P51C-3961:
Estimating Background and Lunar Contribution to Neutrons Detected by the Lunar Reconnaissance Orbiter (LRO) Lunar Exploration Neutron Detector (LEND) Instrument
Friday, 19 December 2014
Timothy A Livengood1, Igor G. Mitrofanov2, Gordon Chin3, William V Boynton4, Larry G. Evans5, Maxim L Litvak2, Timothy P McClanahan6, Roald Sagdeev7, Anton B. Sanin2, Richard D Starr8 and Jao Jiang Su7, (1)University of Maryland College Park, College Park, MD, United States, (2)Space Research Institute RAS, Moscow, Russia, (3)NASA Goddard Space Flight Center, Code 693, Greenbelt, MD, United States, (4)Univ Arizona, Tucson, AZ, United States, (5)Computer Sciences Corporation, Stennis Space Center, MS, United States, (6)NASA Goddard Space Flight Center, Greenbelt, MD, United States, (7)University of Maryland College Park, Physics, College Park, MD, United States, (8)Catholic University of America, Washington, DC, United States
Abstract:
The fraction of hydrogen-bearing species embedded in planetary regolith can be determined from the ratio between measured epithermal neutron leakage flux and the flux measured from similar dry regolith. The Lunar Reconnaissance Orbiter (LRO) spacecraft is equipped with the Lunar Exploration Neutron Detector (LEND) instrument to measure embedded hydrogen in the Moon’s polar regions and elsewhere. We have investigated the relative contribution of lunar and non-lunar (spacecraft-sourced) neutrons by modeling maps of the measured count rate from three of the LEND detector systems using linear combinations of maps compiled from the Lunar Prospector Neutron Spectrometer (LPNS) and the LEND detectors, demonstrating that the two systems are compatible and enabling reference signal to be inferred to enable detecting hydrogen and hydrogen-bearing volatiles. The pole-to-equator contrast ratio in epithermal neutrons indicates that the average concentration of hydrogen in the Moon’s polar regolith above 80° north or south latitude is ~110 ppmw, or 0.10±0.01 wt% water-equivalent hydrogen. Above 88° north or south, the concentration increases to ~140 ppmw, or 0.13±0.02 wt% water-equivalent hydrogen. Nearly identical suppression of neutron flux at both the north and south poles, despite differences in topography and distribution of permanently-shadowed regions, supports the contention that hydrogen is broadly distributed in the polar regions and increasingly concentrated approaching the poles. Similarity in the degree of neutron suppression in low-energy and high-energy epithermal neutrons suggests that the hydrogen fraction is relatively uniform with depth down to ~1 m; the neutron leakage flux is insensitive to greater depth.
