Lunar South Polar Hydrogen Concentrations in the Context of LRO/Diviner Results and Modeling

At the Moon, the leakage flux of cosmic ray-generated epithermal neutrons depends strongly on soil hydrogen content, and less strongly on other factors such as soil temperature [Lawrence et al., 2006]. For the lunar polar regions where warm sunlit features lie adjacent to permanently-shadowed (and very cold) areas, and where significant seasonal changes occur, it is important to take soil temperature into account. Here we investigate the effects of the subsurface soil temperature profile on epithermal leakage flux. As a starting point, we use a model of seasonally-averaged LRO/Diviner subsurface temperature to predict the effect on epithermal leakage flux. An isothermal depth profile is assumed. The relationship between Lunar Prospector epithermal detector count rates and surface temperature comes from Lawrence et al. [2006], and assumes the average soil near the south pole is of typical ferroan anorthosite highlands composition: epi = a + bT, where a = 19.65 counts/sec, b = 2.505x10-3 counts/sec/K, and T is the uniform soil temperature. We have adjusted the constant a to a value consistent with the epithermal measurements at lower latitudes. The temperature-dependent flux is convolved with the Lunar Prospector neutron spectrometer instrument spatial response for 30-km altitude to produce a map of predicted epithermal count rate that would be measured from orbit, in the absence of any soil hydrogen variations. We find that the predicted epithermal neutron count rates vary only over a narrow range: 19.85 to 20.1 counts/sec across a scene 600 x 600 km centered on the pole. The instrument-convolved prediction map shows a count rate low over the trio of large craters Faustini (87.2S, 89E), Shoemaker (88.5S, 50E), and Haworth (87.4S, 355E), and a separate low over Cabaeus (85.6S, 308.9E). The variation in predicted count rate is much smaller than what is observed in smoothed maps of the Lunar Prospector epithermal count rate. For example, the lowest epithermal count rate on the Moon, 18.3 counts/sec, is seen at Cabaeus. Such a low is consistent with concentrations of 1+ wt% water-equivalent hydrogen within the permanently-shadowed area of the crater. We will present the results of a search for seasonal variations in epithermal leakage flux. (This work is supported by the LRO Participating Scientist Program.)