Hydrogen Abundances at the Lunar Poles as Measured with Lunar Prospector Neutron Spectrometer Data

D.J. Lawrence¹, W.C. Feldman¹, R.C. Elphic¹, J.J. Hagerty¹, G.W. McKinney¹, T.H. Prettyman¹, and S. Maurice^2
1 Los Alamos National Laboratory, Los Alamos, NM
² Observatoire Midi-Pyrenees, Toulouse, France

We have carried out revised modeling work of the Lunar Prospector (LP) thermal and epithermal neutron data using the particle transport code MCNPX. With our modeling, we have examined the sensitivity of epithermal neutrons to elements other than hydrogen to investigate if the polar neutron signals can be reasonably caused by any element other than hydrogen. We have found that for standard dry lunar soils, epithemal neutrons are most sensitive to composition variations of iron, gadolinium, and samarium abundances. This is consistent with measured data. However, based on various reasons, such as our current modeling, known mineral compositions of lunar soils, and other global lunar datasets, we continue to conclude that the best explanation for a decrease in the count rate of epithermal neutrons near both lunar poles is the presence of hydrogen. We have further concluded that the average hydrogen abundance is 100 – 150 ppm and is likely buried by 5 to 25 cm of dry lunar soil. This is consistent with previous studies. The localized abundance for small (<20 km) areas of permanently shaded regions remains highly uncertain and could range from 200 ppm H up to 40 wt.% H₂O in some small regions and be consistent with LP neutron data.