津田 雄一

Abstract We report a Fourier transform infrared analysis of functional groups in insoluble organic matter (IOM) extracted from a series of 100–500 μm Ryugu grains collected during the two touchdowns of February 22 and July 11, 2019. IOM extracted from most of the samples is very similar to IOM in primitive CI, CM, and CR chondrites, and shows that the extent of thermal metamorphism in Ryugu regolith was, at best, very limited. One sample displays chemical signatures consistent with a very mild heating, likely due to asteroidal collision impacts. We also report a lower carbonyl abundance in Ryugu IOM samples compared to primitive chondrites, which could reflect the accretion of a less oxygenated precursor by Ryugu. The possible effects of hydrothermal alteration and terrestrial weathering are also discussed. Last, no firm conclusions could be drawn on the origin of the soluble outlier phases, observed along with IOM in this study and in the preliminary analysis of Ryugu samples. However, it is clear that the HF/HCl residues presented in this publication are a mix between IOM and the nitrogen‐rich outlier phase.

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 ⁵⁴ Cr-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 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.

Abstract Returned samples from Cb-type asteroid (162173) Ryugu exhibit very dark spectra in visible and near-infrared ranges, generally consistent with the Hayabusa2 observations. A critical difference is that a structural water absorption of hydrous silicates is around twice as deep in the returned samples compared with those of Ryugu’s surface, suggesting Ryugu surface is more dehydrated. Here we use laboratory experiments data to indicate the spectral differences between returned samples and asteroid surface are best explained if Ryugu surface has (1) higher porosity, (2) larger particle size, and (3) more space-weathered condition, with the last being the most effective. On Ryugu, space weathering by micrometeoroid bombardments promoting dehydration seem to be more effective than that by solar-wind implantation. Extremely homogeneous spectra of the Ryugu’s global surface is in contrast with the heterogeneous S-type asteroid (25143) Itokawa’s spectra, which suggests space weathering has proceeded more rapidly on Cb-type asteroids than S-type asteroids.

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.

Abstract We report μm‐scale nondestructive infrared (IR) hyperspectral results (IR computed tomography, IR‐CT) in 3‐D and IR surface imaging, IR‐S) in 2‐D, at SOLEIL) combined with X‐ray nano‐computed tomography analyses (at SPring‐8) performed on eight small Ryugu fragments extracted from mm‐sized grains coming both from touchdown first and second sites. We describe the multiscale assembly of phyllosilicates, carbonates, sulfides, oxides, and organics. Two types of silicates, as well as diverse kinds of organic matter, were detected inside Ryugu material. Their spatial correlations are described to discuss the role of the mineralogical microenvironments in the formation/evolution of organic matter. In particular, we have shown that there is a redistribution of the organic matter diffuse component during aqueous alteration on the parent body, with a preferential circulation among fine‐grained phyllosilicates.

Abstract 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.

Abstract The infrared spectral characteristics of organic‐rich acid residues prepared from Ryugu samples returned by the JAXA Hayabusa2 mission generally match those from unheated carbonaceous chondrite meteorites, but the residues from Ryugu are richer in methyl and methylene functional groups and have higher CH₂/CH₃ ratios. Moreover, two distinct outlier carbonaceous phases are found; one with spectral characteristics of N‐H functional groups, likely amides, and a second phase containing less nitrogen. Such infrared characteristics of Ryugu organic matter might indicate the pristine nature of the freshly collected samples and reflect the near‐surface chemistry in the parent asteroid.

<p id="p1">The elastic property of asteroids is one of the paramount parameters forunderstanding their physical nature. For example, the rigidity enablesus to discuss the asteroid’s shape and surface features such as cratersand boulders, leading to a better understanding of geomorphological andgeological features on small celestial bodies. The sound velocity allowsus to construct an equation of state that is the most fundamental stepto simulate the formation of small bodies numerically. Moreover, seismicwave velocities and attenuation factors are useful to account forresurfacing caused by impact-induced seismic shaking. The elasticproperty of asteroids thus plays an important role in elucidating theasteroid’s evolution and current geological processes. The Hayabusa2spacecraft brought back the rock samples from C-type asteroid (162173)Ryugu in December 2020. As a part of the initial analysis of returnedsamples, we measured the seismic wave velocity of the Ryugu samplesusing the pulse transmission method. We found that P- and S-wavevelocities of the Ryugu samples were about 2.1 km/s and 1.2 km/s,respectively. We also estimated Young’s modulus of 6.0 – 8.0 GPa. Acomparison of the derived parameters with those of carbonaceouschondrites showed that the Ryugu samples have a similar elastic propertyto the Tagish Lake meteorite, which may have come from a D-typeasteroid. Both Ryugu and Tagish Lake show a high degree of aqueousalteration and few high-temperature components such as chondrules,indicating that they formed in the outer region of the solar system.</p>

Abstract Micrometeorites, a possible major source of Earth’s water, are thought to form from explosive dispersal of hydrated chondritic materials during impact events on their parental asteroids. However, this provenance and formation mechanism have yet to be directly confirmed using asteroid returned samples. Here, we report evidence of mild shock metamorphism in the surface particles of asteroid Ryugu based on electron microscopy. All particles are dominated by phyllosilicates but lack dehydration textures, which are indicative of shock-heating temperatures below ~500 °C. Microfault-like textures associated with extensively shock-deformed framboidal magnetites and a high-pressure polymorph of Fe–Cr–sulfide have been identified. These findings indicate that the average peak pressure was ~2 GPa. The vast majority of ejecta formed during impact on Ryugu-like asteroids would be hydrated materials, larger than a millimetre, originating far from the impact point. These characteristics are inconsistent with current micrometeorite production models, and consequently, a new formation mechanism is required.

Abstract Rock fragments of the Cb-type asteroid Ryugu returned to Earth by the JAXA Hayabusa2 mission share mineralogical, chemical, and isotopic properties with the Ivuna-type (CI) carbonaceous chondrites. Similar to CI chondrites, these fragments underwent extensive aqueous alteration and consist predominantly of hydrous minerals likely formed in the presence of liquid water on the Ryugu parent asteroid. Here we present an in situ analytical survey performed by secondary ion mass spectrometry from which we have estimated the D/H ratio of Ryugu’s hydrous minerals, D/H_Ryugu, to be [165 ± 19] × 10⁻⁶, which corresponds to δD_Ryugu = +59 ± 121‰ (2σ). The hydrous mineral D/H_Ryugu’s values for the two sampling sites on Ryugu are similar; they are also similar to the estimated D/H ratio of hydrous minerals in the CI chondrites Orgueil and Alais. This result reinforces a link between Ryugu and CI chondrites and an inference that Ryugu’s samples, which avoided terrestrial contamination, are our best proxy to estimate the composition of water at the origin of hydrous minerals in CI-like material. Based on this data and recent literature studies, the contribution of CI chondrites to the hydrogen of Earth’s surficial reservoirs is evaluated to be ∼3%. We conclude that the water responsible for the alteration of Ryugu’s rocks was derived from water ice precursors inherited from the interstellar medium; the ice partially re-equilibrated its hydrogen with the nebular H₂ before being accreted on the Ryugu’s parent asteroid.

Abstract 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.

Abstract 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.

Samples of the carbonaceous asteroid (162173) Ryugu were collected and brought to Earth by the Hayabusa2 spacecraft. We investigated the macromolecular organic matter in Ryugu samples and found that it contains aromatic and aliphatic carbon, ketone, and carboxyl functional groups. The spectroscopic features of the organic matter are consistent with those in chemically primitive carbonaceous chondrite meteorites that experienced parent-body aqueous alteration (reactions with liquid water). The morphology of the organic carbon includes nanoglobules and diffuse carbon associated with phyllosilicate and carbonate minerals. Deuterium and/or nitrogen-15 enrichments indicate that the organic matter formed in a cold molecular cloud or the presolar nebula. The diversity of the organic matter indicates variable levels of aqueous alteration on Ryugu’s parent body.

The Hayabusa2 spacecraft collected samples from the surface of the carbonaceous near-Earth asteroid (162173) Ryugu and brought them to Earth. The samples were expected to contain organic molecules, which record processes that occurred in the early Solar System. We analyzed organic molecules extracted from the Ryugu surface samples. We identified a variety of molecules containing the atoms CHNOS, formed by methylation, hydration, hydroxylation, and sulfurization reactions. Amino acids, aliphatic amines, carboxylic acids, polycyclic aromatic hydrocarbons, and nitrogen-heterocyclic compounds were detected, which had properties consistent with an abiotic origin. These compounds likely arose from an aqueous reaction on Ryugu’s parent body and are similar to the organics in Ivuna-type meteorites. These molecules can survive on the surfaces of asteroids and be transported throughout the Solar System.

Abstract 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 (&lt;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.

Abstract Sample-return missions allow the study of materials collected directly from celestial bodies, unbiased by atmospheric entry effects and/or terrestrial alteration and contamination phenomena, using state-of-the-art techniques which are available only in a laboratory environment—but only if the collected material stays pristine. The scarcity of outer-space unaltered material recovered until now makes this material extremely precious for the potential scientific insight it can bring. To maximize the scientific output of current and future sample-return missions, the scientific community needs to plan for ways of storing, handling, and measuring this precious material while preserving their pristine state for as long as the ‘invasiveness’ of measurements allows. In July 2021, as part of the Hayabusa2 (JAXA) “Stone” preliminary examination team, we received several microscopic particles from the asteroid Ryugu, with the goal of performing IR hyper-spectral imaging and IR micro-tomography studies. Here, we describe the sample transfer, handling methods and analytical pipeline we implemented to study this very precious material while minimizing and surveilling their alteration history on Earth. Graphical Abstract

Mass spectrometry of noble gas isotopes from the asteroid materials delivered by robotic space missions requires high sensitivity, high ion transmission, low detection limit, and other characteristics not readily available in commercial instruments.

Abstract 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.

There are a number of deep space probes that are currently in operation with diverse destinations and objectives. For example, the Japanese Hayabusa2 and the U.S. OSIRIS-REx missions are both sample returns, targeting different near-Earth asteroids. Europe’s ExoMars and the U.S. Perseverance are orbiting and roving Mars as precursors of future manned explorations. Conventionally, deep space missions require dedicated launch vehicles for each mission. The interplanetary Earth departure trajectory from the low Earth orbit (LEO) usually lacks flexibility and efficiency. Furthermore, innovative and reusable launch systems have been researched and developed by multiple organizations including private sector organizations such as SpaceX and Blue Origin. It is expected that the unit cost per launch weight to LEO be significantly reduced by rideshare mass transportation executed by using reusable mega launchers in the near future. This study aims to fill the transportation gap between LEO and deep space by realizing a flexible and economical interplanetary Earth departure without sacrificing the arbitrariness of LEO, target V-infinity vector, and target Earth departure epoch. Thus, the one-revolution Earth free-return orbit (1rEFRO) and the consequent Earth gravity assist (EGA) are introduced to separate the velocity increment and direction adjustment. The planetary free-return and EGAs are common in interplanetary missions; however, a comprehensive study on the flexibility, economic efficiency, and arbitrariness of the sequence (1rEFRO + EGA) originating from LEO was not explicitly found. After describing the necessary coordinate frames, LEO’s orbital elements, 1rEFRO, and the terms ‘flexibility’ and ‘economic efficiency’ are defined in Section 2. Then in Section 3, the two-body-based preliminary orbit design method is proposed and formulated. Section 4 aims to reveal LEO’s comprehensiveness as efficient parking orbits when adopting the 1rEFRO + EGA sequence, using the newly proposed “ LEO i-Ω diagram”. Section 5 describes a detailed orbit design constructed based on multi-body propagation and optimization to confirm the feasibility, flexibility, and economics of the solution and the usefulness of the initial solution given by the preliminary design method formulated in Section 3.

On 5 April 2019, the Hayabusa 2 spacecraft performed the first successful artificial impact experiment on an asteroid. The Small Carry-on Impactor (SCI) device was deployed at an altitude of 500 m above Ryugu’s surface. The 2 kg copper projectile hit Ryugu’s surface in 40 min and caused the formation of an artificial crater 14.5 m in diameter. Once the SCI was deployed, the Hayabusa 2 spacecraft performed a two-week escape trajectory reaching altitudes as far as 120 km from Ryugu. The spacecraft then returned to its nominal position at 20 km altitude (Home-Position) from Ryugu for hovering control. This was done to prevent ejecta particles from seriously damaging the spacecraft and compromising its functionality. In this article, we present a method to forecast the daily probability of spacecraft damage along the selected nominal escape trajectory due to the debris cloud formed by an artificial impact. The result of the damage analysis confirmed that the selected escape trajectory experienced a small number of particle collisions under the design threshold, which would not have resulted in damage. Indeed, no damage was reported on the Hayabusa 2 spacecraft and it kept operating normally after the SCI operation. The method here presented serves as a guideline for post-impact mission operations to forecast and estimate the probability of damage to spacecraft or CubeSats operating near a small celestial body after an artificial impact experiment has occurred.

Abstract A fundamental parameter-based quantification scheme for confocal XRF was applied to sub-micron synchrotron radiation X-ray fluorescence (SR-XRF) data obtained at the beamline P06 of the Deutsches Elektronen-Synchrotron (DESY, Hamburg, Germany) from two sections C0033-01 and C0033-04 that were wet cut from rock fragment C0033 collected from Cb-type asteroid (162173) Ryugu by JAXA’s Hayabusa2 mission. Trace-element quantifications show that C0033 bulk matrix is CI-like, whereas individual mineral grains (i.e., magnetite, pyrrhotite, dolomite, apatite and breunnerite) show, depending on the respective phase, minor to strong deviations. The non-destructive nature of SR-XRF coupled with a new PyMca (a Python toolkit for XRF data analysis)-based quantification approach, performed in parallel with the synchrotron experiments, proves to be an attractive tool for the initial analysis of samples from return missions, such as Hayabusa2 and OSIRIS-REx, the latter returning material from a B-type asteroid (101955) Bennu in 2023. Graphical Abstract