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 macro scopic absorption to energy-loss cross sections. Although this parameter ca nnot specify the chemistry of lunar soils uniquely, it provides an index th at can be used to quantitatively separate anorthositic from mafic chemistri es. Hydrogen abundances can best be derived from measurements of the epithe rmal 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 a greement is used to show that the highlands terrane on the farside of the M oon is anorthositic, containing high percentages of plagioclase and possibl y a range of Mg-suite minerals with a high Mg/(Mg + Fe) atomic ratio. Our b est estimate of the abundance of FeO is 2%, with an upper limit of 5%.