The evolution of organic material on Asteroid 162173 Ryugu and its delivery to Earth

The study investigates how organic matter (OM) on the carbonaceous asteroid Ryugu evolved during aqueous alteration and what forms of OM are delivered to Earth. Context: Carbonaceous asteroids like Ryugu and Bennu are abundant and preserve records of early Solar System processes; Ryugu samples are CI chondrite-like, highly hydrated, and rich in organics dominated by phyllosilicates. Prior to sample-return missions, carbonaceous chondrites (CCs) provided most constraints on soluble (SOM) and insoluble organic matter (IOM). Purpose: Using in situ, high-resolution microscopy and spectroscopy, the authors characterize the morphology, distribution, and functional chemistry of OM in Ryugu to test whether aromatic-rich OM is pre-accretionary or results from parent-body processing, and to assess the occurrence of diffuse OM and possible encapsulation of phyllosilicate by organic particles (OPs). Importance: Findings inform the physicochemical evolution of prebiotic organics on planetesimals, challenge prevailing interpretations of OM origins, and constrain the delivery pathways of biologically relevant molecules to early Earth.

• CCs contain the highest concentrations of SOM and IOM (up to ~4 wt% total organic carbon), with hydration generally correlating with TOC.
• IOM in CCs comprises macromolecular solids of small aromatic groups with short, branched aliphatic moieties; occurs as submicron rounded nanoglobules, dendrites, and veins in matrix.
• Two main OP spectral types by C-XANES: (1) 3-peak (aromatic/olefinic C=C, carbonyl C=O, carboxyl/ester COOR) typical of most IOM; (2) 2-peak (C=C and COOR), more aromatic-rich, lacking the carbonyl peak; less abundant overall.
• Prior work shows higher abundance of aromatic-rich 3-peak OPs in type 1 CCs (e.g., CR1 GRO 95577, CI Orgueil) than in type 2/3, suggesting alteration increases aromaticity; challenges pre-accretionary origin of aromatic-rich OM.
• Diffuse OM is widespread in hydrated silicates (phyllosilicates in type 1; hydrated amorphous silicates in type 2/3), characterized as aromatic-poor and carboxylic-rich, potentially hosting SOM (e.g., amino and monocarboxylic acids). In situ characterization of SOM distribution has been limited but IR studies indicate aliphatic and C–O enrichments in phyllosilicate domains.
• Background on Ryugu and Bennu: both rubble-pile, CI-like composition; CI scarcity in meteorite collections may reflect fragility and survival bias during atmospheric entry.

Samples: Ryugu grain A0083 (Radegast) from the first Hayabusa-2 TAG site; comparative CI meteorite Ivuna (microtome and FIB-SEM cross-sections).

Sample preparation:

• Minimal mechanical damage mounting of Radegast on Au divot with tensioned Al foil clamp; uncoated to reduce charging through conductive contact.
• FIB-SEM tomography volume targeted at the grain edge (~60 × 65 × 20–25 µm) using TESCAN AMBER X (Xe PFIB) with 25 nm slice/shift increments (25 × 25 × 25 nm voxels), Pt protective cap, drift correction marker, and ±7° stage rocking to minimize curtaining. Low-kV (5 kV) EDX maps acquired every 4 slices (C Kα, Fe Lα, Si Kα, S Kα, Mg Kα, P Kα, O Kα; up to Ca Kα).
• Lamella extraction at the end of the tomography volume: two ~2 µm thick FIB sections attached to Cu grids, final thinning to ~100 nm using Ga (LYRA3) and Xe (AMBER X) sources. Imaging at low kV/current to minimize radiation damage and avoid polycarbonate-like XANES artifacts (~290.3 eV).
• Ivuna controls: sulfur-embedded ultramicrotomy (Leica UC7) onto SiO grids for STXM-TEM; additional Ivuna and Ryugu microfragments cross-sectioned by FIB-SEM (AMBER X; Quanta 3D without stage rocking for comparison).

Imaging and spectroscopy:

• FIB-SEM tomography: BSE imaging per slice, periodic EDX mapping; alignment in Microscope Image Browser (MIB) and TESCAN 3D; segmentation of OPs in MIB; 3D renders in Thermo Fisher Amira.
• STXM at Photon Factory KEK BL-19A: OD maps (C: 292–280 eV; Fe: 709–705 eV; O: 539–525 eV); XANES stacks 280–320 eV with 0.1 eV steps (3 ms per step); spectra processed with aXis2000 and normalized using ATHENA.
• TEM/STEM (Thermo Fisher Talos F200C, 200 kV): HAADF imaging, STEM-EDX mapping for elemental context; HRTEM bright-field to characterize phyllosilicate (1:1 and 2:1 mixed-layered textures consistent with interlayered serpentine–saponite).

Controls and validation:

• Comparison of Ga vs Xe FIB-prepared lamellae vs Ivuna microtome sections shows consistent OP and diffuse OM functional chemistry, indicating preservation of OM signatures under both FIB methods.

• Ryugu OM occurs as pervasive micron to submicron organic particles (OPs) and widespread diffuse OM within phyllosilicates.
• OP chemistry: All analyzed OPs exhibit aromatic-rich 3-peak C-XANES [(C=C)-(C=O)-(COOR)] with relatively low carbonyl intensity, closely matching CI Ivuna and CR1 GRO 95577; some particles approach 2-peak, highly aromatic character reported previously for Ryugu and Orgueil.
• Diffuse OM chemistry: Aromatic-poor and aliphatic/carboxylic-rich signatures occur throughout hydrated silicate domains (both fine and coarse phyllosilicate), consistent with SOM-bearing material.
• Tomography and 3D rendering reveal OPs ranging from rounded nanoglobules to irregular vermicular forms; many show hollow sub-particle morphologies compounded into larger OPs, consistent with polymerization-driven self-assembly rather than gas bubbling/cavitation.
• Novel observation: OPs encapsulate fibrous phyllosilicate-like material. Some OPs completely surround internal silicate; others have organic walls cross-cut by phyllosilicate filling the particle interior.
• Quantitative spectral contrasts: In type 1 materials (Ryugu, Ivuna, GRO 95577), carbonyl peak absorption can be ~50% lower than in type 2/3 IOM-like spectra (e.g., QUE 91177). For diffuse OM, the aromatic (C=C) peak in type 2/3 (QUE 99177) can be up to ~100% higher than in type 1, while the carboxyl (COOR) peak at 288.8 eV in type 1 can be up to ~40% higher than in QUE 91177.
• Phyllosilicate context: Coarse and fine phyllosilicate domains are present; diffuse OM is less concentrated in coarse domains than in finer phyllosilicate intermixed with Fe and Fe,Ni sulphides. HRTEM confirms mixed-layered 1:1 and 2:1 phyllosilicate (interlayered serpentine–saponite).
• Comparative analyses show Ryugu OM most closely resembles type 1 CCs (CI, CR1) rather than type 2/3, indicating aqueous alteration enhanced aromaticity of OPs and increased carboxylic/aliphatic character of diffuse OM.
• Implication: OP encapsulation of phyllosilicate likely retains diffuse OM and SOM within organic shells, indicating delivery of micron-scale polymeric organic objects carrying biologically relevant molecules to Earth.

The findings indicate that aqueous alteration on Ryugu transformed pre-existing OM into two coupled reservoirs: (1) aromatic-rich OPs (3-peak C-XANES with subdued carbonyl) and (2) diffuse OM within hydrated silicates that is aliphatic/carboxylic-rich and aromatic-poor. The close similarity of Ryugu OM to type 1 CCs (CI Ivuna, CR1 GRO 95577) supports a parent-body alteration origin for aromatic-rich OPs, challenging interpretations of a dominantly pre-accretionary highly aromatic component. Tomographic evidence that OPs can encapsulate phyllosilicate shows a physical mechanism for concentrating and protecting SOM/diffuse OM within organic shells during alteration. This structural association provides a robust vehicle for the delivery of biologically relevant molecules to planetary surfaces. The work links mineralogical evolution (hydration, phyllosilicate formation) to organic functional chemistry and morphology, offering an integrated view of prebiotic organic processing on carbonaceous asteroids and constraining the nature of organic cargo delivered to early Earth.

This study applies coordinated FIB-SEM tomography, STXM, and TEM to Ryugu grain A0083 to reveal that aqueous alteration on a CI-like asteroid produced abundant aromatic-rich OPs and pervasive diffuse OM in phyllosilicate. OPs frequently encapsulate phyllosilicate, implying sequestration of diffuse OM/SOM within micron-scale polymeric organic objects. Spectroscopic comparisons align Ryugu with type 1 CCs (CI, CR1), indicating alteration-driven maturation of OM (enhanced aromaticity in OPs; increased carboxylic/aliphatic features in diffuse OM). These results refine models of organic evolution on planetesimals and support scenarios where Earth received micron-sized organic-polymeric carriers containing biologically relevant molecules. Future work should quantify the prevalence of OP–phyllosilicate encapsulation across more Ryugu/Bennu samples, resolve nanoscale transport pathways for SOM into hydrated silicates, and integrate thermal/aqueous alteration histories to model OM maturation and delivery efficiencies.

• Sample scope: Detailed in situ analyses focus primarily on a single Ryugu grain (A0083) and limited additional fragments, which may not capture full asteroid-scale heterogeneity.
• Preparation artifacts: Although Ga and Xe FIB methods and microtomy controls indicate preserved functional chemistry, FIB curtaining (noted in some cross sections) and potential beam-induced modifications remain possible sources of bias.
• Mineral encapsulation process: The exact mechanisms by which phyllosilicate becomes encapsulated within OPs, and the permeability of OP walls to fluids during alteration, remain unresolved.
• In situ SOM mapping: While diffuse OM likely hosts SOM, direct spatially resolved identification and quantification of specific soluble molecules within hydrated silicates at nanoscale resolution are still limited.
• Comparative baselines: Quantitative spectral comparisons rely on representative spectra from selected meteorites (e.g., QUE 91177/99177, Ivuna, GRO 95577); broader datasets would improve generalizability.