Background and lunar neutron populations detected by LEND and average concentration of near-surface hydrogen near the Moon's poles

Abstract

Neutron flux measurements by the Lunar Exploration Neutron Detector (LEND) on the Lunar Reconnaissance Orbiter (LRO) enable quantifying hydrogen-bearing volatiles in the lunar surface from orbit. Accurately determining hydrogen abundance requires discriminating between the instrument background detection rate and the population of lunar-sourced neutrons that are sensitive to surficial hydrogen. We have investigated the detection rate for lunar and non-lunar (spacecraft-sourced) neutrons in LEND by modeling maps of measured count rate in three LEND detector systems using linear combinations of maps compiled from LEND detectors and from the Lunar Prospector Neutron Spectrometer. We find that 30% of the global-average 24.926±0.020 neutron counts per second (cps) detected by the LEND STN3 thermal-energy neutron sensor are lunar-sourced neutrons in the thermal energy range (E<0.4 eV), 65% are lunar-sourced neutrons in the epithermal and fast energy range (E>0.4 eV), and 5% are from spacecraft-sourced background signal. In the SETN epithermal neutron detector, 90% of the 10.622±0.002 neutron detections per second are consistent with a lunar source of epithermal and fast neutrons combined (E>0.4 eV), with 3% due to lunar-sourced thermal neutron leakage into the detector (E<0.4 eV), and background signal accounting for 7% of total detections. Background signal due to spacecraft-derived neutrons is substantial in the CSETN collimated detector system, accounting for 57% of the global average detection rate of 5.082±0.001 cps, greater than the 48% estimated from cruise-phase data. Lunar-sourced epithermal and fast neutrons account for 43% of detected neutrons, including neutrons in collimation as well as neutrons that penetrate the collimator wall to reach the detector. We estimate a lower limit of 17% of lunar-sourced neutrons detected by CSETN are epithermal neutrons in collimation (0.37 cps), with an upper limit estimate of 54±11% of lunar-sourced neutrons received in collimation, or 1.2±0.2 cps global average. The pole-to-equator contrast ratio in epithermal and high-energy epithermal neutron flux indicates that the average concentration of hydrogen in the polar regolith above 80° north or south latitude is ~105 ppmw (parts per million by weight), or 0.095±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. The similar pattern of neutron flux suppression at both poles suggests that hydrogen concentration generally increases nearer the pole and is not closely associated with a specific feature such as Shackleton Crater at the lunar south pole that has no northern counterpart. Epithermal neutron flux decreases with increasing latitude outside the polar regions, consistent with surface hydration that increases with latitude if that hydration extends to ~13-40 cm into the surface.

Temperatures Near the Lunar Poles and Their Correlation With Hydrogen Predicted by LEND

“…In a direct response to the raised concerns by Lawrence et al (2011) and Mitrofanov et al (2011) presented additional evidence supporting their earlier results. Later studies (Litvak et al, 2012(Litvak et al, , 2016Livengood et al, 2018) could show further that LEND indeed measures a significant amount of collimated epithermal neutrons and represents an appropriate data set to be used for mapping hydrogen at higher spatial resolution than was possible with LPNS (Litvak et al, 2012). Therefore, we decided to use the previously published LEND data (Sanin et al, 2017) to carry out the presented analysis.…”

Volcanically Induced Transient Atmospheres on the Moon: Assessment of Duration, Significance, and Contributions to Polar Volatile Traps

“…Using estimates of mare basalt unit ages (e.g., Hiesinger et al, 2011) and thicknesses (Weider et al, 2010), Needham and Kring (2017) concluded that during a period of peak mare emplacement and volcanic volatile release at ∼3.5 Ga (supporting information Figures S1a and S1b), the maximum atmospheric pressure at the lunar surface could have reached ∼1 kPa (∼1.5 times greater than the current atmospheric surface pressure of Mars) ( Figure S1c) and that this lunar atmosphere could have persisted for ∼70 million years before fully dissipating ( Figure S1c). They further pointed out that even though most of the volcanically released volatiles will have been lost to space, if only 0.1% of the water released during these eruptions migrated to the permanently shadowed polar regions of the Moon, then the resulting hydrogen mass could account for the entire currently observed hydrogen deposits located there (Eke et al, 2009;Livengood et al, 2018).…”