Oxygen isotope evidence from Ryugu samples for early water delivery to Earth by CI chondrites

The origin and delivery pathway of Earth’s water remain unresolved. Hydrated asteroids, sampled on Earth as carbonaceous chondrite meteorites, are prime candidates for delivering volatiles to the inner Solar System. Yet meteorite collections are biased against fragile, highly porous materials likely destroyed during atmospheric entry. The Hayabusa2 mission returned samples from C-type asteroid 162173 Ryugu, enabling direct study of primitive, low-density materials otherwise underrepresented. Prior spectral observations suggested Ryugu’s surface materials resembled thermally/shock-metamorphosed CCs, potentially matching CY (Yamato-type) chondrites, whereas initial curation indicated similarity to CI chondrites. This study uses high-precision bulk oxygen isotope analysis, a robust discriminator among CC groups, to test Ryugu’s affinity to known meteorite groups and assess implications for water delivery to Earth.

- Remote near-IR spectroscopy of Ryugu suggested affinities to thermally/shock metamorphosed CCs and potential resemblance to CY chondrites. - Initial curation and mineralogical assessments of returned Ryugu samples indicated they are most similar to CI chondrites. - Oxygen isotopes have long been used to establish relationships among CC groups and parent bodies. - Other recent studies also reported bulk oxygen isotope data implying a CI-like composition for Ryugu, though with sample-scale heterogeneity and occasional values near the CY field. - CI meteorites (Alais, Ivuna, Orgueil) are rare and known to undergo terrestrial alteration after fall, potentially affecting light stable isotopes; thus Ryugu’s uncontaminated materials offer a benchmark.

- Samples: Subsamples from four Ryugu particles were analyzed (three from Chamber C: C0014, C0068, C0087; one from Chamber A: A0098). Seven bulk analyses were performed on 0.18–1.83 mg aliquots. - Contamination control: Particles were transported in sealed, nitrogen-filled containers; loading and handling occurred in a nitrogen glove box (<0.1% O2). Modified sample holder and chamber allowed transfer under vacuum, overnight bake-out (~95 °C), and BrF5 purges to remove moisture. Samples were never exposed to atmospheric air. - Oxygen extraction: Infrared laser-assisted fluorination with BrF5 using a Photon Machines 50 W CO2 laser. Post-reaction gas was purified cryogenically and over heated KBr. - Mass spectrometry: Thermo Fisher MAT 253 dual-inlet mass spectrometer. Bellows mode used for >~140 µg O2; micro-volume mode for smaller gas amounts. NF fragment interferences at mass 33 were eliminated via cryogenic separation on molecular sieves. - Standards and precision: Internal obsidian standard monitored precision. System precision (2 s.d.): bellows mode ±0.05‰ (δ18O), ±0.10‰ (δ17O), ±0.02‰ (Δ17O); slightly lower in micro-volume mode. - Blank corrections: Pre- and post-reaction blanks measured; corrections applied using quantified blank amounts and compositions (e.g., 2.4 µg O2 correction with δ17O = −5.15‰, δ18O = −9.95‰, Δ17O = 0.02‰). - Comparative materials: Bulk oxygen isotope analyses were also conducted on CI chondrites (Orgueil, Ivuna, Alais) and CY chondrites (Y-82162, B-7904). Weighted means calculated (weighting by liberated O2) for cross-group comparisons. - Mineralogical context: Modal compositions and phase-specific isotope data were used to interpret bulk heterogeneity (dominant phyllosilicates; variable carbonates; magnetite and sulfides; high porosity ~41%).

- Bulk oxygen isotopes of Ryugu: - Weighted mean δ18O = 15.88 ± 4.85‰ (2 s.d.); Δ17O = 0.66 ± 0.09‰ (2 s.d.). - Individual Ryugu analyses span δ18O = 11.46 to 19.30‰; Δ17O ~0.54–0.75‰, indicating mg-scale heterogeneity consistent with variable proportions of magnetite, phyllosilicates, and carbonates. - CI comparison: - CI weighted mean (Alais, Ivuna, Orgueil): δ18O = 15.16 ± 4.05‰ (2 s.d.); Δ17O = 0.53 ± 0.21‰. - Ryugu overlaps CIs and is distinct from CY chondrites (CY δ18O higher and Δ17O different). One CY (B-7904) shows Δ17O near 0 while Y-82162 has Δ17O ~0.46–0.52‰, implying CYs may not be homogeneous. - Calculations show lower CI Δ17O relative to Ryugu can be explained by terrestrial contamination (e.g., interlayer water), shifting CI bulk towards the terrestrial fractionation line. - Mineralogy and physical properties align with CI-like material: phyllosilicates (64–88 vol%), variable carbonates (2–21 vol%), magnetite (3.6–6.8 vol%), sulfides (2.4–5.6 vol%), high porosity (~41%), and low density (~1,528 ± 242 kg m³), comparable to Orgueil. - Broader implications: - Ryugu’s CI-like isotopic signature supports widespread CI-related material among carbonaceous asteroids. - Given fragility, CI-like materials are likely underrepresented in meteorite collections but may be significant contributors to Earth’s water and volatiles. - Ryugu likely not the immediate source of known CI falls based on orbital considerations, but both may derive from similar inner main belt reservoirs (e.g., Eulalia/Polana families).

The oxygen three-isotope data firmly link Ryugu samples to CI chondrites rather than CYs, resolving prior spectral-classification ambiguities. Despite significant intra-sample δ18O variability at the mg scale, the weighted mean composition of Ryugu closely matches CIs. The small systematic difference in Δ17O (Ryugu higher than measured CIs) is plausibly explained by terrestrial contamination of CI meteorites (especially interlayer water), which would pull CI values toward the terrestrial fractionation line. Mineralogical evidence (absence of ferrihydrite and sulfate in Ryugu vs presence in Orgueil) supports this interpretation. Consequently, Ryugu may preserve a more pristine CI-like signature, refining bulk Solar System benchmarks. The prevalence of CI-like material in the main belt and its fragility imply a larger-than-apparent role in delivering volatiles to the inner Solar System. Isotopic similarities in H, N, O, and nucleosynthetic Fe further support the contribution of CI-like bodies to Earth’s water budget. The results also suggest CI chondrite data sets must be critically reassessed for terrestrial contamination when used as Solar System proxies.

This study demonstrates that Ryugu’s bulk oxygen isotope composition is CI-like and distinct from CY chondrites, strengthening the case that CI-related material is widespread among carbonaceous asteroids. Accounting for terrestrial contamination reconciles small Δ17O differences between CIs and Ryugu, indicating Ryugu provides a more pristine reference for volatile-rich outer Solar System material. These findings bolster the hypothesis that CI-like bodies significantly contributed water and other volatiles to the early Earth. Future work should: (1) expand isotopic and chemical analyses of uncontaminated Ryugu samples and upcoming returned samples (e.g., Bennu) to refine Solar System bulk values; (2) investigate CY group heterogeneity; (3) integrate mineralogical, isotopic, and dynamical models to quantify the relative contributions of CI- and CM-like sources to Earth’s water; and (4) model aqueous alteration on small, porous parent bodies to reconcile isochemical alteration with fluid-flow predictions.

- Sample size and representativeness: Only 5.4 g returned from Ryugu; seven mg-scale bulk analyses from four particles were measured, and although orbital spectra suggest representativeness, local heterogeneity remains. - Heterogeneity: Significant mg-scale mineralogical and isotopic heterogeneity affects individual bulk measurements, particularly for small aliquots. - Terrestrial contamination modeling: While calculations plausibly explain CI–Ryugu Δ17O differences via contamination, primary differences cannot be completely excluded. - Comparative meteorites: CY chondrites show internal isotopic diversity, complicating group-level comparisons. - Analytical constraints: Micro-volume analyses have slightly lower precision; blank corrections introduce additional uncertainty though carefully quantified.