A pristine record of outer Solar System materials from asteroid Ryugu's returned sample

Between June 2018 and November 2019, JAXA’s Hayabusa2 spacecraft conducted extensive remote sensing of asteroid Ryugu. Near-IR spectra (NIRS3) initially suggested similarity to thermally or shock-metamorphosed carbonaceous chondrites, notably CY (Yamato-type) chondrites, with Ryugu’s low albedo attributed to carbon-rich components plus grain size, porosity, and space-weathering effects. Hayabusa2 performed two touchdowns (February and July 2019) to collect samples from the surface and near an artificial crater. Initial non-destructive curation indicated Ryugu particles are most similar to CI chondrites with varying alteration levels. These seemingly contradictory classifications (CY vs CI) motivated a detailed isotopic, elemental, and mineralogical characterization to resolve Ryugu’s true bulk composition and origin. The Phase 2 Kochi team received eight particles (~60 mg total) from chambers A and C to elucidate Ryugu’s nature, origin, and evolution and to compare with meteorites, IDPs, and Stardust cometary materials.

Prior work suggested Ryugu’s surface resembled CY chondrites (based on NIRS3 spectra), while CI chondrites are widely used as proxies for bulk Solar System composition. Comparisons to other carbonaceous chondrites (CM, CR, ungrouped C2), IDPs, and cometary samples (Stardust) provide context for isotopic and organic signatures. The study references terrestrial weathering products (ferrihydrite, sulfates) known in CI meteorites, prior assessments of CI compositions, and established oxygen isotope systematics (TFL, CCAM lines) used for classification and origin tracing.

Samples: Eight Ryugu particles (four from Chamber A; four from Chamber C) were studied. Five particles (A0029, A0037, C0009, C0014, C0068) underwent detailed mineralogical/petrological analyses; additional particles (A0098, C0068, C0087) were used for bulk chemistry and XRD. Curation and handling: Samples were recovered and curated in contamination-controlled, pure N2 environments. Transfers used airtight facility-to-facility containers and sapphire/stainless capsules; all preparation (chipping, cutting, mounting) occurred in dry N2 glove boxes (dew point −80 to −60 °C; O2 50–100 ppm). All tools were ultracleaned. Analytical workflow: Coordinated microanalysis combining SR-XCT and SR-XRD-CT (BL20XU/SPring-8), FIB extraction of ~150–200 nm sections, STXM-NEXAFS (BL4U UVSOR) for carbon K-edge functional groups, NanoSIMS (JAMSTEC) for H, C, N isotopic imaging (and Si isotopes for presolar graphite), TEM (JEOL JEM-ARM200F) for microtexture, diffraction, and EDS chemistry, SEM-EDS (JEOL JSM-7100F) and EPMA (JEOL JXA-8200) for mineral chemistry and modal abundances, bulk XRD (Rigaku SmartLab) for phase identification, INAA (Kyoto Univ.) for elemental abundances, and laser fluorination (Open University) for bulk oxygen isotopes. Key analytical details: - SR-XCT: WL mode pixel ~0.848 µm; NH mode pixel ~0.25 µm; XRD and XRD-CT for phase mapping; X-ray energy 30 keV; reconstructed 3D CT and phase maps. - EPMA: 15 keV; 5 nA for phyllosilicates/carbonates; 30 nA for sulfide, magnetite, olivine, pyroxene. Modal abundances from BSE/elemental maps via ImageJ. - XRD: Cu Kα, 40 kV/40 mA, 2θ=3–100°, ~28 h per scan; compared with CI (Orgueil), CM (Y-791198), CY (Y 980115). - Laser fluorination O isotopes: CO2 laser with BrF5; purification with cryo traps and heated KBr; MAT 253 dual-inlet; Δ17O precision ±0.018‰ (2σ). - STXM-NEXAFS: ~50 nm spot; 280–300 eV scans; spectra reduction with aXis2000/in-house software; He backfill ~20 mbar. - NanoSIMS: Cs+ beam ~2 pA (C,N) and ~13 pA (H); multi-collector imaging (256×256 or 128×128 pixels; 30 frames); standards and corrections applied; presolar graphite analyzed for C and Si isotopes. - TEM: 200 kV; SAED, lattice imaging; thickness-corrected k-factors for (Si+Al)-Mg-Fe in phyllosilicates. - INAA: Dual irradiations with thermal/fast fluxes; post-irradiation gamma counting; reference standards for quantification.

- Mineralogy and textures: Ryugu particles are dominated by phyllosilicates (serpentine–saponite intergrowths), with modal abundances ~64–88 vol% (fine- and coarse-grained). Accessory phases: carbonates (~2–21 vol%, mainly dolomite with minor Ca-carbonate and breunnerite), magnetite (~3.6–6.8 vol%; isolated grains, framboids, plaquettes, spherical aggregates), and sulfides (~2.4–5.5 vol%; mostly pyrrhotite with pentlandite). Rare anhydrous silicates (olivine, pyroxene) occur (~0.5 vol%) in C0009. - Bulk composition: XRD patterns of Ryugu (C0087; mix of A0029 and A0037) match CI Orgueil and differ from CY/CM. INAA bulk elemental abundances (A0098, C0068) align with CI chondrites; CM show depletion of volatile elements (e.g., Mn, Zn). Ferrihydrite and sulfates are absent in Ryugu, indicating those CI phases are terrestrial weathering products. - Oxygen isotopes: Bulk O-isotope analysis of C0014 (1.83 mg) compared with Orgueil (n=7; 8.96 mg) and CY Y-82162 (n=7; 5.11 mg) shows Ryugu and CI overlap and are distinct from CY. Weighted means: Δ17O_Ryugu ≈ 0.67‰; Δ17O_Orgueil ≈ 0.58‰; Δ17O_Y-82162 ≈ 0.46‰. Higher Δ17O in Ryugu vs Orgueil may reflect terrestrial contamination in Orgueil. - Organics: STXM-NEXAFS reveals strong aliphatic C–H (287.5 eV), carbonyl (286.5 eV), and carboxyl (288.8 eV) features with weak aromatic C=C (285.2 eV); no 291.7 eV graphene feature, indicating low thermal alteration. Aliphatic-rich organics are locally concentrated within coarse-grained phyllosilicates (notably in C0068), less so in carbonate-rich A0037. - Thermal/alteration conditions: Distribution and preservation of aliphatic C–H imply low maximum parent-body temperatures ~30 °C during aqueous alteration (consistent with cubanite occurrence). Time–temperature kinetics suggest aliphatic bonds would be lost at higher temperatures over geologic times. - NanoSIMS isotopes and presolar grain: A presolar graphite grain (P.G-1) shows extreme 13C enrichment (δ13C ≈ +30,811‰). In C0068, aliphatic-rich regions show δD = 841 ± 394‰ and δ15N = 169 ± 95‰, slightly higher than the surrounding C-rich regions (δD = 528 ± 139‰; δ15N = 67 ± 15‰). - Bulk light-element isotopes (FIB sections A0002, A0037, C0068): δD and δ15N variations overlap with IDPs and are generally higher than CM/CI chondrites; ranges are lower than the broad Stardust comet δD span (−240 to +1,655‰). Ryugu δD, δ15N tend to be lighter than averages for Jupiter-family and Oort-cloud comets but indicate contributions from outer Solar System materials. - Interpretation: Strong CI-like mineralogy and bulk chemistry, CI-like oxygen isotopes, and outer Solar System-like H and N isotopes together show Ryugu samples are pristine, unfractionated, and best match bulk Solar System composition while preserving comet/IDP-like isotopic signatures. - Additional observations: A large organic nanoglobule contains embedded amorphous silicate (a texture not previously reported). Association of organics with coarse phyllosilicates suggests enhanced protection from degradation. Ryugu’s small size (<1 km) implies alteration occurred on a larger precursor body (few tens of km).

The study resolves the initial ambiguity in Ryugu’s classification by demonstrating that, despite remote sensing hints of dehydration (CY-like), the returned samples are mineralogically and isotopically CI-like and lack terrestrial weathering phases found in CI meteorites. Oxygen isotope systematics place Ryugu with CI and separate it from CY, supporting CI-like bulk composition. The observed intimate association of aliphatic-rich organics with coarse phyllosilicates, and their heavy H and N isotopic signatures relative to average CI/CM, suggest incorporation of outer Solar System-derived materials and preservation under low-temperature aqueous alteration (~30 °C). These results address the broader question of volatile delivery to Earth by indicating that Ryugu-like materials could have been one source of organics and water, though their relatively heavy hydrogen isotopic compositions mean additional, lighter-H sources (e.g., solar-wind-implanted water) likely contributed. The discrepancy between spacecraft remote observations (implying dehydration) and sample-based evidence (no dehydration signature) remains unresolved and may relate to surface processes or observation biases. The demonstration that CI meteorites are terrestrially modified underscores the value of pristine returned samples for constraining bulk Solar System composition and early Solar System processes.

Ryugu particles are among the most uncontaminated, chemically unfractionated extraterrestrial materials studied, providing the closest match to bulk Solar System composition. They are CI-like in mineralogy, chemistry, and oxygen isotopes, yet preserve aliphatic-rich organics closely associated with hydrous minerals and exhibit heavy H and N isotopic signatures indicative of outer Solar System inputs. The absence of ferrihydrite/sulfate confirms terrestrial alteration of CI meteorites. Low alteration temperatures (~30 °C) and organics–phyllosilicate associations highlight conditions and mechanisms that preserve primitive volatiles. These findings emphasize the necessity of direct, sterile-return sampling of primitive asteroids. Future work should expand isotopic surveys (especially H and N) across more Ryugu particles to resolve heterogeneities and carriers, reconcile the remote-sensing dehydration signal with sample data, refine thermal/aqueous histories, and further assess the role of Ryugu-like bodies in Earth’s volatile inventory.

- Limited number of analyzed particles and FIB sections restricts statistical power; observed isotopic heterogeneities may reflect sampling bias and fine-scale mixing. - Small particle sizes can introduce heterogeneity and sampling bias in bulk elemental measurements. - The exact carrier of isotopically heavy H and N (aliphatic organics vs adjacent phyllosilicates) cannot be definitively isolated due to submicrometre mixing. - Potential analytical artefacts (e.g., beam-induced amorphization/irradiation effects) may affect interpretations of nanoglobule features. - Discrepancy between remote-sensing indications of dehydration and sample evidence remains unresolved. - Terrestrial contamination in reference meteorites (e.g., Orgueil) complicates direct comparisons. - Time–temperature interpretations of organic preservation are model-dependent and may be complicated by aqueous alteration histories.