Helium reservoirs in iron nanoparticles on the lunar surface

Helium-3 is a rare isotope with applications in cryogenics, neutron detection, medical imaging, and prospective fusion energy. Unlike Earth, the lunar surface has relatively higher 3He due to implantation by the solar wind, which has an average 3He/4He ratio of ~2350 ± 120. Conservative estimates place 3He in lunar soils at a few parts per billion, higher in mare regions than highlands. Understanding how helium is retained and concentrated in lunar regolith is critical for resource assessment and for constraining the Moon’s surface evolution by space weathering. Airless bodies undergo space weathering by solar wind irradiation and micrometeoroid impacts, which pulverize, melt, and vapor-deposit materials, producing amorphization and vesiculation. Lunar helium concentrations correlate with soil exposure age, solar wind fluence, and Ti-Fe content; helium has been directly measured in ilmenite and chromite, oxides with slower helium diffusion than silicates. A key unresolved observation is that agglutinates (glass-welded aggregates) contain more helium than equally sized other soil grains, and helium concentration increases as agglutinate particle size decreases, especially below 100 µm. This study tests the hypothesis that vesicular nanophase metallic iron particles (v-npFe) produced by space weathering act as helium reservoirs, explaining the elevated helium in agglutinates.

Prior work established that helium in lunar soils increases with surface exposure, solar wind fluence, and Fe-Ti content, reflecting enhanced retention in ilmenite and chromite and slower diffusion in oxides relative to silicates. In situ helium has been detected in ilmenite and chromite, while agglutinates exhibit higher helium concentrations than other grains of similar size, with enrichment strongest in smaller agglutinate size fractions. Vesicular or hollow nanophase iron particles have been observed in Apollo samples and in Chang'E-5 materials, though earlier EELS studies often focused on core-loss edges for oxidation state rather than low-loss helium detection. Hollow metal particles in GEMS (Glass with Embedded Metal and Sulfides) in interplanetary dust have been linked to irradiation and condensation processes, suggesting broader relevance of vesiculated metals formed under space weathering. Competing formation mechanisms such as the nanoscale Kirkendall effect have been proposed for hollow/vesicular npFe, but predicted oxide shells and void coalescence textures are often inconsistent with observations in lunar samples.

Samples: Seven Apollo 17 lunar soil samples spanning submature to mature states were analyzed, including <45 µm fractions from soils 72320/1 (Is/FeO 73.0), 72501 (Is/FeO 81.0), 76240/1 (Is/FeO 56.0), 76261 (Is/FeO 58.0), and 79221 (Is/FeO 81.0). Preparation: Grains were prepared for STEM via ultramicrotomy (embedded in epoxy, sectioned to 80–100 nm onto lacey carbon Cu grids) or focused ion beam (FIB) liftouts using an FEI Helios G3, protected with electron- and ion-beam deposited carbon, thinned stepwise to <100 nm, and mounted on Cu half-grids. STEM imaging: Conducted on a Cs-corrected Nion UltraSTEM200-X at 200 keV, 40 pA, ~0.1 nm probe, using HAADF for Z/thickness contrast. Samples were pre-baked under vacuum (~20 °C, 48 h) to remove adsorbed water. EELS: Acquired with a Gatan Enfinium ER dual EELS spectrometer as spectrum images (2048 channels at 0.05 eV/channel; energy resolution 0.45 eV FWHM; collection semi-angle 50 mrad; 2.0 mm aperture). Sequential acquisitions: zero-loss (−10–90 eV), low-loss (12–105 eV), and Fe L3,2 edge (660–760 eV). Processing in DigitalMicrograph corrected scan-related energy drift (using zero-loss peak), removed plural scattering (Fourier-ratio), and removed X-ray spikes (5σ). Backgrounds: Fe L-edge with power-law; He K-edge background via normalization of matrix to vesicle spectra and subtraction, or polynomial/linear fits when necessary. Helium mapping and quantification: The He K-edge (1s→2p) near ~22–24 eV overlaps the Fe bulk plasmon; multiple linear least squares (MLLS) fitting used reference spectra for glass matrix, npFe, and vesicles to isolate components and produce fit-integral maps, validated by low residuals and flat reduced chi-squared. Helium concentrations in vesicles were derived from established EELS methods correlating He K-edge energy shift/intensity with density, accounting for up to ~3 eV edge position variation with He density. Structural characterization: Lattice fringes and FFTs identified α-Fe (e.g., 2.0 Å d-spacing for (110)). Depth distributions of npFe and v-npFe relative to grain surfaces were recorded to assess relationships with expected solar wind implantation depths (~35 nm for 4 keV He2+ in silicates per SRIM) and diffusion.

• Vesicular nanophase metallic iron (v-npFe) particles are widespread across all seven Apollo 17 soil samples examined. Two size populations were identified: large (>15 nm) occurring mainly within agglutinitic glass and melt splashes, and small (<15 nm) occurring in agglutinitic glass, melt splashes, and in solar wind–irradiated and vapor-deposited rims.
• Depth dependence: v-npFe are typically confined to within ~50–100 nm of grain surfaces; in rims, small v-npFe occur only within ~50 nm of the surface. Deeper npFe lack vesiculation. This pattern aligns with solar wind He implantation depths and suggests vesiculation results from He implantation into pre-existing npFe.
• EELS unambiguously identified helium in vesicles within many large v-npFe, with He K-edge detected at ~22.8–24.3 eV. Helium was absent in surrounding glass and solid Fe particle regions.
• Quantified helium concentrations in v-npFe vesicles range from 10 to 24 atoms/nm3. Within a single particle, different vesicles can show varying concentrations (e.g., 15–24 atoms/nm3), implying multiple vesicles with different internal pressures; identical concentrations among vesicles suggest interconnected vesicle networks.
• Structural integrity: Despite extensive vesiculation, α-Fe crystallinity persists without evidence of lattice strain; FFTs show 2.0 Å (110) reflections of α-Fe.
• Selective retention: In cases with vesicular rim matrices coexisting with v-npFe, only v-npFe vesicles contained helium, indicating higher He retention in metallic Fe nanoparticles than in surrounding phases.
• Small v-npFe vesicles (<1 nm) were generally below detection thresholds for He by EELS under the applied conditions, though they likely contain or previously contained He.
• Instances of large v-npFe coalescence and agglomeration were observed; large v-npFe can occur in groups mixed with small npFe/v-npFe or as solitary particles.

The findings demonstrate that vesicular metallic iron nanoparticles formed by space weathering are significant helium repositories in lunar regolith. The depth-dependent occurrence of vesiculation, the presence of helium exclusively within v-npFe vesicles, and the maintenance of metallic α-Fe argue against the Kirkendall oxidation mechanism and support a formation pathway via solar wind He implantation into existing npFe. The stability of helium at the crystalline (α-Fe)–amorphous (glass) interface in a nanoparticle–matrix geometry provides a natural example of He confinement at such interfaces, paralleling engineered alloy interfaces known to trap helium. These results explain previously puzzling trends: elevated helium in agglutinates relative to other soil grains and increasing helium concentrations with decreasing agglutinate size. Agglutinates commonly host large, surface-accessible npFe that can be converted to helium-bearing v-npFe by ongoing irradiation; smaller particles increase the likelihood that npFe reside within the He implantation zone, and their irregular shapes increase accessible surface area. Consequently, progressive space weathering not only alters optical properties through npFe production but also enhances the regolith’s helium retention over time, influencing the lunar exosphere and potential in situ resource utilization. The observations are consistent with vesiculated/hollow metal particles in other space-weathered materials (e.g., GEMS) and suggest helium implantation may contribute to their textures.

This study identifies vesicular nanophase metallic iron particles as widespread, efficient helium reservoirs in lunar soils. Using aberration-corrected STEM and EELS, helium was mapped and quantified within v-npFe vesicles at concentrations of ~10–24 atoms/nm3, with vesiculation confined to near-surface depths consistent with solar wind implantation. The metallic α-Fe nature, He localization within vesicles, and depth dependence indicate formation by He implantation into pre-existing npFe rather than by the Kirkendall effect. These insights reconcile elevated helium contents in agglutinates and their size-dependent enrichment, and they highlight a mechanism by which continued space weathering progressively increases regolith helium retention, with implications for the lunar helium cycle and ISRU. Future work should address: (1) survivability of helium-bearing v-npFe through multiple impact events; (2) the roles of impact energy, melt composition, and cooling rate on v-npFe formation and helium retention; (3) detection and quantification of helium in sub-1 nm vesicles; and (4) practical methods to access this helium reservoir (e.g., crushing or heating) and associated environmental impacts.

• Detection limits: Small v-npFe vesicles (<1 nm) challenge He detection by EELS under the applied conditions. Helium was not always detectable due to potential natural diffusion, thermal effects, sample perforation during preparation, beam-induced diffusion during acquisition, sample thickness effects, beam energy spread, and detector sensitivity.
• Open-system processes: Space weathering is complex; the measured He abundances may be influenced by local exposure histories, implantation fluences, and subsequent impact heating events.
• Mechanistic uncertainties: The persistence of helium-bearing v-npFe through regolith gardening and impacts is unresolved. The influence of melt composition, impact energy, and cooling rates on v-npFe formation and retention needs quantification.
• Sampling: Although seven Apollo 17 soils were examined, broader sampling across terrains and maturities could refine generality and variability of v-npFe-mediated helium sequestration.