The Search for a Diurnal Effect in Lunar Hydrogen Abundance

Monday, 15 December 2014
Luis A Teodoro, BAER Inst, NASA Ames, Moffett Field, CA, United States, David J Lawrence, JHU/APL, Laurel, MD, United States, Richard C Elphic, NASA Ames Research Center, Moffett Field, CA, United States, Vincent R Eke, Durham University, Department of Physics, Durham, United Kingdom, William C Feldman, Planetary Science Institute, Planetary Science Institute, Los Alamos, NM, United States and Sylvestre Maurice, IRAP, Toulouse, France
Mapping the abundance of hydrogen-bearing materials has led to significant advances in our understanding of the sequestration of volatiles at the poles of the Moon. Neutron spectroscopy, and especially mapping of epithermal neutron fluxes, has been central to this endeavor (e.g., Feldman et al., Science, 1998).
In this talk we present a study of the diurnal variation of the Lunar Prospector neutron spectrometer (LPNS) measurements to search for the possible low-latitude mobility of water molecules. This study is prompted by reports of local-time-varying concentrations of H2O/OH, based on near-infrared spectral reflectance data (e.g., Sunshine et al., Science, 2009), as well as reports of a diurnal hydrogen signature in the Lunar Exploration Neutron Detector epithermal neutron fluxes (eg., Livengood et al., ESF, 2014). While the spectral reflectance signatures could be due to small amounts of surficial water or hydroxyl molecules within the instrument view, the neutron result implies the diurnal mobility of volumetrically significant amounts of water and/or hydroxyl. Such an extraordinary finding, if confirmed, could have significant ramifications for our understanding of the H2O/OH distribution and mobility at the lunar surface. 
In this talk, we will show that Lunar Prospector epithermal neutron data exhibit diurnal variations of the same magnitude (1-2% of the average lunar epithermal neutron flux) as those reported by Livengood et al., 2014, but the LPNS variations do not follow the same diurnal trend. Instead, the LPNS variations are systemically anti-correlated with instrument temperature, and are related to very small changes in instrument gain. These findings suggest that, rather than reflecting diurnal changes in hydrogen, the temporal fluctuations in the count rates are due to small residual systematic effects in the data reduction.