater demonstrates the highest hydrogen concentration in the lunar south polar region, corresponding to an estimated content of 0.5 to 4.0% water ice by weight, depending on the thickness of any overlying dry regolith layer. The distribution of hydrogen across the region is consistent with buried water ice from cometary impacts, hydrogen implantation from the solar wind, and/or other as yet unknown sources.

Variations in H2O/OH absorption band strength with time of lunar day exhibit strong dependence on latitude and terrain type.

Abstract. A detailed remote sensing survey of the Reiner Gamma Formation (RGF) region by means of Earth-based telescopic and Clementine multispectral imaging has been made in the UVvisible-near-infrared domain. The spectral mixture analysis reveals the existence of three basic end-members relevant for modeling the observed spectral variations in the RGF vicinity. These are MB (mare background), SWS (southwest swirl), and RGS (Reiner Gamma soil). The first two components exhibit spectral characteristics consistent with a prevailing contribution of mature mare soils for the surroundings (MB) and of immature mare crater-like soils (RGS) at RGF. The third intermediate-albedo component (SWS) has general characteristics of a mature mare soil, but with a redder continuum slope. The reported observation can be modeled by a mechanism which would remove the finest fraction in the soil (particle diameter < 45 [tm) at RGF and redistribute it in the vicinity with a laterally variable proportion and local accumulations such as at SWS site. According to the available set of in situ data documenting variations in the chemical composition, in the distribution of particle sizes, and in the degree of maturity with depth in the mare regolith, the characteristics depicted at RGF are those of a subsurface soil layer from a depth of the order of 0.3 -0.8 m. In our view, the simplest way to account for the whole body of information available from the present work lies in the proposition that in the area of RGF the uppermost layer of the regolith has been optically and mechanically modified by a process involving the fall of fragments of a low-density cometary nucleus previously disrupted by tidal interaction in the Earth-Moon system. We recognize, however, that in the present state of knowledge, one cannot rule out the hypothesized existence of a zone of seismically modified terrain peripheral to the Imbrium or Orientale basins just beneath the mare surface that would be the actual source of the RGF magnetic anomaly.

The design of the Lunar Exploration Neutron Detector (LEND) experiment is presented, which was optimized to address several of the primary measurement requirements of NASA's Lunar Reconnaissance Orbiter (LRO): high spatial resolution hydrogen mapping of the Moon's upper-most surface, identification of putative deposits of appreciable nearsurface water ice in the Moon's polar cold traps, and characterization of the human-relevant space radiation environment in lunar orbit. A comprehensive program of LEND instrument physical calibrations is discussed and the baseline scenario of LEND observations from the primary LRO lunar orbit is presented. LEND data products will be useful for determining the next stages of the emerging global lunar exploration program, and they will facilitate the study of the physics of hydrogen implantation and diffusion in the regolith, test the presence of water ice deposits in lunar cold polar traps, and investigate the role of neutrons within the radiation environment of the shallow lunar surface.