Soluble organic matter Molecular atlas of Ryugu reveals cold hydrothermalism on C-type asteroid parent body

The study investigates the soluble organic matter (SOM) of asteroid Ryugu to understand the temperature history and water–rock interactions on its parent body, placing the results in the context of carbonaceous chondrites (CCs). Hayabusa2 sampled surface (Chamber A) and possibly subsurface (Chamber C) materials in 2019 and returned ~5.4 g to Earth in 2020, allowing direct comparison of asteroid material to meteorites. Prior analyses indicated hydrous silicates (serpentine, saponite), carbonates, sulfides, and magnetite consistent with extensive aqueous alteration and affinities to CI chondrites. Elemental analyses showed Ryugu is rich in C, N, and H, comparable to CI (Ivuna, Orgueil). Targeted organics included amino acids (racemic, non-biological), aliphatic amines, short-chain carboxylic acids, and PAHs. Differences from CI suggest distinct synthesis or alteration pathways and peak temperatures up to ~300 °C inferred from specific amino acids in some meteorites. This work applies non-targeted, ultrahigh-resolution MS and NMR to Ryugu SOM and compares it to 36 CCs to infer the extent and temperature of hydrothermal processing.

Background work established Ryugu’s mineralogy (hydrous silicates, carbonates, sulfides, magnetite) and CI-like characteristics with high C, N, H. Targeted organics in Ryugu included ~15 amino acids (many non-protein, racemic), series of aliphatic amines, short-chain carboxylic acids, and PAHs (e.g., naphthalene, phenanthrene, anthracene, fluoranthene, pyrene, chrysene/triphenylene), with distributions attributed to parent-body alteration and solubility. Aromatic and aliphatic N-heterocycles differed from CM2 Murchison, reflecting distinct redox conditions. Broader studies linked nitrogen-bearing SOM in CCs to residues from interstellar ice analogs, and nucleobases have been identified in various meteorites, indicating inheritance from the interstellar medium. Recent meteorites (Flensburg CI-ung, Tarda C2-ung) lacking organomagnesium compounds and showing strong aqueous alteration provide comparative context for low-temperature, water-rich processing.

Ryugu sample A0106 (surface, Chamber A) was sequentially extracted with solvents of increasing polarity: hexane, dichloromethane, methanol, and water to fractionate SOM by polarity. Extracts were analyzed by direct-injection Fourier transform ion cyclotron resonance mass spectrometry (DI-FT-ICR-MS) using complementary ionization: electrospray ionization in negative and positive modes (ESI(-), ESI(+)) and atmospheric pressure photoionization in positive mode (APPI(+)). Instrument: Bruker Daltonics high-field FT-ICR with 12 T magnet; time-domain transient 4 MWords; mass range 147–1000 amu; 3000 scans per spectrum; ion accumulation 300 ms; source temperature 200 °C; ESI flow 120 µL h−1; APPI flow 500 µL h−1; external calibration with arginine negative cluster ions. One-third of the small solution volume was used for quantitative and 2D nuclear magnetic resonance (NMR) spectroscopy (1H NMR at 800 MHz in CD3OD), assessing aliphatic, olefinic, aromatic, and ammonium environments and reconstructing average aliphatic branching features via 2D correlations. Data mining included assignment of exact masses to CHNOS elemental formulas with conservative filters, construction of mass difference networks (MDINs) using frequent transformations (e.g., CH2, H2, O, H2O, CO2, HCN, NH, NH3, S, SO, SO2, SO3, SO4), and visualization via van Krevelen diagrams. Comparative chemometrics: principal component analysis (PCA) on intensities of annotated m/z in CHNOS space (unit-variance scaled, log-transformed) across methanol extracts of 36 CCs (work set n=25, prediction set n=11), to map Ryugu within a thermal index framework (all TI-type meteorites altered at <300 °C) and identify markers of water alteration versus temperature.

- Extremely high molecular diversity across m/z 120–700 akin to CC SOM, indicating minimal terrestrial contamination; regular homologous patterns observed across ionization modes. - Total >200,000 signals (S/N ≥ 3) reduced by conservative filters to 23,100 CHNOS monoisotopic elemental compositions. Chemical families predominated by N-containing formulas (16,950), followed by CHO/Na (8,843), CHOS/Na (5,647), CH (1,056), and CHS (165). Most abundant molecules extracted with methanol; hexane and water yielded the most apolar and highly polar subsets, respectively. - Sulfur-rich species: intense polythiols (disulfates) S3–S9 in methanol ESI(-); monosulfonates and polysulfanes S3–S8 in less polar extracts. Oxygenation and sulfurization patterns track solvent polarity; sulfonated species favored in polar solvents. - Mass difference network (MDIN): 154,578 paired connections (edges) among 19,675 formulas (nodes) using common transformations. Most abundant transformations were CH2 (20,232 pairs), H2 (19,349), O (17,111), and H2O (16,620), totaling 47% of differences. Nitrogen-based (HCN, NH, NH3) and sulfur-based (S, SO, SO2, SO3, SO4) transitions accounted for 22.6% and 20.0% of transformations, respectively. - 1H NMR: narrow resonances from δH ~0.5–8.5 ppm indicate prevalent branched aliphatic structures terminated by methyl and carboxyl groups; lower abundance aliphatic OCH units and olefinic/aromatic unsaturation. Distinct NH4+ triplet at δH ~4.5 ppm (JNH ~43 Hz) with 1.2% relative abundance, higher than in other meteorite extracts. - PAHs: APPI(+) detected a continuum from aliphatic to highly aromatic multi-ring compounds; ESI modes showed higher oxygenation (3–4 O per molecule) and relative abundance of N-bearing ions, especially in methanol extracts. - Comparative analysis: Ryugu A0106 methanol extract displays van Krevelen patterns similar to Murchison CM2 but with lower high-m/z abundance; oxygenation and functionalization profiles most similar to Orgueil CI for oxygenated hydrocarbons and organosulfur compounds; Murchison and Ryugu share heteroatom abundance profiles. - PCA (R2(cum)=0.51; Q2(cum)=0.39) positions A0106 with meteorites that experienced strong aqueous alteration (CI, CM1/2, CR1, C1/2-ung, C2-ung), indicative of cold hydrothermalism. Loadings associate increased water alteration with saturated/mono-unsaturated long-chain aliphatic carboxylic acids; sulfurization of N-compounds is more abundant at slightly elevated temperatures in TI-type meteorites. - No organomagnesium compounds (CHOMg, CHOSMg) detected, consistent with low-temperature, water-rich alteration and hydrolysis; aligns with observations in Flensburg CI-ung and Tarda C2-ung. - Overall, Ryugu SOM exhibits a continuum in molecular size, polarity, oxygenation, and sulfurization, dominated by N- and S-rich compounds and abundant ammonium, evidencing extensive aqueous processing at low temperatures (~≤150–200 °C, often <150 °C).

The comprehensive molecular atlas of Ryugu’s SOM reveals a structurally connected continuum consistent with extensive low-temperature water–rock interactions on the parent body. High abundances of sulfurized and nitrogen-rich compounds, a strong ammonium signal, and pervasive oxygenation patterns indicate severe aqueous alteration and hydrolysis. The absence of organomagnesium species and the PCA placement alongside CI/CM1-type meteorites corroborate cold hydrothermalism. The observed PAH continuum and homologous series of N-bearing organics suggest multiple origins, including inheritance from organic-rich interstellar ices and subsequent parent-body processing. Serpentinization and carbonation, thermodynamically favored at low temperatures on small bodies, likely generated localized heat (<~150–200 °C), water, and oxidized carbon, driving redox-coupled abiotic synthesis and sulfurization during mineral evolution. These processes can explain the oxidation gradients, carbon saturation ranges, and heteroatom distributions observed, and position Ryugu’s SOM as a potential reservoir of prebiotic molecular precursors.

This study provides a non-targeted, ultrahigh-resolution molecular atlas of Ryugu’s soluble organic matter, demonstrating extreme chemical diversity and a continuous structural network shaped by low-temperature, water-rich alteration on the asteroid’s parent body. Quantitative MS and NMR analyses, combined with comparative chemometrics across 36 carbonaceous chondrites, place Ryugu A0106 within the cold hydrothermal regime (CI/CM1/2-like), marked by abundant N- and S-bearing species, ammonium, oxygenated hydrocarbons, PAHs, and the absence of organomagnesium compounds. The findings support a scenario of multi-stage abiotic synthesis involving inheritance from interstellar ices, water–rock interactions, serpentinization, carbonation, and sulfurization. Potential future work includes expanded structural elucidation of key formula families, co-localized mineral–organic analyses, and broader comparisons across additional Ryugu samples and extraction fractions to further resolve reaction pathways and temporal sequences of alteration.

- Limited sample volume (minimum amounts used for MS and NMR) constrained analyses and necessitated optimization of injection conditions. - Extremely high isomeric complexity yielded Gaussian distributions of signals per nominal mass, making detailed structural annotation of individual formulas nearly impossible. - Certain sodium-containing formula classes (e.g., CHONa, CHNONa, CHOSNa, CHNOSNa) could not be incorporated into the mass difference network with the selected transformations. - Comparisons relied primarily on methanol extracts for multivariate modeling; other solvent fractions may capture additional or distinct molecular populations.