Sensitivity of orbital neutron measurements to the thickness and abundance of surficial lunar water

Recent near-infrared spectral data have shown that surficial water (H2O/OH) exists over large expanses of the lunar surface. These results have led to a reexamination of the hydrogen abundance sensitivity limits of orbital neutron data to detect surficial hydrogen on the lunar surface. A wet-over-dry, two-layer stratigraphy is modeled for the first time using neutron transport codes. For thin layers (<30 g/cm(2)), the epithermal neutron flux increases with increasing hydrogen concentration. This behavior is in contrast to the standard behavior for a single layer or dry-over-wet stratigraphy where the epithermal neutron counting rate decreases with increasing hydrogen concentration. These neutron transport results are applied to a H2O/OH enhancement at Goldschmidt crater. The neutron behavior at Goldschmidt is mostly controlled by spatial variations in neutron absorbing elements. After accounting for variations from neutron absorbing elements, there remain residual neutron enhancements at Goldschmidt crater with marginal statistical significance. If these residual enhancements are due to hydrogen, then their magnitude implies the presence of an upper layer with thickness of similar to 3-30 g/cm(2) (or 1.7-17 cm for an assumed density of 1.8 g/cm(3)) having an enhanced hydrogen abundance of 0.1-1 wt % water equivalent hydrogen. However, more work needs to be done to understand systematic variations of neutron counting rates at the 1-3% signal contrast level before a definitive conclusion can be made that the residual neutron enhancement at Goldschmidt crater is due to enhanced hydrogen abundances.