A solar wind-derived water reservoir on the Moon hosted by impact glass beads

The Moon's surface water has been a topic of significant interest and investigation over the past two decades. Initial evidence from the Lunar Prospector and Chandrayaan-1 missions confirmed the presence of water ice at the poles and hydroxyl/water across the lunar surface. The Lunar Crater Observation and Sensing Satellite (LCROSS) impact experiment provided further evidence of substantial water ice in permanently shadowed regions. The origin and distribution of this water, however, remained largely unknown. Several potential sources have been proposed, including solar wind implantation, outgassing from lunar volcanism, delivery by impacting comets and asteroids, and deposition of volatile-bearing materials. Solar wind implantation, in particular, is considered a likely mechanism for the production of hydroxyl and water through the interaction of solar wind hydrogen ions with surface minerals. The proposed lunar water cycle involves the retention, release, and replenishment of water on the lunar surface, suggesting the existence of a deeper hydrated layer acting as a reservoir. Despite investigations into various potential reservoirs such as mineral grains, agglutinates, and volcanic rocks, the location of this reservoir remained elusive. This study focuses on impact glass beads within lunar soil as a potential location of this previously unidentified reservoir.

Existing literature extensively documents the discovery of water on the Moon, starting with the confirmation of polar water ice by the Lunar Prospector mission. Subsequent missions like Chandrayaan-1 detected hydroxyl and/or water across the lunar surface, highlighting its widespread presence but varying abundances. LCROSS provided direct evidence of high water-ice abundances in permanently shadowed craters. Studies also explore potential sources, such as solar wind implantation, volcanic outgassing, and cometary/asteroid impacts. The concept of a lunar water cycle, involving retention, release, and replenishment, has been proposed, emphasizing the need to identify a deep hydrated layer as a reservoir. Prior studies explored various potential reservoirs within lunar soils, such as mineral grains, agglutinates, and volcanic rocks, but a definitive reservoir remained unidentified, creating the impetus for this study focusing on impact glass beads.

The research utilized lunar soil samples (CE5C0100YJFM00103) collected by the Chang'e-5 mission. 117 individual spherical glass beads were initially characterized using field-emission scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and Raman spectroscopy. 32 beads with smooth surfaces and compositions consistent with CE5 basalts underwent in-situ water abundance and hydrogen isotope analysis using a nanoscale secondary ion mass spectrometer (NanoSIMS). Water abundances were measured along transects across five beads to examine core-to-rim variations. The methodology involved meticulous sample preparation, including mounting, polishing, and cleaning to minimize contamination. SEM provided petrographic observations, while EPMA quantified major and minor element abundances. NanoSIMS analysis quantified water abundances and hydrogen isotope compositions. Raman spectroscopy confirmed the form of water (hydroxyl/molecular water) within the beads. The data was analyzed using a binary mixing model to account for the interplay between pre-existing water and solar wind-derived water, and diffusion modeling was employed to estimate water diffusion timescales. De-gassing modeling was also used to assess the role of volatile loss in the observed isotopic variations.

The study's key findings center on the discovery that impact glass beads within the lunar soil serve as a significant reservoir for water originating from the solar wind. The impact glass beads exhibit a negative correlation between water abundance and δD values, ranging from 0–1,909 µg g⁻¹ and –990‰ to 522‰ respectively. Core-to-rim variations within the beads show elevated water abundances and lower δD values at the rims, consistent with solar wind isotopic signatures. The extremely low δD values at the rims strongly suggest a solar wind origin for the water. De-gassing modeling excluded the possibility of isotopic variations being solely attributed to H2 loss. Instead, the findings support a two-endmember mixing model, where solar wind-derived water diffused inward into the initially water-poor glass beads. Diffusion modeling estimated that water diffusion timescales were remarkably short (less than 15 years at 360K), indicating an efficient recharge mechanism. The estimated total water hosted by impact glass beads in lunar soils reaches up to 2.7 × 10¹⁴ kg, representing a substantial contribution to the Moon's overall water inventory. The study's detailed analysis of hydration profiles, coupled with isotopic data and modeling, unequivocally demonstrates the post-formation ingress of solar wind-derived water into the impact glass beads.

The findings directly address the research question of identifying the missing lunar water reservoir by presenting strong evidence that impact glass beads serve as a crucial reservoir. The short diffusion timescales for solar wind-derived water imply a dynamic process capable of sustaining the lunar water cycle. The significant water inventory within these beads underscores their importance in lunar water dynamics. The results have broad implications for understanding water distribution on airless bodies and for planning future lunar exploration. The dynamic ingress and egress of water within these beads, governed by temperature oscillations, could potentially buffer the observed daily and global variations in surface and exospheric water abundance.

This research successfully identifies impact glass beads as a significant reservoir for solar wind-derived water on the Moon. The rapid diffusion rates and substantial water inventory highlight their role in the lunar water cycle. Future research could focus on expanding analyses to other lunar regions and airless bodies to determine the generality of this finding and further refine models of lunar water dynamics. The potential of impact glass as a water reservoir on other airless bodies should also be investigated.

The study primarily focuses on impact glass beads from the Chang'e-5 landing site. While the findings strongly suggest a broader relevance, further research is needed to confirm the generality of this observation across different lunar regions with varying geological histories. The diffusion modeling relies on assumptions regarding temperature and diffusion coefficients, which might introduce uncertainties. The cosmic ray exposure age used for spallation correction was an estimate, and a more precise age would refine the correction.