Solar Wind Implantation Into the Lunar Regolith: Monte Carlo Simulations of H Retention in a Surface With Defects and the H₂ Exosphere

Abstract

The solar wind implants protons into the top 20–30 nm of lunar regolith grains, and the implanted hydrogen will diffuse out of the regolith but also interact with oxygen in the regolith oxides. We apply a statistical approach to estimate the diffusion of hydrogen in the regolith hindered by forming temporary bonds with regolith oxygen atoms. A Monte Carlo simulation was used to track the temporal evolution of bound OH surface content and the H2 exosphere. The model results are consistent with the interpretation of the Chandrayaan‐1 M3 observations of infrared absorption spectra by surface hydroxyls as discussed in Li and Milliken (2017, https://doi.org/10.1126/sciadv.1701471). The model reproduced the latitudinal concentration of OH by using a Gaussian energy distribution of f(U0 = 0.5 eV, UW = 0.078–0.1 eV) to characterize the activation energy barrier to the diffusion of hydrogen in space weathered regolith. In addition, the model results of the exospheric content of H2 are consistent with observations by the Lyman Alpha Mapping Project on the Lunar Reconnaissance Orbiter. Therefore, we provide support for hydroxyl formation by chemically trapped solar wind protons.

Surface Water at Lunar Magnetic Anomalies

“…At Reiner Gamma, the field strength is comparable to Gerasimovich, yet Reiner Gamma shows a weaker difference in water supression. We attribute this weaker difference due to the slightly lower latitude of Reiner Gamma (7°N vs 22°S), where warmer temperatures will reduce the background water (Benna et al, ; Farrell et al, ; Hendrix et al, ; Jones et al, ; Li & Milliken, ; Tucker et al, ; Wöhler, Grumpe, Berezhnoy, & Shevchenko, ).…”

“…Lunar surface water was then discovered with infrared data from spacecraft measurements (Clark, ; Pieters et al, ; Sunshine et al, ) and later on was confirmed with ground‐based telescope observations (Honniball et al, ); this water is stable in sunlight at middle to high latitudes (e.g., Clark, ; Honniball et al, ; Pieters et al, ; Sunshine et al, ). A major formation mechanism of this type of lunar surface water is attributed to solar wind implantation (Bandfield et al, ; Clark, ; Hendrix et al, ; Jones et al, ; Li & Milliken, ; McCord et al, ; Pieters et al, ; Sunshine et al, ; Tucker et al, ; Wöhler, Grumpe, Berezhnoy, & Shevchenko, ). Water was also directly observed in the Lunar Crater Observation and Sensing Satellite ejecta plumes at Cabeus crater (Colaprete et al, ; Gladstone et al, ; Hayne et al, ; Schultz et al, ).…”

“…Similarly, it is unclear how the reduction in UV photon flux that controls the photoionization loss of water competes with these two other effects. Although models have been suggested to explain some of these effects (Farrell et al, 2017;Jones et al, 2018;Tucker et al, 2019;Wöhler, Grumpe, Berezhnoy, Feoktistova, et al, 2017), there is no consensus on what controls the equilibrium abundance. Measuring the abundances of water across regions where at least one of these variables is controlled would provide insight into the mechanisms that produce surface water.…”

“…Changes in solar wind protons reaching the surface may produce variations in lunar surface water content (Farrell et al, 2017;Jones et al, 2018;Schaible & Baragiola, 2014;Tucker et al, 2019;Zeller et al, 1966), if the contribution of water from impacts and interior degassing is constant in that region and represents a background water signal. Thus, assessing whether water content is suppressed at magnetic anomalies may reveal how much contribution is from solar wind implantation.…”

Magnetic evidence for an extended hydrogen exosphere at Mercury

“…Usually, the sources and sinks for the H atoms are assumed to be related to the surface chemistry [36] caused by solar wind protons which are implanted in the surface regolith, where they diffuse and degas to the exosphere mainly as H 2 molecules [21,42]. A number of exospheric models and computational simulations have been developed to explain these highly complex and interrelated source and loss processes to understand the composition of Mercury's exosphere [51,25,35,52,24].…”