Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples

Decoding the formation of barred olivine chondrules: Realization of numerical replication

Millimeter-sized silicate spherules embedded in primitive meteorites, namely, "chondrules," are the primary solid component of the early solar nebula. They exhibit distinctive solidification textures, formed through rapid cooling from a molten state. The formation conditions of these textures have primarily been inferred on the basis of dynamic crystallization experiments; however, the theoretical verification of the solidification process has been largely neglected. Here, we conducted numerical simulations of the solidification of chondrule melt and successfully reproduced a crystal growth pattern resembling a typical barred olivine chondrule texture. This pattern emerged under conditions of rapid cooling, exceeding 104 kelvins hour-1, which is substantially larger than those inferred experimentally. These results suggest that theories of chondrule formation in the nebula, which have been developed based on experimental results, should be reexamined.

Near-infrared spectral variation in Ryugu particles and implication for rapid space weathering by solar UV radiation

The 2.7 µm absorption band, represented by OH groups, in the near-infrared reflectance spectra of primitive Solar System bodies could be an indicator of the cause and degree of space weathering. We compared the absorption bands of particles sampled from the surface (chamber A) and the excavated subsurface (chamber C) on the C-type asteroid Ryugu by the JAXA Hayabusa2 mission. We developed a fitting method using multiple Gaussians to precisely calculate the band's peak position and depth, providing more reliable information on space weathering effects. We found that the chamber A particles were divided into groups Aα with shorter peak position and deeper band depth (non or poorly space weathered) and Aβ with longer peak position and shallower band depth (rich in space weathering). Chamber C particles are partly show intermediate characteristics between Aα and Aβ, which might imply weak space weathering by solar UV radiation during three months between the excavation by impact experiment and sampling by Hayabusa2. Supplementally, our preliminary experiment of UV irradiation to Ivuna meteorite showed decrease of 2.7 µm band depth and longward shift of the peak position, and further study is needed to investigate the effect of short-term UV space weathering on carbonaceous asteroids.

Impact-driven oxidation of organics explains chondrite shock metamorphism dichotomy

Shocked meteorites can be used to probe the dynamics of the early Solar system. Carbonaceous chondrites are less shocked than ordinary chondrites, regardless of the degree of aqueous alteration. Here, we show that this shock metamorphic dichotomy is a consequence of impact-driven oxidation of organics that are abundant in carbonaceous but not ordinary chondrites. Impact experiments at 3-7 km s-1 using analogs of chondrite matrices reveal evidence of local heating in the matrix up to ~2000 K. Impacts on carbonaceous asteroids cause explosive release of CO and/or CO2, which can efficiently remove evidence of shock. We show that highly shocked materials are lost to space from typical-sized chondrite parent bodies (~100 km in diameter), but are retained on the largest known carbonaceous asteroid, namely, (1) Ceres, due to its stronger gravity. Ceres' surface is thus a witness plate for the ancient impact environment of carbonaceous chondrite parent bodies.

Cross-Scale Multimodal Imaging for Organic Matter in Extraterrestrial Samples

The analysis of extraterrestrial organic matter in samples returned by space missions provides a unique opportunity to study prebiotic chemistry. A comprehensive understanding of the occurrence and composition of organic matter is fundamental to unraveling its origin and evolutionary history. However, the scarcity and complexity of these materials pose considerable analytical challenges. Here, we developed a cross-scale multimodal imaging workflow that integrated mass spectrometry imaging (MSI) and vibrational spectroscopy imaging, including desorption electrospray ionization coupled quadrupole-time-of-flight mass spectrometry (DESI-Q-TOF/MS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), nanoscale secondary ion mass spectrometry (NanoSIMS), focal plane array-Fourier transform infrared spectroscopy (FPA-FTIR), and optical photothermal infrared spectroscopy (O-PTIR). This workflow was applied to the Murchison meteorite, with the objective of establishing spatial associations between mineral phases, molecular composition, functional groups, and isotopic composition on a scale from the millimeter to the submicron. The spatial resolution of DESI has been improved from 100 to 200 to 20 μm, enabling spatial correlation with other imaging techniques. For the first time, the enrichment of organic matter─including CHN, CHO, and CHNO compounds and polycyclic aromatic hydrocarbons (PAHs)─in fine-grained rims (FGRs) surrounding silicate chondrules has been observed. Furthermore, the cross-scale multimodal imaging also reveals differences in organic matter composition between Ca-carbonate and phyllosilicates, as well as spatial heterogeneity within the latter. This workflow provides a new paradigm for studying the complex occurrence and composition of organic matter in various research fields, enhancing our understanding of prebiotic materials in the solar system.

Confocal micro-X-ray fluorescence and total reflection X-ray fluorescence analyses of Ryugu sample in a laboratory environment

This study analyzed Ryugu samples utilizing confocal micro-X-ray fluorescence (CM-XRF) and total reflection X-ray fluorescence (TXRF), in which information of elemental mapping as well as the content of trace elements could be obtained separately, and respectively. In CM-XRF, polycapillary lenses are placed in front of the X-ray tube and detector, and elemental information in the overlapping region of both focus points was obtained. Three-dimensional elemental distributions could be obtained by scanning a sample; the Ryugu sample was placed on an Al plate with one center, and the images of Fe, Ni, Cr, and Ca were obtained. The outlines of these elements were almost equivalent; however, as the energy of Ca Kα was low, its intensity in the elemental mapping was weak. Non-destructive TXRF measurements were performed to measure the trace elements in the Ryugu sample. We then performed quantitative analysis and determined the concentrations of trace elements such as Ga, Ge, and Se. The concentrations of these elements are higher than those in the CI chondrites. Therefore, we verified that the 3D elemental distributions and quantitative measurements in the Ryugu sample were performed without destruction.

Magnesium phosphates experienced high-temperature transition found on the CI-like carbonaceous chondrite Yamato 980115 by Raman microspectroscopy

Magnesium phosphate grains, minor accessory minerals found on the primitive meteorite Yamato 980115 (Y 980115), were investigated by Raman microspectroscopy. All magnesium phosphate grains found in the present study can be assigned to farringtonite, dehydrated magnesium phosphate Mg3(PO4)2-I. Since the Mg3(PO4)2-I is generally formed via the irreversible thermal transition from the polymorphs of Mg3(PO4)2-II and -III at above 750-800 degree Celsius, we can infer that the parent body of the Y 980115 meteorites experienced thermal alteration with such a high temperature. This result is in good accordance with the previous studies and the proposals on the alteration history of Y 980115 by the electron-beam microscope and X-ray diffraction analyses. Furthermore, the hydrated form of the magnesium phosphates of Mg3(PO4)2·4H2O, Mg3(PO4)2·8H2O, and Mg3(PO4)2·22H2O was not found in the present research, also suggesting that the extensive vaporization of the hydrated water molecules with magnesium phosphate occurred by such high-temperature thermal alteration. Since Y 980115 has been historically categorized to heavily aqueously altered CI (Ivuna-type) carbonaceous chondrites but has distinct characteristic to CI meteorites, the present result would provide further evidence to the complexed alteration history of the parent body of Y 980115 meteorite.

A closer look into the structure and magnetism of the recently fallen meteorite Ribbeck

Meteorites are a unique source of geological information about our early Solar System and the difference between planets and asteroids. In this study, meteorite Ribbeck (2024 BX1, SAR 2736) from the recently fallen asteroid (21.01.2024), collected right after the fall, was investigated. This meteorite is classified as a coarse-grained brecciated aubrite. The main mineral phases are enstatite, albite, and forsterite. X-ray structural analysis and Raman Spectroscopy indicate its complex metamorphic history, starting with magmatic crystallization and following metamorphic evolution (e.g., impacting). Energy-dispersive. X-ray spectroscopy (EDS) confirms the presence of unusual minerals such as oldhamite, brezinanite (daubréelite, zolenskyite), wassonite/heideite, and alabandite, that formed under highly reductive conditions, with oxygen fugacity ΔIW (logfO2) in a range of -5 to -7. Similar conditions can be found on the Mercury or the Asteroid 3103 Eger. Magnetic measurements confirm that the primary magnetic components in the meteorite are accessory minerals - sulphides and metallic nodules.

Low-Temperature Aqueous Alteration of Chondrites

Chondritic meteorites (chondrites) contain evidence for the interaction of liquid water with the interiors of small bodies early in Solar System history. Here we review the processes, products and timings of the low-temperature aqueous alteration reactions in CR, CM, CI and ungrouped carbonaceous chondrites, the asteroids Ryugu and Bennu, and hydrated dark clasts in different types of meteorites. We first consider the nature of chondritic lithologies and the insights that they provide into alteration conditions, subdivided by the mineralogy and petrology of hydrated chondrites, the mineralogy of hydrated dark clasts, the effects of alteration on presolar grains, and the evolution of organic matter. We then describe the properties of the aqueous fluids and how they reacted with accreted material as revealed by physicochemical modelling and hydrothermal experiments, the analysis of fluid inclusions in aqueously formed minerals, and isotope tracers. Lastly, we outline the chronology of aqueous alteration reactions as determined using the 53Mn-53Cr and 129I-129Xe systems.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11214-024-01132-8.

An evaporite sequence from ancient brine recorded in Bennu samples

Evaporation or freezing of water-rich fluids with dilute concentrations of dissolved salts can produce brines, as observed in closed basins on Earth1 and detected by remote sensing on icy bodies in the outer Solar System2,3. The mineralogical evolution of these brines is well understood in regard to terrestrial environments4, but poorly constrained for extraterrestrial systems owing to a lack of direct sampling. Here we report the occurrence of salt minerals in samples of the asteroid (101955) Bennu returned by the OSIRIS-REx mission5. These include sodium-bearing phosphates and sodium-rich carbonates, sulfates, chlorides and fluorides formed during evaporation of a late-stage brine that existed early in the history of Bennu's parent body. Discovery of diverse salts would not be possible without mission sample return and careful curation and storage, because these decompose with prolonged exposure to Earth's atmosphere. Similar brines probably still occur in the interior of icy bodies Ceres and Enceladus, as indicated by spectra or measurement of sodium carbonate on the surface or in plumes2,3.

Seismic resurfacing of 433 Eros indicative of a highly dissipative interior for large near-Earth asteroids

An asteroid's interior dictates how its space environment changes its shape and surface, which provides a record of the collisional and dynamical evolution of the Solar System. Knowledge of asteroid interiors also enables the formulation of mitigation strategies against hazardous asteroids. Despite this importance, asteroid interiors remain poorly understood. Asteroids between 0.2 km and 10 km in diameter are thought to have rubble-pile interiors, transitioning to fractured interiors at 10 km scales, then to coherent and differentiated interiors at ≳100 km scales. The asteroid 433 Eros, the only 10-km-scale asteroid explored by a rendezvous mission, is an exemplar of asteroids at intermediate scales. Recent exploration of smaller rubble piles has provided insight into physical properties that are shared with Eros-sized objects. Here we quantify the seismic and physical properties of Eros' interior through measurements and modelling of crater degradation and erasure from the impact that formed the 7.5-km-diameter Shoemaker crater on Eros. Our results indicate that Eros' deep interior has a seismic wave scattering length of 0.5 ± 0.1 km and effective quality factor E Q < 63 - 13 + 18 at frequencies >0.06 Hz. Contrary to the established view of Eros as a fractured shard, our findings suggest that Eros' interior properties are consistent with those of a rubble-pile asteroid.

Survivability and life support in sealed mini-ecosystems with simulated planetary soils

Establishing a sustainable life-support system for space exploration is a formidable challenge due to the vast distances, high costs, and environmental differences from Earth. Building upon the lessons from the Biosphere 2 experiment, we introduce the novel "Ecosphere" and "Biosealed" systems, self-sustaining ecosystems within customizable, enclosed containers. These systems incorporate terrestrial ecosystems and groundwater layers, offering a potential model for transplanting Earth-like biomes to extraterrestrial environments. Over 4 years, we conducted rigorous experiments and analyses to understand the dynamics of these enclosed ecosystems. We successfully mitigated moisture deficiency, a major obstacle to plant growth, by incorporating groundwater layers. Additionally, we quantified microbial communities proliferating in specific soils, including simulated lunar and Ryugu asteroid regolith, enhance plant cultivation in space environments. Metagenomic analysis of these simulated space soils revealed diverse microbial populations and their crucial role in plant growth and ecosystem stability. Notably, we identified symbiotic relationships between plants and Cyanobacteria, enhancing oxygen production, and demonstrated the potential of LED lighting as an alternative light source for plant cultivation in sun-limited space missions. We also confirmed the survival of fruit flies within these systems, relying on plant-produced oxygen and photosynthetic bacteria. Our research provides a comprehensive framework for developing future space life-support systems. The novelty of our work lies in the unique design of our enclosed ecosystems, incorporating groundwater layers and simulated extraterrestrial soils, and the detailed analysis of microbial communities within these systems. These findings offer valuable insights into the challenges and potential solutions for establishing sustainable human habitats in space, including the importance of microbial management and potential health concerns related to microbial exposure.

A primordial noble gas component discovered in the Ryugu asteroid and its implications

Ryugu is the C-type asteroid from which material was brought to Earth by the Hayabusa2 mission. A number of individual grains and fine-grained samples analysed so far for noble gases have indicated that solar wind and planetary (known as P1) noble gases are present in Ryugu samples with concentrations higher than those observed in CIs, suggesting the former to be more primitive compared to the latter. Here we present results of analyses of three fine-grained samples from Ryugu, in one of which Xe concentration is an order of magnitude higher than determined so far in other samples from Ryugu. Isotopically, this Xe resembles P1, but with a much stronger isotopic fractionation relative to solar wind and significantly lower 36Ar/132Xe ratio than in P1. This previously unknown primordial noble gas component (here termed P7) provides clues to constrain how the solar composition was fractionated to form the planetary components.

Breunnerite grain and magnesium isotope chemistry reveal cation partitioning during aqueous alteration of asteroid Ryugu

Returned samples from the carbonaceous asteroid (162173) Ryugu provide pristine information on the original aqueous alteration history of the Solar System. Secondary precipitates, such as carbonates and phyllosilicates, reveal elemental partitioning of the major component ions linked to the primordial brine composition of the asteroid. Here, we report on the elemental partitioning and Mg isotopic composition (25Mg/24Mg) of breunnerite [(Mg, Fe, Mn)CO3] from the Ryugu C0002 sample and the A0106 and C0107 aggregates by sequential leaching extraction of salts, exchangeable ions, carbonates, and silicates. Breunnerite was the sample most enriched in light Mg isotopes, and the 25Mg/24Mg value of the fluid had shifted lower by ~0.38‰ than the initial value (set to 0‰) before dolomite precipitation. As a simple model, the Mg2+ first precipitated in phyllosilicates, followed by dolomite precipitation, at which time ~76-87% of Mg2+ had been removed from the primordial brine. A minor amount of phyllosilicate precipitation continued after dolomite precipitation. The element composition profiles of the latest solution that interacted with the cation exchange pool of Ryugu were predominantly Na-rich. Na+ acts as a bulk electrolyte and contributes to the stabilization of the negative surface charge of phyllosilicates and organic matter on Ryugu.

Mechanical properties of rubble pile asteroids (Dimorphos, Itokawa, Ryugu, and Bennu) through surface boulder morphological analysis

Planetary defense efforts rely on estimates of the mechanical properties of asteroids, which are difficult to constrain accurately from Earth. The mechanical properties of asteroid material are also important in the interpretation of the Double Asteroid Redirection Test (DART) impact. Here we perform a detailed morphological analysis of the surface boulders on Dimorphos using images, the primary data set available from the DART mission. We estimate the bulk angle of internal friction of the boulders to be 32.7 ± 2. 5° from our measurements of the roundness of the 34 best-resolved boulders ranging in size from 1.67-6.64 m. The elongated nature of the boulders around the DART impact site implies that they were likely formed through impact processing. Finally, we find striking similarities in the morphology of the boulders on Dimorphos with those on other rubble pile asteroids (Itokawa, Ryugu and Bennu). This leads to very similar internal friction angles across the four bodies and suggests that a common formation mechanism has shaped the boulders. Our results provide key inputs for understanding the DART impact and for improving our knowledge about the physical properties, the formation and the evolution of both near-Earth rubble-pile and binary asteroids.

Evidence of both molecular cloud and fluid chemistry in Ryugu regolith

The sulfur chemistry of (162173) Ryugu particles can be a powerful tracer of molecular cloud chemistry and small body processes, but it has not been well explored. We report identification of organosulfurs and a sulfate grain in two Ryugu particles, A0070 and A0093. The sulfate grain shows oxygen isotope ratios (δ17O = -11.0 ± 4.3 per mil, δ18O = -7.8 ± 2.3 per mil) that are akin to silicates in Ryugu but exhibit mass-independent sulfur isotopic fractionation (Δ33S = +5 ± 2 per mil). A methionine-like coating on the sulfate grain is isotopically anomalous (δ15N = +62 ± 2 per mil). Both the sulfate and organosulfurs can simultaneously form and survive during aqueous alteration within Ryugu's parent body, under reduced conditions, low temperature, and a pH >7 in the presence of N-rich organic molecules. This work extends the heliocentric zone where anomalous sulfur, formed by selective photodissociation of H2S gas in the molecular cloud, is found.

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

The recent return of samples from asteroid 162173 Ryugu provides a first insight into early Solar System prebiotic evolution from known planetary bodies. Ryugu's samples are CI chondrite-like, rich in water and organic material, and primarily composed of phyllosilicate. This phyllosilicate surrounds micron to submicron macromolecular organic particles known as insoluble organic matter. Using advanced microscopy techniques on Hayabusa-2 samples, we find that aqueous alteration on Ryugu produced organic particles richer in aromatics compared to less altered carbonaceous chondrites. This challenges the view that aromatic-rich organic matter formed pre-accretion. Additionally, widespread diffuse organic material occurs in phyllosilicate more aliphatic-, carboxylic-rich, and aromatic-poor than the discrete organic particles, likely preserving the soluble organic material. Some organic particles evolved to encapsulate phyllosilicate, indicating that aqueous alteration on Ryugu led to the containment of soluble organic matter within these particles. Earth therefore has been, and continues to be, delivered micron-sized polymeric organic objects containing biologically relevant molecules.

Primordial aqueous alteration recorded in water-soluble organic molecules from the carbonaceous asteroid (162173) Ryugu

We report primordial aqueous alteration signatures in water-soluble organic molecules from the carbonaceous asteroid (162173) Ryugu by the Hayabusa2 spacecraft of JAXA. Newly identified low-molecular-weight hydroxy acids (HO-R-COOH) and dicarboxylic acids (HOOC-R-COOH), such as glycolic acid, lactic acid, glyceric acid, oxalic acid, and succinic acid, are predominant in samples from the two touchdown locations at Ryugu. The quantitative and qualitative profiles for the hydrophilic molecules between the two sampling locations shows similar trends within the order of ppb (parts per billion) to ppm (parts per million). A wide variety of structural isomers, including α- and β-hydroxy acids, are observed among the hydrophilic molecules. We also identify pyruvic acid and dihydroxy and tricarboxylic acids, which are biochemically important intermediates relevant to molecular evolution, such as the primordial TCA (tricarboxylic acid) cycle. Here, we find evidence that the asteroid Ryugu samples underwent substantial aqueous alteration, as revealed by the presence of malonic acid during keto-enol tautomerism in the dicarboxylic acid profile. The comprehensive data suggest the presence of a series for water-soluble organic molecules in the regolith of Ryugu and evidence of signatures in coevolutionary aqueous alteration between water and organics in this carbonaceous asteroid.

Numerical simulations suggest asteroids (101955) Bennu and (162173) Ryugu are likely second or later generation rubble piles

Rubble pile asteroids are widely understood to be composed of reaccumulated debris following a catastrophic collision between asteroids in the main asteroid belt, where each disruption can make a family of new asteroids. Near-Earth asteroids Ryugu and Bennu have been linked to collisional families in the main asteroid belt, but surface age analyses of each asteroid suggest these bodies are substantially younger than their putative families. Here we show, through a coupled collisional and dynamical evolution of members of these families, that neither asteroid was likely to have been created at the same time as the original family breakups, but rather are likely remnants of later disruptions of original family members, making them second, or later, generation remnants. Our model finds about 80% and 60% of asteroids currently being delivered to near-Earth orbits from the respective families of New Polana and Eulalia are second or later generation. These asteroids delivered today in the 0.5-1 km size range have median ages since their last disruption that are substantially younger than the family age, reconciling their measured crater retention ages with membership in these families.

Recurrent planetesimal formation in an outer part of the early solar system

The formation of planets in our solar system encompassed various stages of accretion of planetesimals that formed in the protoplanetary disk within the first few million years at different distances to the sun. Their chemical diversity is reflected by compositionally variable meteorite groups from different parent bodies. There is general consensus that their formation location is roughly constrained by a dichotomy of nucleosynthetic isotope anomalies, relating carbonaceous (C) meteorite parent bodies to the outer protoplanetary disk and the non-carbonaceous (NC) parent bodies to an origin closer to the sun. It is a common idea, that in these inner parts of the protoplanetary disks, planetesimal accretion processes were faster. Testing such scenarios requires constraining formation ages of meteorite parent bodies. Although isotopic age dating can yield precise formation ages of individual mineral constituents of meteorites, such ages frequently represent mineral cooling ages that can postdate planetesimal formation by millions or tens of millions of years, depending on the cooling history of individual planetesimals at different depths. Nevertheless, such cooling ages provide a detailed thermal history which can be fitted by thermal evolution models that constrain the formation age of individual parent bodies. Here we apply state-of-the-art thermal evolution models to constrain planetesimal formation times particular in the outer solar system formation region of C meteorites. We infer a temporally distributed accretion of various parent bodies from < 0.6 Ma to ≈ 4 Ma after solar system formation, with 3.7 Ma and 2.5 - 2.75 Ma for the parent bodies of CR1-3 chondrites and the Flensburg carbonaceous chondrite, and < 0.6 and < 0.7 Ma for the parent bodies of differentiated achondrites NWA 6704 and NWA 011, respectively. This implies that accretion processes in the C reservoir started as early as in the NC reservoir and were operating throughout typical protoplanetary disk lifetimes, thereby producing differentiated parent bodies with carbonaceous compositions in addition to undifferentiated C chondrite parent bodies. The accretion times correlate inversely with the degree of the meteorites' alteration, metamorphism, or differentiation. The accretion times for the CM, CI, Ryugu, and Tafassite parent bodies of 3.8 Ma, 3.8 Ma, 1 - 3 Ma, and 1.1 Ma, respectively, fit well into this correlation in agreement with the thermal and alteration conditions suggested by these meteorites. Our results indicate that individual planetesimals formed rapidly (i.e., within < 1 Ma), however, distinct planetesimals formed recurrently throughout the total lifetime of the protoplanetary disk. Rapid individual formation is consistent with streaming instabilities assisted by gravitational collapse. However, obviously not the total dust inventory was consumed at early disk evolution stages, so there must have been some delay mechanisms, e.g. collisional destruction of precursor aggregates due to high impact velocities induced by radial drift phenomena. This counterbalance enabled late ( > 2 - 3 Ma) accretion of C planetesimals beyond the snow line which escaped severe planetesimal heating and volatile loss, hence, preserving their volatiles, especially water. Only this delayed formation of water-rich planetesimals allowed Earth to accrete sufficient water to become a habitable planet, preventing it from being a bone dry planet.

Nonmagnetic framboid and associated iron nanoparticles with a space-weathered feature from asteroid Ryugu

Extraterrestrial minerals on the surface of airless Solar System bodies undergo gradual alteration processes known as space weathering over long periods of time. The signatures of space weathering help us understand the phenomena occurring in the Solar System. However, meteorites rarely retain the signatures, making it impossible to study the space weathering processes precisely. Here, we examine samples retrieved from the asteroid Ryugu by the Hayabusa2 spacecraft and discover the presence of nonmagnetic framboids through electron holography measurements that can visualize magnetic flux. Magnetite particles, which normally provide a record of the nebular magnetic field, have lost their magnetic properties by reduction via a high-velocity (>5 km s-1) impact of a micrometeoroid with a diameter ranging from 2 to 20 μm after destruction of the parent body of Ryugu. Around these particles, thousands of metallic-iron nanoparticles with a vortex magnetic domain structure, which could have recorded a magnetic field in the impact event, are found. Through measuring the remanent magnetization of the iron nanoparticles, future studies are expected to elucidate the nature of the nebular/interplanetary magnetic fields after the termination of aqueous alteration in an asteroid.

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.

Alexander CM, Amari S, Amelin Y, Bajo KI, Bizzarro M, Bouvier A, Carlson RW, Chaussidon M, Choi BG, Dauphas N, Davis AM, Fujiya W, Fukai R, Gautam I, Haba MK, Hibiya Y, Hidaka H, Hoppe P, Huss GR, Iizuka T, Ireland TR, Ishikawa A, Itoh S, Kawasaki N, Kita NT, Kitajima K, Kleine T, Krot S, Liu MC, Masuda Y, Moynier F, Nguyen A, Nittler L, Pack A, Park C, Piani L, Qin L, Rocco TD, Russell SS, Sakamoto N, Schönbächler M, Tafla L, Tang H, Terada K, Usui T, Wada S, Wadhwa M, Walker RJ, Yamashita K, Yin QZ, Yoneda S, Young ED, Zhang AC, Nakamura T, Naraoka H, Noguchi T, Okazaki R, Sakamoto K, Yabuta H, Abe M, Miyazaki A, Nakato A, Nishimura M, Okada T, Yada T, Yogata K, Nakazawa S, Saiki T, Tanaka S, Terui F, Tsuda Y, Watanabe SI, Yoshikawa M, Tachibana S, Yurimoto H. Analysis of Cation Composition in Dolomites on the Intact Particles Sampled from Asteroid Ryugu

Characterization of the elemental distribution of samples with rough surfaces has been strongly desired for the analysis of various natural and artificial materials. Particularly for pristine and rare analytes with micrometer sizes embedded on specimen surfaces, non-invasive and matrix effect-free analysis is required without surface polishing treatment. To satisfy these requirements, we proposed a new method employing the sequential combination of two imaging modalities, i.e., microenergy-dispersive X-ray fluorescence (micro-XRF) and Raman micro-spectroscopy. The applicability of the developed method is tested by the quantitative analysis of cation composition in micrometer-sized carbonate grains on the surfaces of intact particles sampled directly from the asteroid Ryugu. The first step of micro-XRF imaging enabled a quick search for the sparsely scattered and micrometer-sized carbonates by the codistributions of Ca2+ and Mn2+ on the Mg2+- and Fe2+-rich phyllosilicate matrix. The following step of Raman micro-spectroscopy probed the carbonate grains and analyzed their cation composition (Ca2+, Mg2+, and Fe2+ + Mn2+) in a matrix effect-free manner via the systematic Raman shifts of the lattice modes. The carbonates were basically assigned to ferroan dolomite bearing a considerable amount of Fe2+ + Mn2+ at around 10 atom %. These results are in good accordance with the assignments reported by scanning electron microscopy-energy-dispersive X-ray spectroscopy, where the thin-sectioned and surface-polished Ryugu particles were applicable. The proposed method requires neither sectioning nor surface polishing; hence, it can be applied to the remote sensing apparatus on spacecrafts and planetary rovers. Furthermore, the non-invasive and matrix effect-free characterization will provide a reliable analytical tool for quantitative analysis of the elemental distribution on the samples with surface roughness and chemical heterogeneity at a micrometer scale, such as art paintings, traditional crafts with decorated shapes, as well as sands and rocks with complex morphologies in nature.

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.

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.

Chemical evolution of primordial salts and organic sulfur molecules in the asteroid 162173 Ryugu

Samples from the carbonaceous asteroid (162173) Ryugu provide information on the chemical evolution of organic molecules in the early solar system. Here we show the element partitioning of the major component ions by sequential extractions of salts, carbonates, and phyllosilicate-bearing fractions to reveal primordial brine composition of the primitive asteroid. Sodium is the dominant electrolyte of the salt fraction extract. Anions and NH4+ are more abundant in the salt fraction than in the carbonate and phyllosilicate fractions, with molar concentrations in the order SO42- > Cl- > S2O32- > NO3- > NH4+. The salt fraction extracts contain anionic soluble sulfur-bearing species such as Sn-polythionic acids (n < 6), Cn-alkylsulfonates, alkylthiosulfonates, hydroxyalkylsulfonates, and hydroxyalkylthiosulfonates (n < 7). The sulfur-bearing soluble compounds may have driven the molecular evolution of prebiotic organic material transforming simple organic molecules into hydrophilic, amphiphilic, and refractory S allotropes.

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.

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.

Oil Inclusions Found in Skeleton Crystals of Quartz Indicated the Existence of Organic Matter Surrounding Ancient Growth Environments

In nature, minerals record various origins and information for geology and geobiochemistry. Here, we investigated the origin of organic matter and growth mechanism of quartz with oil inclusion revealing fluorescence under short ultraviolet (UV) light, obtained from the clay vein at Shimanto-cho, Kochi, Shikoku Island, Japan. Geological investigation indicated that the oil-quartz was formed in hydrothermal metamorphic veins found in the late Cretaceous interbedded sandstone and mudstone. The obtained oil-quartz crystals are mostly double-terminated. Micro-X-ray computed tomography (microCT) indicated that oil-quartz crystals have various veins originating as skeleton structures along the quartz crystal {111} and {1-11} faces. Spectroscopic and chromatographic studies indicated that aromatic ester and tetraterpene (lycopene) molecules, which revealed fluorescence, were detected. Large molecular weight sterol molecules, such as C40, were also detected in the vein of oil-quartz. This investigation indicated that organic inclusions in mineral crystals would form with ancient microorganism culture environments.

New analytical method for the evaluation of heterogeneity in cation compositions of dolomites by micro-XRF and Raman spectroscopies

Dolomite (CaMg(CO₃)₂) is an abundant carbonate mineral contained in sedimentary rocks and plays significant roles in water and carbon cycle in geo/cosmochemical environments. Since the cation compositions of carbonates are sensitive to the aqueous environment where they were precipitated and persisted, quantitative analysis of their cation compositions provides valuable information on the aqueous environments and their changes. The difficulty for the analysis of natural dolomite is that Mg²⁺ is continuously substituted by Fe²⁺ or Mn²⁺, and hence they sometimes possess micrometer-scale heterogeneity. Such heterogeneity carries quite important information on the gradual changes in aqueous environments due to changes in thermodynamic conditions and/or aqueous chemical compositions. In the present study, we explored a new quantitative scale to assess such heterogeneity of cation composition in natural dolomite and ferroan dolomite by combining X-ray fluorescence (XRF) and Raman spectroscopy. While the Fe + Mn content differed spot-by-spot, it was found that the Raman wavenumber and Fe + Mn content linearly correlated with each other. Since the spatial resolution of micro-Raman spectroscopy is as high as 1 μm, it does not require vacuum conditions, and is free from so-called matrix effect faced in other methods utilizing X-Rays and electron beams, the proposed qualitative analytical scale can provide a useful tool to assess the cation compositions in dolomites found in nature.

Coordination Changes in Densified Aluminate Glass upon Compression up to 65 GPa: A View from Solid-State Nuclear Magnetic Resonance

Deciphering the structural evolution in irreversibly densified oxide glasses is crucial for fabricating functional glasses with tunable properties and elucidating the nature of pressure-induced anomalous plastic deformation in glasses. High-resolution NMR spectroscopy quantifies atomic-level structural information on densified glasses; however, its application is limited to the low-pressure range due to technical challenges. Here, we report the first high-resolution NMR spectra of oxide glass compressed by diamond anvil cells at room temperature, extending the pressure record of such studies from 24 to 65 GPa. The results constrain the densification path through coordination transformation of Al cations. Based on a statistical thermodynamic model, the stepwise changes in the Al fractions of oxide glasses and the effects of network polymerization on the densification paths are quantified. These results extend the knowledge on densification of the previously unattainable pressure conditions and contribute to understanding the origin of mechanical strengthening of the glasses.

Noble gases and nitrogen in samples of asteroid Ryugu record its volatile sources and recent surface evolution

The near-Earth carbonaceous asteroid (162173) Ryugu is expected to contain volatile chemical species that could provide information on the origin of Earth's volatiles. Samples of Ryugu were retrieved by the Hayabusa2 spacecraft. We measured noble gas and nitrogen isotopes in Ryugu samples and found that they are dominated by presolar and primordial components, incorporated during Solar System formation. Noble gas concentrations are higher than those in Ivuna-type carbonaceous (CI) chondrite meteorites. Several host phases of isotopically distinct nitrogen have different abundances among the samples. Our measurements support a close relationship between Ryugu and CI chondrites. Noble gases produced by galactic cosmic rays, indicating a ~5 million year exposure, and from implanted solar wind record the recent irradiation history of Ryugu after it migrated to its current orbit.

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.

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.

First asteroid gas sample delivered by the Hayabusa2 mission: A treasure box from Ryugu

The Hayabusa2 spacecraft returned to Earth from the asteroid 162173 Ryugu on 6 December 2020. One day after the recovery, the gas species retained in the sample container were extracted and measured on-site and stored in gas collection bottles. The container gas consists of helium and neon with an extraterrestrial 3He/4He and 20Ne/22Ne ratios, along with some contaminant terrestrial atmospheric gases. A mixture of solar and Earth's atmospheric gas is the best explanation for the container gas composition. Fragmentation of Ryugu grains within the sample container is discussed on the basis of the estimated amount of indigenous He and the size distribution of the recovered Ryugu grains. This is the first successful return of gas species from a near-Earth asteroid.

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.

Graphical Abstract

Supplementary Information

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