Overview of Results from the Lunar Reconnaissance Orbiter (LRO) Lunar Exploration Neutron Detector (LEND) Instrument. R. Z. Sagdeev1, W. V. Boynton2, G. Chin3, M. Litvak4, T. A. Livengood5, T. P. McClanahan3, I. G. Mitrofanov4, and A. B. Sanin4. 1University of Maryland, College Park, MD, 2Lunar and Planetary Laboratory, Tucson, AZ, 3NASA/GSFC, Greenbelt, MD, 4Institute for Space Research, Moscow, Russia, 5CRESST/UMD/GSFC, Greenbelt, MD.

The Lunar Exploration Neutron Detector (LEND)

on the Lunar Reconnaissance Orbiter (LRO) is tasked with evaluating the quantity of hydrogen-bearing spe-cies within the uppermost meter of the lunar regolith; investigating the presence and distribution of possible water-ice deposits at the bottom of permanently shad-owed regions (PSRs) near the poles; and determining the neutron contribution to total radiation dose at an altitude of 50 km above the Moon [1]. To fulfill these goals, LEND has been mapping the distribution of thermal and epithermal neutron leakage flux since LRO entered the polar mapping orbit at 50 km altitude in September 2009 [2]. In December 2011, LRO moved to an elliptical orbit with 30 km periselene over the south pole to map it in greater detail, with apose-lene above the north pole. During the commissioning phase of the mission, July–September 2009, LEND obtained preliminary mapping of hydrogen/water de-posits near the lunar south pole which contributed to selecting the site for the successful LCROSS impactor mission [3].

Global maps of neutron leakage flux measured with LEND show regional variations in thermal (energy range < 0.015 eV) and fast neutrons (>0.5 MeV), and yield a global map of epithermal neutron flux [2]. Spa-tial resolution of the collimated detector has been shown consistent with the design value of 5 km radius for half the detected lunar neutrons, with the remainder spatially diffuse [4]. Statistically significant neutron-suppressed regions (NSRs) are not closely related to PSRs [5]. Outside of the NSRs, hydrogen content in-creases directly with latitude at both poles. Thermal volatilization of water deposits may be responsible for increasing H concentrations nearer the poles because it is minimized at the low surface temperature of the poles. Significant neutron suppression regions (NSRs) relative to neighboring regions have been found in three large PSRs, Shoemaker and Cabeus in the south and Rozhdestvensky U in the north [6]. Some small PSRs display excess neutron emission in comparison to the sunlit vicinity. On average, PSRs other than these three do not contain significantly more hydrogen than sunlit areas around them at the same latitude.

Correlation between neutron suppression measured by LEND and illumination models for the Moon's po-lar regions suggests that insolation at the Moon’s poles is an important factor in locally modulating hydrogen concentrations [7]. The highest concentrations of hy-

drogen appear to be found on poleward-facing vs. equivalent equatorward slopes, although some local-ized high-latitude variations in hydrogen concentration exist that are not explained via insolation.

Fig. 1: Count rate differences relative to smoothed region near north and south poles to latitude 82°. The maps are made by subtracting the background and taking differences from the local count rates at the same latitude. Neutron-suppression regions (NSRs) appear in green and blue. Larger PSRs are outlined. Some NSRs are associated with PSRs (Shoemaker and Cabeus), but many are not.

The long duration of the LRO mission and steady

nadir-pointing geometry enable investigations of low-amplitude regional-scale neutron suppressions in new investigations. Epithermal neutron flux is slightly sup-pressed near the dawn terminator at near-equatorial latitude, with least suppression in local lunar mid-afternoon, implying a mobile population of hydrogen-bearing volatiles near the terminator that resides tran-siently in the regolith [8]. The observed pattern sup-ports hypothesized mineral hydration at the terminator in the form of H2O/OH.

References: [1] Mitrofanov et al. (2008) Astrobi-

ology 8, 793–804. [2] Litvak et al. (2012) JGR-Planets 117, E00H22. [3] Mitrofanov (2010) Science 330, 483–486. [4] Boynton et al. (2012) JGR-Planets 117, E00H33. [5] Mitrofanov et al. (2012) JGR-Planets 117, E00H27. [6] Sanin (2012) JGR-Planets 117, E00H26. [7] McClanahan et al. (2013) Lunar and Planetary Science Conference 44, No. 1719, id.2374. [8] Livengood et al. (2013) LPI 44, No. 1659, id.2643.

7057.pdfAnnual Meeting of the Lunar Exploration Analysis Group (2013)