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1. Evidence for Widespread Hydrogen Sequestration within the Moon’s South Pole Cold Traps

Author

T. P. McClanahan, A. M. Parsons, T. A. Livengood, J. J. Su, G. Chin, D. Hamara, K. Harshman, and R. D. Starr

Subjects
Lunar surface, Lunar origin, Lunar composition, Ice formation, Astronomy, QB1-991
Abstract

Hydrogen-bearing volatiles are observed to be concentrated, likely in the form of water ice, within most of the Moon's permanently shadowed regions (PSRs), poleward of 77° S. Results show that instrumental blurring of the Moon’s epithermal neutron flux correlates the PSRs’ observed hydrogen concentration by their areal density. Epithermal neutron observations of 502 PSRs are positively correlated indicating that they have similar expected hydrogen concentrations, 0.28 ± 0.03 wt% water-equivalent hydrogen, relative to neutron background observations (lower bounds). The correlation arises from the PSRs’ proportional detection attributed to their similar hydrogen distributions and their areal fraction of the collimated instrument footprint of the Collimated Sensor for Epithermal Neutrons (CSETN), which is part of the Lunar Exploration Neutron Detector on board the Lunar Reconnaissance Orbiter (LRO). The lowest hydrogen concentration areas coincide with low PSR areal densities that occur with highly illuminated and warm, equator-facing sloped surfaces. Results show that the maximum hydrogen concentrations observed within the Haworth, Shoemaker, and Faustini PSRs coincide with their coldest surface temperatures, below 75 K that occur near the base of their poleward-facing slopes. Anomalously enhanced hydrogen concentrations around the Cabeus-1 PSR suggest at least two lunar hydrogen sources. The uncollimated neutron counting rate map is subtracted from CSETN’s collimated neutron map using a novel spatial bandpass filter. The results indicate water ice and perhaps other hydrogen-bearing volatiles are being randomly distributed to the surface and the PSRs’ low sublimation rates likely maximize their residence times and elevate their surface concentrations. CSETN’s corrected south polar map is correlated to coregistered maximum temperature and topography maps made by LRO’s Diviner and Lunar Orbiter Laser Altimeter instruments.

Published
2024
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2. Background and Lunar Neutron Populations Detected by LEND and Average Concentration of Near-Surface Hydrogen near the Moon's Poles

Author

T A Livengood, I G Mitrofanov, G Chin, W V Boynton, J G Bodnarik, L G Evans, K P Harshman, M L Litvak, T P Mcclanahan, R Z Sagdeev, A B Sanin, R D Starr, and J J Su

Subjects
Lunar And Planetary Science And Exploration
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 80deg north or south latitude is ∼105 ppmw (parts per million by weight), or 0.095 +/- 0.01 wt% water-equivalent hydrogen. Above 88deg 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.

Published
2017
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3. The variations of neutron component of lunar radiation background from LEND/LRO observations

Author

I. G. Mitrofanov, Maxim Mokrousov, R. Z. Sagdeev, Т.P. McClanahan, M. L. Litvak, T. A. Livengood, A. V. Malakhov, R. D. Starr, B. N. Bakhtin, Larry G. Evans, K. Harshman, J.G. Bodnarik, A. B. Sanin, William V. Boynton, and Gordon Chin

Subjects
Physics, 010504 meteorology & atmospheric sciences, Astrophysics::High Energy Astrophysical Phenomena, Astrophysics::Instrumentation and Methods for Astrophysics, Magnetic flux leakage, Astronomy, Astronomy and Astrophysics, Cosmic ray, Astrophysics, Radiation, 01 natural sciences, law.invention, Orbiter, Space and Planetary Science, law, Neutron flux, Physics::Space Physics, 0103 physical sciences, Neutron detection, Neutron, Astrophysics::Earth and Planetary Astrophysics, 010303 astronomy & astrophysics, 0105 earth and related environmental sciences, Background radiation
Abstract

Lunar neutron flux data measured by the Lunar Exploration Neutron Detector (LEND) on board NASA's Lunar Reconnaissance Orbiter (LRO) were analyzed for the period 2009-2014.We have re-evaluated the instrument's collimation capability and re-estimated the neutron counting rate measured in the Field of View (FOV) of the LEND collimated detectors, and found it to be 1.070.1counts per second. We derived the spectral density of the neutron flux for various lunar regions using our comprehensive numerical model of orbital measurements. This model takes into account the location of the LEND instrument onboard LRO to calculate the surface leakage neutron flux and its propagation to the instrument detectors. Based on this we have determined the lunar neutron flux at the surface to be approx. 2 neutrons/ [sq cm/ sec] in the epithermal energy range, 0.4e V to 1keV. We have also found variations of the lunar neutron leakage flux with amplitude as large as a factor of two, by using multi-year observations to explore variations in the Galactic Cosmic Ray (GCR) flux during the 23rd-24th solar cycles.

Published
2016
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4. Evidence for the sequestration of hydrogen-bearing volatiles towards the Moon’s southern pole-facing slopes

Author

A. B. Sanin, T. P. McClanahan, M. L. Litvak, A. M. Parsons, G. M. Milikh, J.G. Bodnarik, T. A. Livengood, William V. Boynton, A. V. Malakhov, K. Harshman, Jao-Jang Su, Erwan Mazarico, Dmitry Golovin, D. K. Hamara, R. D. Starr, G. Nandikotkur, Gordon Chin, Larry G. Evans, R. Z. Sagdeev, G. Droege, I. G. Mitrofanov, and J. Murray

Subjects
Radiometer, Ices, Mineralogy, Flux, Astronomy and Astrophysics, Regolith, Latitude, Astrobiology, Neutron flux, Space and Planetary Science, Moon, surface, Environmental science, Neutron detection, Neutron, Regoliths, Moon, Diviner
Abstract

The Lunar Exploration Neutron Detector (LEND) onboard the Lunar Reconnaissance Orbiter (LRO) detects a widespread suppression of the epithermal neutron leakage flux that is coincident with the pole-facing slopes (PFS) of the Moon’s southern hemisphere. Suppression of the epithermal neutron flux is consistent with an interpretation of enhanced concentrations of hydrogen-bearing volatiles within the upper meter of the regolith. Localized flux suppression in PFS suggests that the reduced solar irradiation and lowered temperature on PFS constrains volatility to a greater extent than in surrounding regions. Epithermal neutron flux mapped with LEND’s Collimated Sensor for Epithermal Neutrons (CSETN) was analyzed as a function of slope geomorphology derived from the Lunar Orbiting Laser Altimeter (LOLA) and the results compared to co-registered maps of diurnally averaged temperature from the Diviner Lunar Radiometer Experiment and an averaged illumination map derived from LOLA. The suppression in the average south polar epithermal neutron flux on equator-facing slopes (EFS) and PFS (85–90°S) is 3.3 ± 0.04% and 4.3 ± 0.05% respectively (one-sigma-uncertainties), relative to the average count-rate in the latitude band 45–90°S. The discrepancy of 1.0 ± 0.06% between EFS and PFS neutron flux corresponds to an average of ∼23 parts-per-million-by-weight (ppmw) more hydrogen on PFS than on EFS. Results show that the detection of hydrogen concentrations on PFS is dependent on their spatial scale. Epithermal flux suppression on large scale PFS was found to be enhanced to 5.2 ± 0.13%, a discrepancy of ∼45 ppmw hydrogen relative to equivalent EFS. Enhanced poleward hydration of PFS begins between 50°S and 60°S latitude. Polar regolith temperature contrasts do not explain the suppression of epithermal neutrons on pole-facing slopes. The Supplemental on-line materials include supporting results derived from the uncollimated Lunar Prospector Neutron Spectrometer and the LEND Sensor for Epithermal Neutrons.

Published
2015
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