On the origin and evolution of the asteroid Ryugu: A comprehensive geochemical perspective

High surface area and interconnected nanoporosity of clay-rich astromaterials

Porosity affects key astromaterial processes from disruption in our atmosphere and impact with the ground, to the comminution of boulders by thermal and impact processes and slope mechanics on asteroid surfaces, to access and utilization of in-situ resources. Whereas the bulk porosity of clay-rich meteorites is well established, the magnitude of their surface area and nano-scale porosity is poorly known. Here we use N2 BET gas adsorption to measure the specific surface area and nanoscale pore distribution in several clay-rich meteorites. Two recent falls Tarda (C2-ung) and Aguas Zarcas (CM2) have specific surface areas of 72.5 and 16.5 m2/g, respectively. However, the specific surface area of Tarda ranges from 33.7 to 81.6 m2/g depending on outgassing conditions. The Tarda surface area is dominated by an interconnected network of ~ 3-nm-sized pores, whereas Aguas Zarcas shows a lower density of ~ 3 nm pores and broader size distribution around 40 nm. In contrast, Ivuna and Orgueil (CI1) have surface areas of ~ 15 to 18 m2/g: the low values compared to Tarda are likely due to the neoformation of pore-blocking minerals during atmospheric exposure. These data suggest that returned samples from asteroids Ryugu and Bennu, which are mineralogically and texturally similar to Tarda, also have interconnected nano-scale porosity with high surface area.

Prebiotic Vitamin B3 Synthesis in Carbonaceous Planetesimals

Aqueous chemistry within carbonaceous planetesimals is promising for synthesizing prebiotic organic matter essential to all life. Meteorites derived from these planetesimals delivered these life building blocks to the early Earth, potentially facilitating the origins of life. Here, we studied the formation of vitamin B3 as it is an important precursor of the coenzyme NAD(P)(H), which is essential for the metabolism of all life as we know it. We propose a new reaction mechanism based on known experiments in the literature that explains the synthesis of vitamin B3. It combines the sugar precursors glyceraldehyde or dihydroxyacetone with the amino acids aspartic acid or asparagine in aqueous solution without oxygen or other oxidizing agents. We performed thermochemical equilibrium calculations to test the thermodynamic favorability. The predicted vitamin B3 abundances resulting from this new pathway were compared with measured values in asteroids and meteorites. We conclude that competition for reactants and decomposition by hydrolysis are necessary to explain the prebiotic content of meteorites. In sum, our model fits well into the complex network of chemical pathways active in this environment.

Microstructural and chemical features of impact melts on Ryugu particle surfaces: Records of interplanetary dust hit on asteroid Ryugu

The Hayabusa2 spacecraft delivered samples of the carbonaceous asteroid Ryugu to Earth. Some of the sample particles show evidence of micrometeoroid impacts, which occurred on the asteroid surface. Among those, particles A0067 and A0094 have flat surfaces on which a large number of microcraters and impact melt splashes are observed. Two impact melt splashes and one microcrater were analyzed to unveil the nature of the objects that impacted the asteroid surface. The melt splashes consist mainly of Mg-Fe-rich glassy silicates and Fe-Ni sulfides. The microcrater trapped an impact melt consisting mainly of Mg-Fe-rich glassy silicate, Fe-Ni sulfides, and minor silica-rich glass. These impact melts show a single compositional trend indicating mixing of Ryugu surface materials and impactors having chondritic chemical compositions. The relict impactor in one of the melt splashes shows mineralogical similarity with anhydrous chondritic interplanetary dust particles having a probable cometary origin. The chondritic micrometeoroids probably impacted the Ryugu surface during its residence in a near-Earth orbit.

Aqueous breakdown of aspartate and glutamate to n-ω-amino acids on the parent bodies of carbonaceous chondrites and asteroid Ryugu

Amino acids in carbonaceous chondrites may have seeded the origin of life on Earth and possibly elsewhere. Recently, the return samples from a C-type asteroid Ryugu were found to contain amino acids with a similar distribution to Ivuna-type CI chondrites, suggesting the potential of amino acid abundances as molecular descriptors of parent body geochemistry. However, the chemical mechanisms responsible for the amino acid distributions remain to be elucidated particularly at low temperatures (<50°C). Here, we report that two representative proteinogenic amino acids, aspartic acid and glutamic acid, decompose to β-alanine and γ-aminobutyric acid, respectively, under simulated geoelectrochemical conditions at 25°C. This low-temperature conversion provides a plausible explanation for the enrichment of these two n-ω-amino acids compared to their precursors in heavily aqueously altered CI chondrites and Ryugu's return samples. The results suggest that these heavily aqueously altered samples originated from the water-rich mantle of their water/rock differentiated parent planetesimals where protein α-amino acids were decomposed.

Synthesis and applications of helical polymers with dynamic and static memories of helicity

This review mainly highlights our studies on the synthesis of one-handed helical polymers with a static memory of helicity based on the noncovalent helicity induction with a helical-sense bias and subsequent memory of the helicity approach that we developed during the past decade. Apart from the previous approaches, an excess one-handed helical conformation, once induced by nonracemic molecules, is immediately retained ("memorized") after the complete removal of the nonracemic molecules, accompanied by a significant amplification of the asymmetry, providing novel switchable chiral materials for chromatographic enantioseparation and asymmetric catalysis as well as a highly sensitive colorimetric and fluorescence chiral sensor. A conceptually new one-handed helix formation in a racemic helical polymer composed of racemic repeating units through the deracemization of the pendants is described.

Spectroscopic and Biophysical Methods to Determine Differential Salt-Uptake by Primitive Membraneless Polyester Microdroplets

α-Hydroxy acids are prebiotic monomers that undergo dehydration synthesis to form polyester gels, which assemble into membraneless microdroplets upon aqueous rehydration. These microdroplets are proposed as protocells that can segregate and compartmentalize primitive molecules/reactions. Different primitive aqueous environments with a variety of salts could have hosted chemistries that formed polyester microdroplets. These salts could be essential cofactors of compartmentalized prebiotic reactions or even directly affect protocell structure. However, fully understanding polyester-salt interactions remains elusive, partially due to technical challenges of quantitative measurements in condensed phases. Here, spectroscopic and biophysical methods are applied to analyze salt uptake by polyester microdroplets. Inductively coupled plasma mass spectrometry is applied to measure the cation concentration within polyester microdroplets after addition of chloride salts. Combined with methods to determine the effects of salt uptake on droplet turbidity, size, surface potential and internal water distribution, it was observed that polyester microdroplets can selectively partition salt cations, leading to differential microdroplet coalescence due to ionic screening effects reducing electrostatic repulsion forces between microdroplets. Through applying existing techniques to novel analyses related to primitive compartment chemistry and biophysics, this study suggests that even minor differences in analyte uptake can lead to significant protocellular structural change.

Water circulation in Ryugu asteroid affected the distribution of nucleosynthetic isotope anomalies in returned sample

Studies of material returned from Cb asteroid Ryugu have revealed considerable mineralogical and chemical heterogeneity, stemming primarily from brecciation and aqueous alteration. Isotopic anomalies could have also been affected by delivery of exogenous clasts and aqueous mobilization of soluble elements. Here, we show that isotopic anomalies for mildly soluble Cr are highly variable in Ryugu and CI chondrites, whereas those of Ti are relatively uniform. This variation in Cr isotope ratios is most likely due to physicochemical fractionation between 54Cr-rich presolar nanoparticles and Cr-bearing secondary minerals at the millimeter-scale in the bulk samples, likely due to extensive aqueous alteration in their parent bodies that occurred [Formula: see text] after Solar System birth. In contrast, Ti isotopes were marginally affected by this process. Our results show that isotopic heterogeneities in asteroids are not all nebular or accretionary in nature but can also reflect element redistribution by water.

Determination of mass-dependent chromium isotopic compositions in geological samples by double spike-total evaporation-thermal ionization mass spectrometry (DS-TE-TIMS)

BACKGROUND

Chromium isotopes have been used to trace geochemical and cosmochemical processes in the past. However, the presence of multivalent Cr species has made it difficult to isolate Cr from geological samples, particularly for samples with a low Cr mass fraction.

RESULTS

Here, a simple three-step ion exchange chromatography procedure is presented to separate Cr from various sample matrices, ranging from ultramafic to felsic rocks. Throughout each of the column chromatography step, 1 mL of cation exchange resin AG50W-X8 (200-400 mesh) was used as the stationary phase and oxalic acid as a chelating agent, was used in addition to the inorganic acids. This method yielded high recoveries of Cr [93 ± 8% (2SD, N = 7)] regardless of the lithology. The total procedural blank of Cr was <0.5 ng. We also developed a double spike-total evaporation-thermal ionization mass spectrometry (DS-TE-TIMS) technique that significantly reduced sample consumption to ∼20 ng of Cr per each measurement of mass-dependent 53Cr/52Cr.

SIGNIFICANCE

This study achieved a 2SD external precision of 0.02‰ for the analysis of NIST NBS3112a and of 0.01-0.07‰ for the geological samples. This study enabled high-precision Cr isotope analysis in geological samples with various matrix and Cr compositions using relatively small sample volumes.

A Comparative Study of Methods for Detecting Extraterrestrial Life in Exploration Missions to Mars and the Solar System II: Targeted Characteristics, Detection Techniques, and Their Combination for Survey, Detection, and Analysis

We present a comparative study of the methods used in the search for extraterrestrial microorganism life, including a summary table where different life-detection techniques can be easily compared as an aid to mission and instrument design aimed at life detection. This is an extension of previous study, where detection techniques for a series of target characteristics and molecules that could constitute a positive life detection were evaluated. This comparison has been extended with a particular consideration to sources of false positives, the causes of negative detection, the results of detection techniques when presented regarding terrestrial life, and additional science objectives that could be achieved outside the primary aim of detecting life. These additions address both the scientific and programmatic side of exploration mission design, where a successful proposal must demonstrate probable outcomes and be able to return valuable results even if no life is found. The applicability of the life detection techniques is considered for Earth life, Earth-independent life (life emerging independently from that on Earth,) and Earth-kin life (sharing a common ancestor with life on Earth), and techniques effective in detecting Earth life should also be useful in the detection of Earth-kin life. However, their applicability is not guaranteed for Earth-independent life. As found in our previous study, there exists no realistic single detection method that can conclusively determine the discovery of extraterrestrial life, and no method is superior to all others. In this study, we further consider combinations of detection techniques and identify imaging as a valuable addition to molecule detection methods, even in cases where there is insufficient resolution to observe the detailed morphology of a microbial cell. The search for extraterrestrial life is further divided into a survey-and-detection and analysis-and-conclusion step. These steps benefit from different detection techniques, but imaging is necessary for both parts.

Origin of Homochirality in Amino Acids Induced by Lyman-α Irradiation in the Early Stage of the Milky Way

The enantiomeric excess (ee) of l-form amino acids found in the Murchison meteorite poses some issues about the cosmic origin of their chirality. Circular dichroism (CD) spectra of amino acids in the far-ultraviolet (FUV) at around 6.8 eV (182 nm) indicate that the circularly polarized light can induce ee through photochemical reactions. Here, we resort to ab initio calculations to extract the CD spectra up to the vacuum-ultraviolet (VUV) region (∼11 eV), and we propose a novel equation to compute the ee applicable to a wider range of light frequency than what is available to date. This allows us to show that the strength of the induced ee (|ee|) in the 10 eV VUV region is comparable to the one in the 6.8 eV FUV region. This feature is common for some key amino acids (alanine, 2-aminobutyric acid, and valine). In space, intense Lyman-α (Lyα) light of 10.2 eV is emitted from star forming regions. This study provides a theoretical basis that Lyα emitter from an early starburst in the Milky Way plays a crucial role in initiating the ee of amino acids.

Visualization of nanoscale magnetic domain states in the asteroid Ryugu

In the samples collected from the asteroid Ryugu, magnetite displays natural remanent magnetization due to nebular magnetic field, whereas contemporaneously grown iron sulfide does not display stable remanent magnetization. To clarify this counterintuitive feature, we observed their nanoscale magnetic domain structures using electron holography and found that framboidal magnetites have an external magnetic field of 300 A m-1, similar to the bulk value, and its magnetic stability was enhanced by interactions with neighboring magnetites, permitting a disk magnetic field to be recorded. Micrometer-sized pyrrhotite showed a multidomain magnetic structure that was unable to retain natural remanent magnetization over a long time due to short relaxation time of magnetic-domain-wall movement, whereas submicron-sized sulfides formed a nonmagnetic phase. These results show that both magnetite and sulfide could have formed simultaneously during the aqueous alteration in the parent body of the asteroid Ryugu.

The Evolution of Life Is a Road Paved with the DNA Quadruplet Symmetry and the Supersymmetry Genetic Code

Symmetries have not been completely determined and explained from the discovery of the DNA structure in 1953 and the genetic code in 1961. We show, during 10 years of investigation and research, our discovery of the Supersymmetry Genetic Code table in the form of 2 × 8 codon boxes, quadruplet DNA symmetries, and the classification of trinucleotides/codons, all built with the same physiochemical double mirror symmetry and Watson-Crick pairing. We also show that single-stranded RNA had the complete code of life in the form of the Supersymmetry Genetic Code table simultaneously with instructions of codons' relationship as to how to develop the DNA molecule on the principle of Watson-Crick pairing. We show that the same symmetries between the genetic code and DNA quadruplet are highly conserved during the whole evolution even between phylogenetically distant organisms. In this way, decreasing disorder and entropy enabled the evolution of living beings up to sophisticated species with cognitive features. Our hypothesis that all twenty amino acids are necessary for the origin of life on the Earth, which entirely changes our view on evolution, confirms the evidence of organic natural amino acids from the extra-terrestrial asteroid Ryugu, which is nearly as old as our solar system.

Igneous processes in the small bodies of the Solar System I. Asteroids and comets

Igneous processes were quite widespread in the small bodies of the Solar System (SBSS) and were initially fueled by short-lived radioisotopes, the proto-Sun, impact heating, and differentiation heating. Once they finished, long-lived radioisotopes continued to warm the active bodies of the Earth, (possibly) Venus, and the cryovolcanism of Enceladus. The widespread presence of olivine and pyroxenes in planets and also in SBSS suggests that they were not necessarily the product of igneous processes and they might have been recycled from previous nebular processes or entrained in comets from interstellar space. The difference in temperature between the inner and the outer Solar System has clearly favored thermal annealing of the olivine close to the proto-Sun. Transport of olivine within the Solar System probably occurred also due to protostellar jets and winds but the entrainment in SBSS from interstellar space would overcome the requirement of initial turbulent regime in the protoplanetary nebula.

Abundant presolar grains and primordial organics preserved in carbon-rich exogenous clasts in asteroid Ryugu

Preliminary analyses of asteroid Ryugu samples show kinship to aqueously altered CI (Ivuna-type) chondrites, suggesting similar origins. We report identification of C-rich, particularly primitive clasts in Ryugu samples that contain preserved presolar silicate grains and exceptional abundances of presolar SiC and isotopically anomalous organic matter. The high presolar silicate abundance (104 ppm) indicates that the clast escaped extensive alteration. The 5 to 10 times higher abundances of presolar SiC (~235 ppm), N-rich organic matter, organics with N isotopic anomalies (1.2%), and organics with C isotopic anomalies (0.2%) in the primitive clasts compared to bulk Ryugu suggest that the clasts formed in a unique part of the protoplanetary disk enriched in presolar materials. These clasts likely represent previously unsampled outer solar system material that accreted onto Ryugu after aqueous alteration ceased, consistent with Ryugu's rubble pile origin.

Organic Matter in the Asteroid Ryugu: What We Know So Far

The Hayabusa2 mission was tasked with returning samples from the C-complex asteroid Ryugu (1999 JU3), in order to shed light on the formation, evolution and composition of such asteroids. One of the main science objectives was to understand whether such bodies could have supplied the organic matter required for the origin of life on Earth. Here, a review of the studies concerning the organic matter within the Ryugu samples is presented. This review will inform the reader about the Hayabusa2 mission, the nature of the organic matter analyzed and the various interpretations concerning the analytical findings including those concerning the origin and evolution of organic matter from Ryugu. Finally, the review puts the findings and individual interpretations in the context of the current theories surrounding the formation and evolution of Ryugu. Overall, the summary provided here will help to inform those operating in a wide range of interdisciplinary fields, including planetary science, astrobiology, the origin of life and astronomy, about the most recent developments concerning the organic matter in the Ryugu return samples and their relevance to understanding our solar system and beyond. The review also outlines the issues that still remain to be solved and highlights potential areas for future work.

Enantiomeric Excesses of Aminonitrile Precursors Determine the Homochirality of Amino Acids

High enantiomeric excesses (ee's) of l-amino acids, including non-proteinogenic amino acid isovaline (Iva), were discovered in the Murchison meteorite, but the detailed molecular mechanism responsible for the observed ee of amino acids remains elusive and inconsistent, because Iva has an inverted circular dichroism (CD) spectrum with respect to α-H amino acids, e.g., alanine. To address this issue, we resort to accurate ab initio calculations for amino acids and their precursors in the Strecker synthesis. We evaluated their photolysis-induced ee in the range 5-11 eV including the Lyman alpha emission line (Lyα), the typical intensive 10.2 eV radiation ascribed to the early phase of galactic evolution. We show that only the aminonitrile precursors are characterized by positive ee in the Lyα region, explaining why right-handed circularly polarized Lyα (R-CP-Lyα) induces homologous l-amino acids. This study shows that the homochirality of amino acids is produced at the aminonitrile precursors stage.

Uracil in the carbonaceous asteroid (162173) Ryugu

The pristine sample from the near-Earth carbonaceous asteroid (162173) Ryugu collected by the Hayabusa2 spacecraft enabled us to analyze the pristine extraterrestrial material without uncontrolled exposure to the Earth's atmosphere and biosphere. The initial analysis team for the soluble organic matter reported the detection of wide variety of organic molecules including racemic amino acids in the Ryugu samples. Here we report the detection of uracil, one of the four nucleobases in ribonucleic acid, in aqueous extracts from Ryugu samples. In addition, nicotinic acid (niacin, a B₃ vitamer), its derivatives, and imidazoles were detected in search for nitrogen heterocyclic molecules. The observed difference in the concentration of uracil between A0106 and C0107 may be related to the possible differences in the degree of alteration induced by energetic particles such as ultraviolet photons and cosmic rays. The present study strongly suggests that such molecules of prebiotic interest commonly formed in carbonaceous asteroids including Ryugu and were delivered to the early Earth.

Insights into the formation and evolution of extraterrestrial amino acids from the asteroid Ryugu

All life on Earth contains amino acids and carbonaceous chondrite meteorites have been suggested as their source at the origin of life on Earth. While many meteoritic amino acids are considered indigenous, deciphering the extent of terrestrial contamination remains an issue. The Ryugu asteroid fragments (JAXA Hayabusa2 mission), represent the most uncontaminated primitive extraterrestrial material available. Here, the concentrations of amino acids from two particles from different touchdown sites (TD1 and TD2) are reported. The concentrations show that N,N-dimethylglycine (DMG) is the most abundant amino acid in the TD1 particle, but below detection limit in the other. The TD1 particle mineral components indicate it experienced more aqueous alteration. Furthermore, the relationships between the amino acids and the geochemistry suggest that DMG formed on the Ryugu progenitor body during aqueous alteration. The findings highlight the importance of aqueous chemistry for defining the ultimate concentrations of amino acids in primitive extraterrestrial samples.

Chondrule-like objects and Ca-Al-rich inclusions in Ryugu may potentially be the oldest Solar System materials

Chondrule-like objects and Ca-Al-rich inclusions (CAIs) are discovered in the retuned samples from asteroid Ryugu. Here we report results of oxygen isotope, mineralogical, and compositional analysis of the chondrule-like objects and CAIs. Three chondrule-like objects dominated by Mg-rich olivine are ¹⁶O-rich and -poor with Δ¹⁷O (=δ¹⁷O - 0.52 × δ¹⁸O) values of ~ -23‰ and ~ -3‰, resembling what has been proposed as early generations of chondrules. The ¹⁶O-rich objects are likely to be melted amoeboid olivine aggregates that escaped from incorporation into ¹⁶O-poor chondrule precursor dust. Two CAIs composed of refractory minerals are ¹⁶O-rich with Δ¹⁷O of ~ -23‰ and possibly as old as the oldest CAIs. The discovered objects (<30 µm) are as small as those from comets, suggesting radial transport favoring smaller objects from the inner solar nebula to the formation location of the Ryugu original parent body, which is farther from the Sun and scarce in chondrules. The transported objects may have been mostly destroyed during aqueous alteration in the Ryugu parent body.

Insights into the molecular structure and infrared spectrum of the prebiotic species aminoacetonitrile

Aminoacetonitrile is an interstellar molecule with a prominent prebiotic role, already detected in the chemically-rich molecular cloud Sagittarius B2(N) and postulated to be present in the atmosphere of the largest Saturn's moon, Titan. To further support its observation in such remote environments and laboratory experiments aimed at improving our understanding of interstellar chemistry, we report a thorough spectroscopic and structural characterization of aminoacetonitrile. Equilibrium geometry, fundamental bands as well as spectroscopic and molecular parameters have been accurately computed by exploiting a composite scheme rooted in the coupled-cluster theory that accounts for the extrapolation to the complete basis set limit and core-correlation effects. In addition, a semi-experimental approach that combines ground-state rotational constants for different isotopic species and calculated vibrational corrections has been employed for the structure determination. From the experimental side, we report the analysis of the three strongest fundamental bands of aminoacetonitrile observed between 500 and 1000 cm^-1 in high-resolution infrared spectra. More generally, all computed band positions are in excellent agreement with the present and previous experiments. The only exception is the ν₁₅ band, for which we provide a revision of the experimental assignment, now in good agreement with theory.

Ancestral Sequence Reconstruction of the Ribosomal Protein uS8 and Reduction of Amino Acid Usage to a Smaller Alphabet

Understanding the origin and early evolution of proteins is important for unveiling how the RNA world developed into an RNA-protein world. Because the composition of organic molecules in the Earth's primitive environment was plausibly not as diverse as today, the number of different amino acids used in early protein synthesis is likely to be substantially less than the current 20 proteinogenic residues. In this study, we have explored the thermal stability and RNA binding of ancestral variants of the ribosomal protein uS8 constructed from a reduced-alphabet of amino acids. First, we built a phylogenetic tree based on the amino acid sequences of uS8 from multiple extant organisms and used the tree to infer two plausible amino acid sequences corresponding to the last bacterial common ancestor of uS8. Both ancestral proteins were thermally stable and bound to an RNA fragment. By eliminating individual amino acid letters and monitoring thermal stability and RNA binding in the resulting proteins, we reduced the size of the amino acid set constituting one of the ancestral proteins, eventually finding that convergent sequences consisting of 15- or 14-amino acid alphabets still folded into stable structures that bound to the RNA fragment. Furthermore, a simplified variant reconstructed from a 13-amino-acid alphabet retained affinity for the RNA fragment, although it lost conformational stability. Collectively, RNA-binding activity may be achieved with a subset of the current 20 amino acids, raising the possibility of a simpler composition of RNA-binding proteins in the earliest stage of protein evolution.

Proceedings of the Japan Academy - History, database, and trend

A catalog of 13,591 papers published by the Japan Academy in three phases over a century in the Proceedings of the Imperial Academy (1913-1945), the Proceedings of the Japan Academy (1945-1977), and Proceedings of the Japan Academy, divided in Series A and B (1977-2022), is made available for public access. The catalog contains information about the authors, the title of the paper, published year, volume, issue, start page, end page, the field of sciences, and the academy member who introduced the paper in the monthly academy meeting. This article reports some analyses of the catalog and discusses the trends and background of the academies' publications during the past century.

A dehydrated space-weathered skin cloaking the hydrated interior of Ryugu

Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe³⁺ to Fe²⁺ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (-OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss.

Oxygen isotopes of anhydrous primary minerals show kinship between asteroid Ryugu and comet 81P/Wild2

The extraterrestrial materials returned from asteroid (162173) Ryugu consist predominantly of low-temperature aqueously formed secondary minerals and are chemically and mineralogically similar to CI (Ivuna-type) carbonaceous chondrites. Here, we show that high-temperature anhydrous primary minerals in Ryugu and CI chondrites exhibit a bimodal distribution of oxygen isotopic compositions: ¹⁶O-rich (associated with refractory inclusions) and ¹⁶O-poor (associated with chondrules). Both the ¹⁶O-rich and ¹⁶O-poor minerals probably formed in the inner solar protoplanetary disk and were subsequently transported outward. The abundance ratios of the ¹⁶O-rich to ¹⁶O-poor minerals in Ryugu and CI chondrites are higher than in other carbonaceous chondrite groups but are similar to that of comet 81P/Wild2, suggesting that Ryugu and CI chondrites accreted in the outer Solar System closer to the accretion region of comets.

Ryugu's nucleosynthetic heritage from the outskirts of the Solar System

Little is known about the origin of the spectral diversity of asteroids and what it says about conditions in the protoplanetary disk. Here, we show that samples returned from Cb-type asteroid Ryugu have Fe isotopic anomalies indistinguishable from Ivuna-type (CI) chondrites, which are distinct from all other carbonaceous chondrites. Iron isotopes, therefore, demonstrate that Ryugu and CI chondrites formed in a reservoir that was different from the source regions of other carbonaceous asteroids. Growth and migration of the giant planets destabilized nearby planetesimals and ejected some inward to be implanted into the Main Belt. In this framework, most carbonaceous chondrites may have originated from regions around the birthplaces of Jupiter and Saturn, while the distinct isotopic composition of CI chondrites and Ryugu may reflect their formation further away in the disk, owing their presence in the inner Solar System to excitation by Uranus and Neptune.

Rare earth element identification and quantification in millimetre-sized Ryugu rock fragments from the Hayabusa2 space mission

Millimetre-sized primordial rock fragments originating from asteroid Ryugu were investigated using high energy X-ray fluorescence spectroscopy, providing 2D and 3D elemental distribution and quantitative composition information on the microscopic level. Samples were collected in two phases from two sites on asteroid Ryugu and safely returned to Earth by JAXA's asteroid explorer Hayabusa2, during which time the collected material was stored and maintained free from terrestrial influences, including exposure to Earth's atmosphere. Several grains of interest were identified and further characterised to obtain quantitative information on the rare earth element (REE) content within said grains, following a reference-based and computed-tomography-assisted fundamental parameters quantification approach. Several orders of magnitude REE enrichments compared to the mean CI chondrite composition were found within grains that could be identified as apatite phase. Small enrichment of LREE was found for dolomite grains and slight enrichment or depletion for the general matrices within the Ryugu rock fragments A0055 and C0076, respectively.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1186/s40623-022-01705-3.