Shallowly-Buried Hydrogenation in the Lunar Regolith: Using Albedo Protons to Refine Latitude and Local Time Trends

The CRaTER instrument on LRO has tentatively detected excess hydrogen or hydrogen-bearing molecules in the lunar regolith both near the dawn terminator [Schwadron et al., 2017] and near the poles [Schwadron et al., 2016], and we are exploring multiple avenues to better quantify the degree of lunar hydrogenation in the top 10 cm of regolith as a function of latitude and time of day.

Horizon-viewing observations by CRaTER are sensitive to forward-scattering knock-on collisions of galactic cosmic rays (GCRs) with hydrogen in the top few cm of soil [de Wet et al, 2018]. Our first horizon-viewing data from 2015 revealed that the yield of 100 MeV lunar albedo protons at local lunar sunrise is double that at sunset [Schwadron et al., 2017], suggesting enhanced hydrogenation in the cool, pre-sunrise lunar regolith. We are planning horizon-viewing observations over all lunar night-time hours in 2019 to test whether this hydrogenation gradually builds up during the lunar night, or whether it occurs in a pre-sunrise "standing wave" as hypothesized by Schorghofer [2014]. The former case would suggest that the entire night-side of the Moon efficiently traps hydrogen, while the latter case would suggest that the dawn hydrogen enhancement is mostly a manifestation of a localized diffusion "wind" from the warm side of the dawn terminator to the cold side.

Schwadron et al. [2016] used an early form of CRaTER data analysis [Wilson et al., 2012] to create a coarse plot of the lunar albedo proton yield vs. latitude, and found a 1% enhancement of the yield near the poles compared to the equator, suggesting enhanced polar hydrogen in the top 10 cm of regolith. Planned horizon-viewing observations over the lunar poles, combined with the superior data reduction technique of Schwadron et al. [2017] should provide better statistics and higher spatial resolution of the high-latitude trend of hydration in the top few cm of lunar regolith.

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