Chemical information content of lunar thermal and epithermal neutrons

Simulations of equilibrium thermal and epithermal neutron flux spectra for the Moon have been studied to determine their chemical information content, Most of this information resides in a single parameter, the ratio of macroscopic absorption to energy-loss cross sections. Although this parameter cannot specify the chemistry of lunar soils uniquely, it provides an index that can be used to quantitatively separate anorthositic from mafic chemistries. Hydrogen abundances can best be derived from measurements of the epithermal neutron flux, which is strongly related to its macroscopic energy-loss cross section. The ratio of thermal to epithermal neutron flux measured by Lunar Prospector is shown to agree quantitatively with simulations, This agreement is used to show that the highlands terrane on the farside of the Moon is anorthositic, containing high percentages of plagioclase and possibly a range of Mg-suite minerals with a high Mg/(Mg + Fe) atomic ratio. Our best estimate of the abundance of FeO is 2%, with an upper limit of 5%.