Hiroshi Naraoka

Most carbonaceous chondrite (CC) meteorites contain abundant chondrules, millimeter-scale spherical inclusions of previously molten material. Ivuna-type carbonaceous chondrites (CIs) do not contain chondrules, nor do samples of the carbonaceous asteroid Ryugu. CIs and Ryugu share isotopic properties distinct from those of other CCs, implying that their parent bodies formed at a different time or place than those of other CCs. We applied manganese-chromium dating and oxygen isotope thermometry to Ryugu and Ivuna samples. The results indicate that the parent bodies of Ryugu and Ivuna formed less than 2 million years after the beginning of Solar System formation—before the majority of chondrules in CCs and the parent bodies of other CCs formed. This implies that Ryugu and CIs are remnants of an earlier generation of carbonaceous planetesimals.

A diverse variety of organic matter exists in space. Since these extraterrestrial organic materials preserve chemical information from the early solar system, their identification has been extensively studied. However, detailed structural information has remained limited. Here we investigate organic matter from the carbonaceous asteroid Ryugu using a high-resolution atomic force microscope (AFM). We directly resolve the chemical structures of individual organic molecules from the asteroid. We find a wide variety of polycyclic aromatic hydrocarbons (PAHs), many of which are unexpectedly large in size. The largest one is composed of approximately 100 fused rings, significantly larger than the extraterrestrial PAHs identified in previous ensemble-level analyses. These PAHs exhibit non-planar structures incorporating five-, six-, seven-, and even eight-membered rings. Such complex structures can be resolved in detail only through single-molecule AFM analysis.

Organic molecules delivered from extraterrestrial materials may have played a key role in supplying building blocks for life on Earth. Here we report all five canonical nucleobases—purines (adenine and guanine) and pyrimidines (cytosine, thymine and uracil)—in samples returned from the C-type asteroid (162173) Ryugu by JAXA’s Hayabusa2 mission and compare the results with data from similar extraterrestrial material. Ryugu samples contain nearly equal amounts of purines and pyrimidines, whereas Murchison is enriched in purines and Bennu and Orgueil in pyrimidines. Samples from Ryugu, Bennu and Orgueil, which have a similar mineralogy and elemental composition, show purine-to-pyrimidine ratios negatively correlating with ammonia. These observations indicate that the nucleobases in these samples may have formed via a shared pathway depending on the physicochemical environment of the respective parent bodies. The detection of diverse nucleobases in asteroid and meteorite materials demonstrates their widespread presence throughout the Solar System and reinforces the hypothesis that carbonaceous asteroids contributed to the prebiotic chemical inventory of early Earth.

Ethanol contamination recently identified in Ryugu samples was traced to fluoroelastomer O-rings used in the Facility-to-Facility Transfer Container (FFTC). To eliminate this contamination source, the O-ring preparation procedure was revised by minimizing ultrasonic cleaning with high-purity ethanol, introducing vacuum baking at 150°C, and storing O-rings under vacuum or dry nitrogen until use. Thermal desorption–comprehensive two-dimensional gas chromatography–time-of-flight mass spectrometry showed that residual ethanol in the O-rings decreased by more than four orders of magnitude to <1 μg per ring compared to the conventional cleaning procedure. Transport experiments further demonstrated that samples carried in the improved FFTC exhibited ethanol, acetaldehyde, and acetone levels comparable to those transported in an O-ring-free metal-seal container. The revised protocol provides an effective and practical approach for contamination-controlled transport of returned extraterrestrial samples.

Ryugu samples and carbonaceous chondrites contain organic matter that is expected to record the molecular evolution from the presolar stage to the early Solar System. In contrast to CI chondrites, Ryugu samples are well suited for investigating pristine organic components because they have not undergone terrestrial weathering. In this study, we analyzed volatile organic components released from Ryugu grains by vacuum pyrolysis and gas chromatography/mass spectroscopy (GC/MS). Two of three Ryugu grains released volatile organic fragments at ~80–120°C; however, such releases were not observed in other carbonaceous chondrites. The GC/MS of released gases from another Ryugu grain revealed that the gases were dominated by ethanol, with a concentration of ~100 μmol/g, while other components were scarcely detected. We found that O-rings used to seal a Facility to Facility Transfer Container (FFTC) that had been used for sample transportation contained ethanol absorbed during cleaning. Transportation tests of blank samples revealed that blank samples transported using FFTC released markedly more ethanol than those transported in a container without an O-ring. These results indicate that the O-rings of the FFTC were the contamination source. Ethanol derived from O-rings was likely trapped in the interlayers of phyllosilicates in Ryugu samples and released via heating in vacuum. This contamination could affect the reported bulk abundances and isotopic compositions of carbon and hydrogen in Ryugu samples. Further investigations using intact Ryugu samples are expected to clarify the representative characteristics of Ryugu.

Soluble organic matter (SOM) in carbonaceous chondrites is an important aspect of understanding the origin and evolution of extraterrestrial organic matter. Interactions with minerals and water on planetesimals are essential for the formation and evolution of SOM, yet the detailed processes remain poorly understood. To elucidate these processes, it is necessary to establish relationships between SOM and minerals, particularly secondary minerals. In this study, nitrogen (N)-heterocyclic compounds (CHN compounds) in the methanol extracts of powdered Murchison meteorite fragments were characterized by high-performance liquid chromatography coupled with high-resolution mass spectrometry (HPLC/HRMS). Their abundance patterns and chemical characteristics were examined in direct relation to mineral compositions determined by X-ray diffraction (XRD) from the same samples. Five homologous series of CHN compounds (alkylpyridines: CnH2n-4N+ or CnH2n-6N+, alkylpiperidines: CnH2n+2N+, and alkylimidazoles: CnH2n-1N2+ or CnH2n-3N2+, which were identified as positive ions) were identified from all of the samples. XRD analyses revealed inter-sample variability in the abundance of phyllosilicate, anhydrous silicates, sulfides, and carbonates. Several CHN homologous series exhibited systematic relationships with alteration minerals, particularly phyllosilicates and magnetite, whereas correlations with minor phases were generally weak. These relationships are consistent with water–mineral–organic interactions on the Murchison parent body, such as mineral-surface/interlayer interactions, fluid-driven chromatographic effects during aqueous alteration, redox-dependent alteration , and the potential heterogeneity of precursor organic molecules. Although this present approach is not universally applicable to all classes of SOM and requires careful evaluation of potential mineral alteration during extraction, it enables direct comparisons between soluble organics and minerals, thereby providing new insights into water–mineral–organic interactions on planetesimals.

The successful sample return from asteroid (162173) Ryugu by Hayabusa2 has contributed to our understanding of the solar system evolution. Over the course of the initial sample analysis, various measurements were conducted, such as mineralogical observation, chemical analysis, and mechanical property measurement. These pieces of information allow us to give constraints on the essential conditions of Ryugu's formation and evolution processes (e.g., thermal environment, aqueous alteration, formation of a rubble‐pile body), leading to a clearer view of the early solar system. Here, we report the initial results of the elastic properties of Ryugu particles (e.g., P‐ and S‐wave velocities and Young's modulus) obtained via ultrasonic pulse transmission measurement. Our measurement results showed 2.15 0.05 km/s and 1.25 0.05 km/s for the compressional and shear waves, respectively. Regarding Young's modulus, we obtained 7.1 0.6 GPa, consistent with the previously measured value via a nanoindentation test. Compared with the elastic properties of other carbonaceous chondrites (Tagish Lake, Tarda, Ivuna, and Murchison meteorites), we found that Ryugu had distinctly lower rigidity than Ivuna—the most similar material to Ryugu with respect to chemical and mineralogical features. Instead, Tagish Lake showed the closest elastic properties to Ryugu samples. The affinities in chemical and mineralogical features indicate the genetic relationship between Ryugu and Ivuna. On the other hand, the difference in elastic properties might indicate their formation and evolution processes proceeded differently (e.g., formation depth, degree of alteration).

Organic molecules delivered from extraterrestrial materials may have played a key role in supplying building blocks for life on Earth. Here we report the detection of all five canonical nucleobases—adenine, guanine, cytosine, thymine, and uracil—in samples returned from the C-type asteroid (162173) Ryugu by JAXA’s Hayabusa2 mission. The definitive identification of a complete set of nucleobases in carbonaceous asteroid samples—following detection in Bennu—strongly suggests that these essential components of DNA and RNA are widespread throughout the Solar System. Comparative analysis with Bennu and the Orgueil meteorite shows purine and pyrimidine abundances vary and correlate with ammonia concentrations, linking to physicochemical environments of their parent bodies and providing new insights into extraterrestrial nucleobase formation pathways. The identification of diverse nucleobases in asteroidal and meteoritic materials reinforces the hypothesis that carbonaceous asteroids contributed to the prebiotic chemical inventory of early Earth.

Organic matter in meteorites reveals clues about early Solar System chemistry and the origin of molecules important to life, but terrestrial exposure complicates interpretation. Samples returned from the B-type asteroid Bennu by the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer mission enabled us to study pristine carbonaceous astromaterial without uncontrolled exposure to Earth’s biosphere. Here we show that Bennu samples are volatile rich, with more carbon, nitrogen and ammonia than samples from asteroid Ryugu and most meteorites. Nitrogen-15 isotopic enrichments indicate that ammonia and other N-containing soluble molecules formed in a cold molecular cloud or the outer protoplanetary disk. We detected amino acids (including 14 of the 20 used in terrestrial biology), amines, formaldehyde, carboxylic acids, polycyclic aromatic hydrocarbons and N-heterocycles (including all five nucleobases found in DNA and RNA), along with ~10,000 N-bearing chemical species. All chiral non-protein amino acids were racemic or nearly so, implying that terrestrial life’s left-handed chirality may not be due to bias in prebiotic molecules delivered by impacts. The relative abundances of amino acids and other soluble organics suggest formation and alteration by low-temperature reactions, possibly in NH ₃ -rich fluids. Bennu’s parent asteroid developed in or accreted ices from a reservoir in the outer Solar System where ammonia ice was stable.

Nucleosynthetic isotope variations are powerful tracers to determine genetic relationships between meteorites and planetary bodies. They can help to link material collected by space missions to known meteorite groups. The Hayabusa 2 mission returned samples from the Cb‐type asteroid (162173) Ryugu. The mineralogical, chemical, and isotopic characteristics of these samples show strong similarities to carbonaceous chondrites and in particular CI chondrites. The nucleosynthetic isotope compositions of Ryugu overlap with CI chondrites for several elements (e.g., Cr, Ti, Fe, and Zn). In contrast to these isotopes, which are of predominately supernovae origin, s‐process variations in Mo isotope data are similar to those of carbonaceous chondrites, but even more s‐process depleted. To further constrain the origin of this depletion and test whether this signature is also present for other s‐process elements, we report Zr isotope compositions for three bulk Ryugu samples (A0106, A0106‐A0107, C0108) collected from the Hayabusa 2 mission. The data are complemented with that of terrestrial rock reference materials, eucrites, and carbonaceous chondrites. The Ryugu samples are characterized by distinct ⁹⁶Zr enrichment relative to Earth, indicative of a s‐process depletion. Such depletion is also observed for carbonaceous chondrites and eucrites, in line with previous Zr isotope work, but it is more extreme in Ryugu, as observed for Mo isotopes. Since s‐process Zr and Mo are coupled in mainstream SiC grains, these distinct s‐process variations might be due to SiC grain depletion in the analyzed materials, potentially caused by incomplete sample digestion, because the Ryugu samples were dissolved on a hotplate only to avoid high blank levels for other elements (e.g., Cr). However, local depletion of SiC grains cannot be excluded. An alternative, equally possible scenario is that aqueous alteration redistributed anomalous, s‐process‐depleted, Zr on a local scale, for example, into Ca‐phosphates or phyllosilicates.