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

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.

Keywords

• Lunar surface
• Lunar origin
• Lunar composition
• Ice formation