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

Multiple missions and observations have established that water exists at or near the lunar surface in various forms, including hydroxyl/water absorption bands at 2.8–3.0 µm, polar ice deposits, exospheric water released by meteoroid impacts, and molecular water on the sunlit surface. Proposed sources include solar wind implantation, volcanic outgassing, pyroclastic deposits and minerals, and exogenous delivery by comets and asteroids. A global lunar surface water cycle has been proposed involving retention, migration to colder regions, and loss to space. To sustain such a cycle, a subsurface hydrated reservoir within lunar soils is required, but it has not been identified. The study aims to test whether impact glass beads in lunar soils constitute this missing water reservoir by characterizing their water contents, hydrogen isotope compositions, and core-to-rim profiles in Chang’e-5 (CE5) returned samples.

Prior work detected lunar polar ice via neutron spectroscopy and impact experiments, and mapped widespread OH/H2O absorptions across the lunar surface with spatial and temporal variability. The LADEE mission detected exospheric water from meteoroid impacts, and ground-based observations identified molecular water on the sunlit Moon. Proposed water sources and processes include solar wind H implantation forming OH/H2O on mineral surfaces, indigenous water related to volcanism (including in pyroclastic glasses and melt inclusions), exogenous delivery by comets and carbonaceous chondrites, and impact-produced agglutinates. However, earlier studies focusing on fine mineral grains, agglutinates, volcanic rocks, and pyroclastic glass beads did not fully account for a soil reservoir capable of buffering the lunar surface water cycle. Geochronology indicates CE5 impact glass beads formed over the last ~2 Gyr with age peaks linked to nearby craters, suggesting ongoing production of such beads in the regolith.

Samples: CE5 lunar soil (CE5C0100YJFM00103, ~1 g) collected by the CE5 lander robotic arm. A total of 150 grains (~50 µm to ~1 mm) were hand-picked; 117 were spherical/ovoid glass beads. Beads were mounted in epoxy, prepared as double-polished sections (~100 µm thick), epoxy removed to improve vacuum, and cleaned and baked. Petrography/chemistry: Field-emission SEM (FEI Nova NanoSEM 450, Thermo Apreo; 15 kV, 2.0–6.4 nA) and EPMA (CAMECA SXFive; 20 kV, 10 nA, 10 µm beam; standards VG-2, MPI-DING-GOR128) characterized textures and major/minor elements. Homogeneous beads with mare-like compositions were selected. Water/isotopes: 32 impact glass beads with smooth surfaces and CE5-basalt-like chemistry were selected for in situ H2O and δD analyses using CAMECA NanoSIMS 50L in two sessions. Analysis pits 7×7 µm (central 5×5 µm collected), ~0.5 nA Cs−, pre-sputter 2 min with 2 nA, electronic gating (49% blanking), pressure 1.5–1.9×10^−10 Torr. Secondary ions measured: 1H−, 2D−, 12C−, 16O−. Liquid N2 cold trap used in session 2. Charging compensated by electron flood. Background and calibration: Instrument H2O background determined using San Carlos olivine (1.4 µg g−1 H2O): session 1 Hbg ~815±457 cps (29.8±14.8 µg g−1 H2O), session 2 Hbg ~215±40 cps (11.7±2 µg g−1 H2O). Water abundances computed from background-subtracted H/O and calibration slopes established on standards: Durango apatite, Kovdor apatite, SWIFT MORB glass, and basaltic glasses 519-4-1 and 1833-11. δD corrected for instrumental mass fractionation using SWIFT MORB; reproducibility monitored with standards. δD reported relative to SMOW (D/H=1.5576×10^−4). Cosmogenic spallation corrections used D production rate 2.17×10^−12 mol g^−1 Myr^−1 with assumed 50 Ma exposure; alternate slower rate tested with negligible impact. Raman spectroscopy: WITec alpha 300 R, 484 nm laser, 9.2 mW, 5 µm spot, 500 s integration; measured OH/H2O band (~3300–3700 cm−1) in selected bead (CE5#33,076) and MORB glass standards (MRN-G1, EPR-G3) to confirm water speciation. Profiles and modelling: Six NanoSIMS transects across five beads (CE5#33,003; #33,036 profiles 1 and 2; #33,046; #33,052; #33,076) measured core-to-rim H2O and δD. Diffusion of OH/H2O modelled using Fick’s second law (complementary error function solution) with D=20.84 µm^2 yr^−1 at T=360 K (latitude ~45°N, depth <3 cm). Boundary condition Cs set per bead (500–3000 µg g−1) and initial C0 ~10 µg g−1. For bead CE5#33,036 Profile 2, two-stage model included post-diffusion degassing proportional to surface abundance to reproduce rim depletion. De-gassing Rayleigh modelling assessed and ruled out as primary cause of δD-H2O trends. Two-endmember mixing model quantified mixing between initial low-H2O, higher-δD component (H2O 5–50 µg g−1; δD up to +500‰) and solar wind water (δD ~ −980 to −990‰; H2O up to ~2000 µg g−1).

- CE5 impact glass beads contain 0–1,909 µg g−1 equivalent H2O and display δD values from −990±6‰ to +522±440‰, with a strong negative correlation between H2O and δD. - Core-to-rim profiles show higher H2O at rims (commonly up to ~2,000 µg g−1) decreasing toward cores, while δD decreases from cores to rims; core H2O is near instrumental background (~10–30 µg g−1), implying initial dehydration during bead formation and subsequent hydration. - Extremely low rim δD values (~−990‰) match solar wind hydrogen, distinguishing this water from volcanic, chondritic, or cometary sources. - De-gassing cannot explain the observed H2O–δD range; two-endmember mixing with a solar wind endmember reproduces the trend and constrains pre-hydration bead water to <50 µg g−1. - Diffusion modelling of observed profiles at 360 K yields short diffusion durations of ~1–15 years, indicating rapid inward diffusion and storage of solar wind-derived water in beads. - Evidence of post-diffusion rim water loss in one profile suggests beads can both store and release water depending on temperature conditions. - Estimated bulk water abundance of individual beads: 132–1,570 µg g−1; modal abundance of beads in soils (~3–5 vol%) implies a contribution of ~4–78 µg g−1 equivalent water to bulk regolith, consistent with CE5 lander reflectance-derived estimates. - Global inventory: impact glass beads in lunar soils could collectively host up to ~2.7 × 10^14 kg of water. - Bead compositions are largely consistent with local CE5 mare basalts and show volatile depletion (Na2O, K2O) consistent with high-temperature formation prior to solar wind hydration.

The observations directly identify lunar impact glass beads as an efficient near-surface reservoir for solar wind-derived water. The inward diffusion profiles and solar-wind-like δD at rims demonstrate post-formation hydration, providing the missing reservoir required to buffer a dynamic lunar surface water cycle. Short diffusion timescales at realistic surface temperatures suggest that beads can be recharged rapidly and repeatedly, while observed rim depletion indicates that water can be released to the exosphere during temperature increases or impact events. This cyclical ingress/egress mechanism can explain global and diurnal variations in surface OH/H2O and exospheric water observed by remote sensing and LADEE. Compared to mare basalts, where water is primarily in apatite and less susceptible to diurnal cycling, impact glass beads are better suited to participate in surface-exosphere exchange. The findings imply that similar impact-generated glasses on other airless bodies (e.g., Mercury, Vesta, asteroids) may also act as reservoirs for implanted water, with implications for volatile cycles and resource availability.

This study identifies and characterizes lunar impact glass beads as a substantial, solar wind-derived water reservoir within lunar soils. NanoSIMS measurements reveal rimward enrichment in water with solar-wind-like δD and coreward depletion, consistent with rapid inward diffusion over years to decades. Modelling and inventories indicate that these beads can store and cyclically exchange water with the lunar exosphere, likely driving and buffering the lunar surface water cycle. The total water hosted by such beads on the Moon may be as high as ~2.7 × 10^14 kg. These results suggest that impact glass reservoirs may be widespread on other airless bodies. Future work should constrain the spatial distribution and modal abundance of beads globally, refine diffusion/degassing kinetics under varying thermal regimes, directly determine exposure ages for improved spallation corrections, and assess saturation limits and retention across different glass compositions and grain sizes.

- Core water contents approach instrumental background, limiting precise quantification of initial (pre-hydration) water in beads. - Diffusion modelling employs simplifying assumptions (e.g., constant rim concentration Cs, unknown bead orientation/exposure history, use of silica-glass diffusion coefficient, fixed peak temperature of 360 K for shallow depths), introducing uncertainties in absolute timescales. - Cosmogenic spallation corrections use assumed exposure ages (50 Ma) due to lack of measured exposure ages for individual beads, adding uncertainty to absolute δD values (though corrections are small). - Limited number of detailed profiles (six across five beads) may not capture the full variability of beads in different settings. - Potential matrix effects and environmental variations are mitigated but not entirely eliminable despite calibration and background corrections.