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Abstract 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.

Abstract 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.

Abstract Primordial carbon delivered to the early earth by asteroids and meteorites provided a diverse source of extraterrestrial organics from pre-existing simple organic compounds, complex solar-irradiated macromolecules, and macromolecules from extended hydrothermal processing. Surface regolith collected by the Hayabusa2 spacecraft from the carbon-rich asteroid 162173 Ryugu present a unique opportunity to untangle the sources and processing history of carbonaceous matter. Here we show carbonaceous grains in Ryugu can be classified into three main populations defined by spectral shape: Highly aromatic (HA), Alkyl-Aromatic (AA), and IOM-like (IL). These carbon populations may be related to primordial chemistry, since C and N isotopic compositions vary between the three groups. Diffuse carbon is occasionally dominated by molecular carbonate preferentially associated with coarse-grained phyllosilicate minerals. Compared to related carbonaceous meteorites, the greater diversity of organic functional chemistry in Ryugu indicate the pristine condition of these asteroid samples.

Abstract 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.

Abstract 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.

The chirality of amino acids in extraterrestrial materials may provide an insight into the origin of the essential l-enantiopure amino acids in the terrestrial biosphere. In 2020, the Hayabusa2 mission succeeded in bringing back surface materials from the C-type asteroid (162173) Ryugu to the Earth. Amino acids were one of the targeted organic molecules to be studied in the Ryugu samples. To analyze the various structural isomers of amino acids, which were expected to be present, from the limited amount of the returned samples, the development of a highly-sensitive and selective analytical method was necessary. In the present study, a three-dimensional high-performance liquid chromatography (3D-HPLC) system has been developed for the enantioselective determination of five proteinogenic and three non-proteinogenic amino acids in the Ryugu samples, in which amino acids in the sample were separated by reversed-phase, anion-exchange and enantioselective columns after the fluorescence derivatization with 4-fluoro-7-nitro-2,1,3-benzoxadiozole. The applicability of the analytical system to the extraterrestrial samples was evaluated by analyzing several types of carbonaceous meteorites before applying the system to the Ryugu samples. In the analysis of the Ryugu samples, all of the target amino acids were successfully determined quantitatively. Non-proteinogenic amino acids including 2-amino-n-butyric acid, isovaline and norvaline, rarely present in the terrestrial environment, were found as almost racemic mixtures with 47.1 to 55.2%l.

Abstract The bulk density of grains from a celestial body is a fundamental property related to its composition and structure, contributing to the understanding of its evolutionary history. In this study, we provide the bulk density of 637 grains returned from the C-type near-Earth asteroid 162173 Ryugu. This is the largest number of grains to date for the curation activity, corresponding to 38 wt.% of the total returned samples (approximately 5.4 g). Although several densities of the Ryugu grains were reported, the volume estimation of some samples showed uncertainties. Therefore, we applied a new volume estimation model calibrated by X-ray micro-computed tomography (XCT) to the Ryugu grains to more accurately estimate their bulk density. The obtained average bulk density of 637 Ryugu grains was 1.79 ± 0.31 g/cm3 (1σ variation) for weights of 0.5‒100 mg (sub-mm ‒to 10 mm) irrespective of their 3D shapes characterized by three axial length ratios, considered to be a representative of the returned samples. The bulk density distributions of the grains in Chambers A and C were statistically distinguishable, with mean values of 1.81 ± 0.30 and 1.76 ± 0.33 g/cm3 (1σ variations), respectively. Despite the small difference, bulk density may have differed by sampling site. The obtained average bulk density value of A + C samples was almost the same as that of 16 Ryugu grains estimated based on CT scanned data, and was consistent with the densities of CI chondrites (1.57–1.91 g/cm3). The axial ratios of the grains in Chambers A and C were similar and those of the 724 returned samples and the flying particles ejected during the sampling operations were also similar, suggesting that relatively small Ryugu materials (mm‒cm in size) are similar in shape. The minor difference between the Rygu grains and flying particles could be attributed to events such as scraping during sampling operations and transportation. Graphical Abstract

Abstract Developing a cleanroom and clean chambers (CCs) for Hayabusa2 returned samples has been discussed with the committee for Hayabusa2 sample curation facility since 2015. One major difference from the specifications of the CCs used for Itokawa samples is that a part of samples was decided to be handled and preserved in vacuum to avoid terrestrial nitrogen contamination with organics or unknown materials that might easily react with the samples. Thus, the CCs for Hayabusa2 samples were divided into two CCs for vacuum processes and three CCs for purified nitrogen conditions. The cleanroom was built in summer 2017, while the CCs were installed in the summer of 2018. After the installation of the CCs, instruments for initial descriptions, sample containers, handling tools for powder and particle samples, and jigs to assist handling samples were developed in parallel with functional checks and repeated rehearsals between the fall of 2018 and the fall of 2020. The curatorial works on Hayabusa2-retuned samples were conducted as previously planned. Simultaneously, contaminations and influences of inorganics, organics, microbial, and magnetic constructs have been assessed to evaluate their potential effects on the analysis of the returned samples. Additionally, the tools used to touch samples directly have been demagnetized to avoid sample magnetization during their handling and the tool magnetization was measured before and after their usages. The series of developments and experiences from the curatorial works of Hayabusa2-returned samples represent valuable implications for future sample return missions. Graphical Abstract

Abstract The sample from the near-Earth carbonaceous asteroid (162173) Ryugu is analyzed in the context of carbonaceous meteorites soluble organic matter. The analysis of soluble molecules of samples collected by the Hayabusa2 spacecraft shines light on an extremely high molecular diversity on the C-type asteroid. Sequential solvent extracts of increasing polarity of Ryugu samples are analyzed using mass spectrometry with complementary ionization methods and structural information confirmed by nuclear magnetic resonance spectroscopy. Here we show a continuum in the molecular size and polarity, and no organomagnesium molecules are detected, reflecting a low temperature and water-rich environment on the parent body approving earlier mineralogical and chemical data. High abundance of sulfidic and nitrogen rich compounds as well as high abundance of ammonium ions confirm the water processing. Polycyclic aromatic hydrocarbons are also detected in a structural continuum of carbon saturations and oxidations, implying multiple origins of the observed organic complexity, thus involving generic processes such as earlier carbonization and serpentinization with successive low temperature aqueous alteration.

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.

Abstract 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 SO4 2− > Cl− > S2O3 2− > NO3 − > NH4 +. The salt fraction extracts contain anionic soluble sulfur-bearing species such as S n -polythionic acids (n

Abstract The JAXA Astromaterials Science Research Group developed a web-based database system for the Hayabusa2-returned samples from C-type asteroid Ryugu. The Ryugu Sample Database System database (RS-DBS) is designed as an online catalog for users of wide scientific communities to choose their preferred samples and propose the sample loan through the JAXA Ryugu Sample Announcement of Opportunity. Ryugu samples can be sorted and given identification numbers as individual particles larger than 1 mm and aggregate samples consisting of less than 1 mm particle through the Phase1 curation (i.e., the initial description). The RS-DBS lists all samples with analytical data such as a microscopy image, size, mass, spectroscopic data, and shape model obtained by the initial description at the JAXA curation facility. The list also includes research results conducted by previous projects (i.e., the Hayabusa2 initial analysis team and Phase2 curation teams). The RS-DBS, built with open-source technologies, archives the data securely and long-term on the Data Archives and Transmission System (DARTS) at ISAS/JAXA. Graphical Abstract

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 Hayabusa2 mission provided a unique data set of asteroid Ryugu that covers a wide range of spatial scale from the orbiter remote sensing instruments to the returned samples. The MASCOT lander that was delivered onto the surface of Ryugu aimed to provide context for these data sets by producing in situ data collected by a camera (MasCam), a radiometer (MARA), a magnetometer (MasMag) and a spectrometer (MicrOmega). In this work, we evaluate the success of MASCOT as an integrated lander to bridge the gap between orbiter and returned sample analysis. We find that MASCOT’s measurements and derivatives thereof, including the rock morphology, colour in the visible wavelengths, possible meteorite analogue, density, and porosity of the rock at the landing site are in good agreement with those of the orbiter and the returned samples. However, it also provides information on the spatial scale (sub-millimetres to centimetres) at which some physical properties such as the thermal inertia and reflectance undergo scale-dependent changes. Some of the in situ observations such as the presence of clast/inclusions in rocks and the absence of fine particles at the landing site was uniquely identified by MASCOT. Thus, we conclude that the delivery of an in situ instrument like MASCOT provides a valuable data set that complements and provides context for remote sensing and returned sample analyses. Graphical Abstract

Abstract Hayabusa2 spacecraft successfully collected rock samples from the surface of C-type near-Earth asteroid 162173 Ryugu through two touchdowns and brought them back to Earth in 2020. At the Extraterrestrial Sample Curation Center in JAXA, we performed initial description of all samples to obtain fundamental information and prepare the database for sample allocation. We propose morphological classifications for the returned samples based on the initial description of 205 grains described in the first 6 months. The returned samples can be distinguished by four morphological characteristics: dark, glossy, bright, and white. According to coordinated study to provide an initial description and detailed investigation by scanning electron microscopy and X-ray diffraction analysis in this study, these features reflect the differences in the degree of space weathering and mineral assemblages. The degree of space weathering of the four studied grain types is heterogeneous: weak for A0042 (dark group) and C0041 (white group); moderate for C0094 (glossy); and severe for A0017 (bright). The white phase, which is the mineral characteristic of the white group grains, is identified as large carbonate minerals. This is the first effort to classify Ryugu returned samples into distinct categories. Based on these results, researchers can estimate sample characteristics only from the information on the JAXA curation public database. It will be an important reference for sample selection for further investigation. Graphical Abstract

Abstract Surface and subsurface materials of C-type near-Earth asteroid 162173 Ryugu were collected and successfully returned to the Earth in the Hayabusa2 mission. Fourier Transform Infrared Spectroscopy (FTIR) has been conducted to characterize these returned samples as one of the initial descriptions in a non-destructive manner under a purified nitrogen condition without terrestrial contamination. We selected the individual grains and aggregate samples that were not severely influenced by the reflection of incident beam at the sapphire dish and analyzed their reflectance spectra using the primary component analysis (PCA). The result indicates that Ryugu returned samples are highly homogeneous with only a little heterogeneity. The average spectrum of the main PCA group is represented by four absorption bands at 2.7, 3.05, 3.4, and 3.95 μm. The spectral feature is consistent with that obtained from bulk FTIR measurements, indicating potential presence of hydroxyl, organics, and carbonates. Rarely observed types of grains with unique spectra are categorized into three groups: significantly high reflectance, carbonates, and hydroxyl compounds with broad OH absorption. Graphical Abstract

Uracil was identified in the sample returned from the asteroid Ryugu. Having been provided to the early Earth as a component in such asteroidal materials, these molecules might have played a role for prebiotic chemical evolution on the early Earth

Amino acid concentrations from 2 particles returned from different touchdown sites on the surface of Ryugu are reported. Differences in chemistry suggest different levels of aqueous alteration are recorded at the 2 sampled locations.

Abstract Evaluating the molecular distribution of organic compounds in pristine extraterrestrial materials is cornerstone to understanding the abiotic synthesis of organics and allows us to better understand the molecular diversity available during the formation of our solar system and before the origins of life on Earth. In this work, we identify multiple organic compounds in solvent extracts of asteroid Ryugu samples A0106 and C0107 and the Orgueil meteorite using two-dimensional gas chromatography and time-of-flight high resolution mass spectrometry (GC×GC–HRMS). Our analyses found similarities between the molecular distribution of organic compounds in Ryugu and the CI carbonaceous chondrite Orgueil. Specifically, several PAHs and organosulfides were found in Ryugu and Orgueil suggesting an interstellar and parent body origin for these compounds. We also evaluated the common relationship between Ryugu, Orgueil, and comets, such as Wild-2; however, until comprehensive compound-specific isotopic analyses for these organic species are undertaken, and until the effects of parent body processes and Earth’s weathering processes on meteoritic organics are better understood, their parent–daughter relationships will remain unanswered. Finally, the study of organic compounds in Ryugu samples and the curation practices for the future preservation of these unvaluable materials are also of special interest for future sample return missions, including NASA’s OSIRIS-REx asteroid sample return mission. Graphical Abstract

Chondrule-like objects and CAIs in the Ryugu samples are early generations of chondrules and possibly oldest solids in the Solar System. They were transported from the inner solar nebula to the formation location of the Ryugu original parent body.

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

Abstract 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

The analysis of samples returned from the C-type asteroid Ryugu has drastically advanced our knowledge of the evolution of early solar system materials. However, no consensus has been obtained on the chronological data, which is important for understanding the evolution of the asteroid Ryugu. Here, the aqueous alteration age of Ryugu particles was determined by the Mn–Cr method using bulk samples, yielding an age of 4.13 + 0.62/−0.55 Myr after the formation of Ca–Al-rich inclusions (CAI). The age corresponds to 4563.17 + 0.60/−0.67 Myr ago. The higher ^55 Mn/ ^52 Cr, ε ^54 Cr, and initial ε ^53 Cr values of the Ryugu samples relative to any carbonaceous chondrite samples implies that its progenitor body formed from the least thermally processed precursors in the outermost region of the protoplanetary disk. Despite accreting at different distances from the Sun, the hydrous asteroids (Ryugu and the parent bodies of CI, CM, CR, and ungrouped C2 meteorites) underwent aqueous alteration during a period of limited duration (3.8 ± 1.8 Myr after CAI). These ages are identical to the crystallization age of the carbonaceous achondirtes NWA 6704/6693 within the error. The ε ^54 Cr and initial ε ^53 Cr values of Ryugu and NWA 6704/6693 are also identical, while they show distinct Δ' ^17 O values. This suggests that the precursors that formed the progenitor bodies of Ryugu and NWA 6703/6693 were formed in close proximity and experienced a similar degree of thermal processing in the protosolar nebula. However, the progenitor body of Ryugu was formed by a higher ice/dust ratio, than NWA6703/6693, in the outer region of the protoplanetary disk.

The nucleosynthetic isotope composition of planetary materials provides a record of the heterogeneous distribution of stardust within the early solar system. In 2020 December, the Japan Aerospace Exploration Agency Hayabusa2 spacecraft returned to Earth the first samples of a primitive asteroid, namely, the Cb-type asteroid Ryugu. This provides a unique opportunity to explore the kinship between primitive asteroids and carbonaceous chondrites. We report high-precision μ ^26 Mg* and μ ^25 Mg values of Ryugu samples together with those of CI, CM, CV, and ungrouped carbonaceous chondrites. The stable Mg isotope composition of Ryugu aliquots defines μ ^25 Mg values ranging from –160 ± 20 ppm to –272 ± 30 ppm, which extends to lighter compositions relative to Ivuna-type (CI) and other carbonaceous chondrite groups. We interpret the μ ^25 Mg variability as reflecting heterogeneous sampling of a carbonate phase hosting isotopically light Mg ( μ ^25 Mg ∼ –1400 ppm) formed by low temperature equilibrium processes. After correcting for this effect, Ryugu samples return homogeneous μ ^26 Mg* values corresponding to a weighted mean of 7.1 ± 0.8 ppm. Thus, Ryugu defines a μ ^26 Mg* excess relative to the CI and CR chondrite reservoirs corresponding to 3.8 ± 1.1 and 11.9 ± 0.8 ppm, respectively. These variations cannot be accounted for by in situ decay of ^26 Al given their respective ^27 Al/ ^24 Mg ratios. Instead, it requires that Ryugu and the CI and CR parent bodies formed from material with a different initial ^26 Al/ ^27 Al ratio or that they are sourced from material with distinct Mg isotope compositions. Thus, our new Mg isotope data challenge the notion that Ryugu and CI chondrites share a common nucleosynthetic heritage.

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 δ D _Ryugu = +59 ± 121‰ (2 σ ). 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 H _2 before being accreted on the Ryugu’s parent asteroid.

Both poles of asteroid Ryugu, the target of space mission Hayabusa2, preserve the least processed material by space weathering. Here, the authors show detection of 700 nm absorption band in the polar spectra of Ryugu, that allows to constrain the hydrothermal history of its spectrally blue parent body.

Abstract The thermal infrared imager (TIR) onboard the Hayabusa2 spacecraft performed thermographic observations of the asteroid 162173 Ryugu (1999 JU $$_3$$ 3 ) from June 2018 to November 2019. Our previous reports revealed that the surface of Ryugu was globally filled with porous materials and had high surface roughness. These results were derived from making the observed temperature maps of TIR using a projection method onto the shape model of Ryugu as geometric corrections. The pointing directions of TIR were calculated using an interpolation of data from the SPICE kernels (NASA/NAIF) during the periods when the optical navigation camera (ONC) and the light detection and ranging (LIDAR) observations were performed. However, the mapping accuracy of the observed TIR images was degraded when the ONC and LIDAR were not performed with TIR. Also, the orbital and attitudinal fluctuations of Hayabusa2 increased the error of the temperature maps. In this paper, to solve the temperature image mapping problems, we improved the correction method by fitting all of the observed TIR images with the surface coordinate addressed on the high-definition shape model of Ryugu (SFM 800k v20180804). This correction adjusted the pointing direction of TIR by rotating the TIR frame relative to the Hayabusa2 frame using a least squares fit. As a result, the temperature maps spatially spreading areas were converged within high-resolved $$0.5^\circ$$ 0 . 5 ∘ by $$0.5^\circ$$ 0 . 5 ∘ maps. The estimated thermal inertia, for instance, was approximately 300 $$\sim$$ ∼ 350 Jm $$^{-2}$$ - 2 s $$^{-0.5}$$ - 0.5 K $$^{-1}$$ - 1 at the hot area of the Ejima Saxum. This estimation was succeeded in case that the surface topographic features were larger than the pixel scale of TIR. However, the thermal inertia estimation of smooth terrains, such as the Urashima crater, was difficult because of surface roughness effects, where roughness was probably much smaller than the pixel scale of TIR.

We present oxygen isotopic analyses of fragments of the near-Earth C _b -type asteroid Ryugu returned by the Hayabusa2 spacecraft that reinforce the close correspondence between Ryugu and CI chondrites. Small differences between Ryugu samples and CI chondrites in ${{\rm{\Delta }}}^{{\prime} 17}{\rm{O}}$ can be explained at least in part by contamination of the latter by terrestrial water. The discovery that a randomly sampled C-complex asteroid is composed of CI-chondrite-like rock, combined with thermal models for formation prior to significant decay of the short-lived radioisotope ^26 Al, suggests that if lithified at the time of alteration, the parent body was small (≪50 km radius). If the parent planetesimal was large (>50 km in radius), it was likely composed of high-permeability, poorly lithified sediment rather than consolidated rock.

Abstract We have investigated the influence of thin thermally opaque dust layers on the thermal emission of rocks and regolith and determined the thermal response of these dust-covered surfaces to diurnal insolation cycles. Results are computed for Hayabusa2’s target asteroid (162173) Ryugu, which was observed by thermal infrared instruments on the orbiter and in situ. We show that even a very thin (10..100 μm) fine-grained porous dust layer with thermal inertia of 25 J m−2 K−1 s−1/2 can have a significant influence on surface temperatures and alter the apparent thermal inertia of the underlying material derived under the simplified assumption of a homogenous half space by more than 20%. The masking of the underlying material is complete at about 1 diurnal skin depth, corresponding to ~ 10 mm on Ryugu. Between 0.1 and 1 diurnal skin depths, we find a thermal lag smaller than what would be predicted for a surface consisting of dust only. If a dust cover were present on Ryugu, this should be clearly visible in the data returned by the orbiter’s thermal infrared imager (TIR) and the MASCOT lander’s radiometer (MARA), which observed a single boulder at the landing site. However, this appears not to be the case, and dust seems to play a minor role in the thermal emission from the asteroid.

Abstract Asteroid (162173) Ryugu is the target object of Hayabusa2, an asteroid exploration and sample return mission led by Japan Aerospace Exploration Agency (JAXA). Ground-based observations indicate that Ryugu is a C-type near-Earth asteroid with a diameter of less than 1 km, but the knowledge of its detailed properties is very limited prior to Hayabusa2 observation. This paper summarizes our best understanding of the physical and dynamical properties of Ryugu based on ground-based remote sensing and theoretical modeling before the Hayabusa2’s arrival at the asteroid. This information is used to construct a design reference model of the asteroid that is used for the formulation of mission operation plans in advance of asteroid arrival. Particular attention is given to the surface properties of Ryugu that are relevant to sample acquisition. This reference model helps readers to appropriately interpret the data that will be directly obtained by Hayabusa2 and promotes scientific studies not only for Ryugu itself and other small bodies but also for the solar system evolution that small bodies shed light on.

The Near-Earth Asteroid 162173 Ryugu is a C-type asteroid which preserves information about the ancient Solar System and is considered enriched in volatiles such as water and organics associated with the building blocks of life, and it is a potentially hazardous object that might impact Earth. Hayabusa2 is the asteroid explorer organized by the Japan Aerospace Exploration Agency to rendezvous with the asteroid and collect surface materials to return them to Earth. Thermography has been carried out from Hayabusa2 during the asteroid proximity phase, to unveil the thermophysical properties of the primitive Solar System small body, which offered a new insight for understanding the origin and evolution of the Solar System, and demonstrated the technology for future applications in space missions. Global, local, and close-up thermal images taken from various distances from the asteroid strongly contributed to the mission success, including suitable landing site selection, safe assessment during descents into the thermal environments and hazardous boulder abundance, and the detection of deployable devices against the sunlit asteroid surface. Potential applications of thermography in future planetary missions are introduced.