佐伯 孝尚

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

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 measure noble gas and nitrogen isotopes in Ryugu samples, finding they are dominated by pre-solar 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 heterogeneous abundances between the samples. Our measurements support a close relationship between Ryugu and CI chondrites. Noble gases produced by galactic cosmic rays, indicating ~5 Myr exposure, and from implanted solar wind, record the recent irradiation history of Ryugu after it migrated to its current orbit.

The Hayabusa2 spacecraft has returned samples from the Cb-type asteroid (162173) Ryugu to Earth. Previous petrological and chemical analyses support a close link between Ryugu and CI chondrites that are presumed to be chemically the most primitive meteorites with a solar-like composition. However, Ryugu samples are highly enriched in Ca compared to typical CI chondrites. To identify the cause of this discrepancy, here we report stable Ca isotopic data (expressed as δ44/40CaSRM915a) for returned Ryugu samples collected from two sites. We found that samples from both sites have similar δ44/40CaSRM915a (0.58 ± 0.03 % and 0.55 ± 0.08 %, 2 s.d.) that fall within the range defined by CIs. This isotopic similarity suggests that the Ca budget of CIs and Ryugu samples is dominated by carbonates, and the variably higher Ca contents in Ryugu samples are due to the abundant carbonates. Precipitation of carbonates on Ryugu likely coincided with a major episode of aqueous activity dated to have occurred ∼5 Myr after Solar System formation. Based on the pristine Ryugu samples, the average δ44/40CaSRM915a of the Solar System is defined to be 0.57 ± 0.04 % (2 s.d.).

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

Abstract Chondrule-like objects and Ca-Al-rich inclusions (CAIs) are discovered in the retuned samples from asteroid Ryugu. Three chondrule-like objects, which are ¹⁶O-rich and -poor with D¹⁷O (= d¹⁷O – 0.52 × d¹⁸O) values of ~ − 23‰ and ~ − 3‰, are dominated by Mg-rich olivine, resembling what proposed as earlier 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 spinel, hibonite, and perovskite are ¹⁶O-rich with D¹⁷O of ~ − 23‰ and possibly as old as the oldest CAIs. The chondrule-like objects and CAIs (< 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.

Abstract Volatile and organic-rich C-type asteroids may have been one of the main sources of Earth’s water. Our best insight into their chemistry is currently provided by carbonaceous chondritic meteorites, but the meteorite record is biased: only the strongest types survive atmospheric entry and are then modified by interaction with the terrestrial environment. Here we present the results of a detailed bulk and microanalytical study of pristine Ryugu particles, brought to Earth by the Hayabusa2 spacecraft. Ryugu particles display a close compositional match with the chemically unfractionated, but aqueously altered, CI (Ivuna-type) chondrites, which are widely used as a proxy for the bulk Solar System composition. The sample shows an intricate spatial relationship between aliphatic-rich organics and phyllosilicates and indicates maximum temperatures of ~30 °C during aqueous alteration. We find that heavy hydrogen and nitrogen abundances are consistent with an outer Solar System origin. Ryugu particles are the most uncontaminated and unfractionated extraterrestrial materials studied so far, and provide the best available match to the bulk Solar System composition.

Abstract Hayabusa2 spacecraft successfully collected rock samples through two touchdowns from the surface of C-type near-Earth asteroid 162173 Ryugu and brought them back to Earth in 2020. At the Extraterrestrial Sample Curation Center in JAXA, we performed initial description to all samples to obtain the 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 of 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 grains is heterogeneous: weak for A0042 (dark group) and C0041 (white group); moderate for C0094 (glossy); and severe for A0017 (bright). The white phase in a grain belonging to white group is identified as large carbonate minerals. This is the first effort to classify Ryugu returned samples. Based on these results, researchers can estimate sample characteristics only from the information on the JAXA curation public database. It could be an important reference for sample selection for further investigation.

Abstract We have conducted a NanoSIMS-based search for presolar material in samples recently returned from C-type asteroid Ryugu as part of JAXA's Hayabusa2 mission. We report the detection of all major presolar grain types with O- and C-anomalous isotopic compositions typically identified in carbonaceous chondrite meteorites: 1 silicate, 1 oxide, 1 O-anomalous supernova grain of ambiguous phase, 38 SiC, and 16 carbonaceous grains. At least two of the carbonaceous grains are presolar graphites, whereas several grains with moderate C isotopic anomalies are probably organics. The presolar silicate was located in a clast with a less altered lithology than the typical extensively aqueously altered Ryugu matrix. The matrix-normalized presolar grain abundances in Ryugu are ${4.8}_{-2.6}^{+4.7}$ ppm for O-anomalous grains, ${25}_{-5}^{+6}$ ppm for SiC grains, and ${11}_{-3}^{+5}$ ppm for carbonaceous grains. Ryugu is isotopically and petrologically similar to carbonaceous Ivuna-type (CI) chondrites. To compare the in situ presolar grain abundances of Ryugu with CI chondrites, we also mapped Ivuna and Orgueil samples and found a total of 15 SiC grains and 6 carbonaceous grains. No O-anomalous grains were detected. The matrix-normalized presolar grain abundances in the CI chondrites are similar to those in Ryugu: ${23}_{-6}^{+7}$ ppm SiC and ${9.0}_{-3.6}^{+5.4}$ ppm carbonaceous grains. Thus, our results provide further evidence in support of the Ryugu–CI connection. They also reveal intriguing hints of small-scale heterogeneities in the Ryugu samples, such as locally distinct degrees of alteration that allowed the preservation of delicate presolar material.

ABSTRACT The JAXA Hayabusa2 mission accomplished the formation of an artificial crater on the asteroid Ryugu. The aim of this work is to analyse the area surrounding the artificial crater and reveal spectral variability compared to the same region before the crater formation, to mineralogically and physically characterize the subsurface exposed material. The crater’s investigation focused on the analysis of two regions corresponding to the inner part of crater (the pit and the crater wall/floor), two areas related to ejecta deposited close to the crater, two areas of ejecta moved far from the crater, and two external areas. Each area was investigated both before and after the crater formation, by the study of the photometrically corrected spectral parameters: the 1.9 $\mu$m reflectance, the near-infrared spectral slope, and the depth of the bands at 2.7 and 2.8 $\mu$m. The subsurface material of the post-crater areas shows deeper absorption bands, a decrease in reflectance, and a reddening in spectral slope with respect to the surface material of pre-crater areas. The subsurface regolith could have experienced a lower OH devolatilization due to space weathering and/or could be composed of finer dark grains than the surface layer. The ejecta reached distances of $\sim $20 m from the impact point, mainly moving in the northern direction; nevertheless, a few ejecta also reached the south-eastern part of crater.

2020年12月6日に小惑星探査機「はやぶさ2」はC型小惑星リュウグウ表層物質を収めた再突入カ プセルを地球に帰還させた．回収された再突入カプセルに収められた試料コンテナは，オーストラリア現地でのガス採取を実施した後，JAXA相模原キャンパスの惑星物質試料受入設備に搬入され，チェンバー導入前の部品取り外し・洗浄等のプロセスを経てクリーンチェンバー内で真空中での開封・高純度窒素環境下での帰還試料の取り出し･初期記載が行われた．これらのリュウグウ帰還試料の初期記載の結果，これまでに回収されたどの隕石よりも反射率が低く，全体密度が小さい事が判明した．また，赤外反射スペクトルの吸収特性から水酸基を含む含水鉱物と炭酸塩鉱物，及びCH結合に富む有機物が試料中に含まれることが明らかになった．これらの情報を既知の隕石と比較すると，CIコンドライト隕石に最も似ていると言える．また探査機搭載機器によって得られた可視･近赤外スペクトルと比較した結果，帰還試料はリュウグウ表層全体を代表している事が分かった．取り出された試料の一部は既に初期分析チーム，2次キュレーションチーム，NASAへ配分され，更に国際公募研究による配布が予定されている．本稿では一連の試料取り扱いプロセス・初期記載内容について述べる．

Carbonaceous meteorites are thought to be fragments of C-type (carbonaceous) asteroids. Samples of the C-type asteroid (162173) Ryugu were retrieved by the Hayabusa2 spacecraft. We measure the mineralogy, bulk chemical and isotopic compositions of Ryugu samples. They are mainly composed of materials similar to carbonaceous chondrite meteorites, particularly the CI (Ivuna-type) group. The samples consist predominantly of minerals formed in aqueous fluid on a parent planetesimal. The primary minerals were altered by fluids at a temperature of 37 ± 10°C, (Stat.) (Syst.) million years after formation of the first solids in the Solar System. After aqueous alteration, the Ryugu samples were likely never heated above ~100°C. The samples have a chemical composition that more closely resembles the Sun’s photosphere than other natural samples do.

The Hayabusa2 spacecraft investigated the C-type (carbonaceous) asteroid (162173) Ryugu. The mission performed two landing operations to collect samples of surface and subsurface material, the latter exposed by an artificial impact. We present images of the second touchdown site, finding that ejecta from the impact crater was present at the sample location. Surface pebbles at both landing sites show morphological variations ranging from rugged to smooth, similar to Ryugu’s boulders, and shapes from quasi-spherical to flattened. The samples were returned to Earth on 6 December 2020. We describe the morphology of >5 grams of returned pebbles and sand. Their diverse color, shape, and structure are consistent with the observed materials of Ryugu; we conclude that they are a representative sample of the asteroid.

The Hayabusa2 extended mission is designed to rendezvous with the fast-rotating asteroid 1998 KY26 in 2031. The asteroid's diameter and rotation period are as small as 20-40 m and 10:7 min, respectively. Consequently, the low gravity and rapid rotation of the asteroid form distinctive dynamical environments. This paper rst characterizes orbital and surface environments about 1998 KY26. Then, possible orbital operations are investigated, particularly focusing on the hovering and descent operations. Our research, therefore, contributes to exploring one of the smallest members of the solar system.

Context. JAXA's Hayabusa2 mission rendezvoused the Ryugu asteroid for 1.5 years to clarify the carbonaceous asteroids' record for Solar System origin and evolution. Aims. We studied the photometric behavior of the spectral parameters characterizing the near-infrared (NIR) spectra of Ryugu provided by the Hayabusa2/NIRS3 instrument, that is to say 1.9 µm reflectance, 2.7 and 2.8 µm band depths (ascribed to phyllosilicates), and NIR slope. Methods. For each parameter, we applied the following empirical approach: (1) retrieval of the equigonal albedo by applying the Akimov disk function (this step was only performed for the reflectance photometric correction); (2) retrieval of the median spectral parameter value at each phase angle bin; and (3) retrieval of the phase function by a linear fit. Results. Ryugu's phase function shows a steepness similar to Ceres, according to the same taxonomy of the two asteroids. Band depths decrease with increasing phase angle: this trend is opposite to that observed on other asteroids explored by space missions and is ascribed to the very dark albedo. NIR and visible phase reddening are similar, contrary to other asteroids, where visible phase reddening is larger: this could be due to surface darkness or to particle smoothness. Albedo and band depths are globally uncorrelated, but locally anticorrelated. A correlation between darkening and reddening is observed.

MASCOT (‘Mobile Asteroid Surface Scout’) is a 10 kg mobile surface science package part of JAXA’s Hayabusa2 sample return mission. The mission was launched in December 2014 from Tanegashima Space Center, Japan. The Hayabusa2 spacecraft reached the target asteroid in summer 2018. After a mapping phase of the asteroid and a landing site selection process the MASCOT lander was deployed to the surface on the 3rd of October 2018. MASCOT operated successfully for about 17 h on the surface of Ryugu. It performed three relocation manoeuvres and one “Mini-Move” and returned 128 MBytes of data. MASCOT has been developed by the German Aerospace Center (DLR) in cooperation with the Centre National d’Etudes Spatiales (CNES). The main objectives were to perform in-situ investigations of the asteroid surface and to support the sampling site selection for the mother spacecraft. These objectives could be reached successfully. On 6th December 2020 Hayabusa2 successfully returned asteroid samples to the Earth.

<title>Abstract</title>C-type asteroids¹ are considered to be primitive small Solar System bodies enriched in water and organics, providing clues to the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing^2–7 and on-asteroid measurements^8,9 with Hayabusa2 (ref. ¹⁰). However, the ground truth provided by laboratory analysis of returned samples is invaluable to determine the fine properties of asteroids and other planetary bodies. We report preliminary results of analyses on returned samples from Ryugu of the particle size distribution, density and porosity, spectral properties and textural properties, and the results of a search for Ca–Al-rich inclusions (CAIs) and chondrules. The bulk sample mainly consists of rugged and smooth particles of millimetre to submillimetre size, confirming that the physical and chemical properties were not altered during the return from the asteroid. The power index of its size distribution is shallower than that of the surface boulder observed on Ryugu¹¹, indicating differences in the returned Ryugu samples. The average of the estimated bulk densities of Ryugu sample particles is 1,282 ± 231 kg m⁻³, which is lower than that of meteorites¹², suggesting a high microporosity down to the millimetre scale, extending centimetre-scale estimates from thermal measurements^5,9. The extremely dark optical to near-infrared reflectance and spectral profile with weak absorptions at 2.7 and 3.4 μm imply a carbonaceous composition with indigenous aqueous alteration, matching the global average of Ryugu^3,4 and confirming that the sample is representative of the asteroid. Together with the absence of submillimetre CAIs and chondrules, these features indicate that Ryugu is most similar to CI chondrites but has lower albedo, higher porosity and more fragile characteristics.

<title>Abstract</title>Ryugu is a carbonaceous rubble-pile asteroid visited by the Hayabusa2 spacecraft. Small rubble pile asteroids record the thermal evolution of their much larger parent bodies. However, recent space weathering and/or solar heating create ambiguities between the uppermost layer observable by remote-sensing and the pristine material from the parent body. Hayabusa2 remote-sensing observations find that on the asteroid (162173) Ryugu both north and south pole regions preserve the material least processed by space weathering, which is spectrally blue carbonaceous chondritic material with a 0–3% deep 0.7-µm band absorption, indicative of Fe-bearing phyllosilicates. Here we report that spectrally blue Ryugu’s parent body experienced intensive aqueous alteration and subsequent thermal metamorphism at 570–670 K (300–400 °C), suggesting that Ryugu’s parent body was heated by radioactive decay of short-lived radionuclides possibly because of its early formation 2–2.5 Ma. The samples being brought to Earth by Hayabusa2 will give us our first insights into this epoch in solar system history.

Many small boulders with reflectance values higher than 1.5 times the average reflectance have been found on the near-Earth asteroid 162,173 Ryugu. Based on their visible wavelength spectral differences, Tatsumi et al. (2021, Nature Astronomy, 5, doi:10.1038/s41550-020-1179-z) defined two bright boulder classes: C-type and S-type. These two classifications of bright boulders have different size distributions and spectral trends. In this study, we measured the spectra of 79 bright boulders and investigated their detailed spectral properties. Analyses obtained a number of important results. First, S-type bright boulders on Ryugu have spectra that are similar to those found for two different ordinary chondrites with different initial spectra that have been experimentally space weathered the same way. This suggests that there may be two populations of S-type bright boulders on Ryugu, perhaps originating from two different impactors that hit Ryugu's parent body. Second, the model space-weathering ages of meter-size S-type bright boulders, based on spectral change rates derived in previous experimentally irradiated ordinary chondrites, are 105–106 years, which is consistent with the crater retention age (<106 years) of the ~1-m deep surface layer on Ryugu. This agreement strongly suggests that Ryugu's surface is extremely young, implying that the samples acquired from Ryugu's surface should be fresh. Third, the lack of a serpentine absorption in the S-type clast embedded in one of the large brecciated boulders indicates that fragmentation and cementation that created the breccias occurred after the termination of aqueous alteration. Fourth, C-type bright boulders exhibit a continuous spectral trend similar to the heating track of low-albedo carbonaceous chondrites, such as CM and CI. Other processes, such as space weathering and grain size effects, cannot primarily account for their spectral variation. Furthermore, the distribution of the spectra of general dark boulders, which constitute >99.9% of Ryugu's volume, is located along the trend line in slope/UV-index diagram that is occupied by C-type bright boulders. These results indicate that thermal metamorphism might be the dominant cause for the spectral variety among the C-type bright boulders on Ryugu and that general boulders on Ryugu may have experienced thermal metamorphism under a much narrower range of conditions than the C-type bright boulders. This supports the hypothesis that Ryugu's parent body experienced uniform heating due to radiogenic energy rather than impact heating.

Deployable payloads have been used to expand mission opportunities and enable autonomous navigation and guidance under uncertain terrain conditions. A spacecraft may deploy an artificial marker toward the surface of the target body. An estimation of the deployment trajectory of markers can provide important information for guidance and navigation. We propose a method estimating the deployment trajectory using only images and evaluate its accuracy and reliability. In addition, to determine the spacecraft's relative attitude with respect to the asteroid surface, we introduce an attitude estimation method using the spacecraft's own shadow. The results shown in this paper confirm that the estimated trajectory is sufficiently accurate and thus the proposed method is a viable option for future missions for determining the trajectory of deployable payloads using only images of a deployment.

One of the major challenges in the Hayabusa2 sample-return mission was the second touchdown on the asteroid Ryugu, which was designed to collect subsurface materials near the artificial crater formed by a small carry-on impactor. Due to engineering and scientific requirements, a narrow area with a radius as small as 3.5 m was selected as the target landing site. To achieve pinpoint touchdown at the selected site, an artificial landmark called a target marker (TM) was used for optical navigation. The key to a successful touchdown was precise deployment of the TM in the microgravity environment of the asteroid. This study therefore investigates a viable trajectory for TM deployment, incorporating the uncertainty in the impact and rebound motions of the TM. Following the TM deployment operation, a detailed survey of the landing site around the TM settlement point is performed to assess the terrain conditions. To guarantee the observation quality and spacecraft safety, multi-impulse low-altitude trajectories are introduced in this paper, along with covariance analyses based on the high-fidelity polyhedral gravity model of Ryugu. Subsequently, a pinpoint touchdown trajectory that satisfies various engineering requirements, such as landing accuracy and velocity, is designed, taking advantage of optical TM tracking. The feasibility of the touchdown sequence is further validated by a Monte Carlo dispersion analysis. Consequently, Hayabusa2 successfully touched down within the target site on 11 July 2019. The current research also conducts a post-operation trajectory reconstruction based on the flight data to demonstrate the actual guidance performance in the TM deployment, landing site observations, and pinpoint touchdown. (C) 2021 COSPAR. Published by Elsevier B.V.

<title>Abstract</title> C-type asteroids are considered to be primitive small Solar-System bodies enriched in water and organics, providing clues for understanding the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing and on-asteroid measurements with Hayabusa2, but further studies are expected by direct analyses of returned samples. Here we describe the bulk sample mainly consisting of rugged and smooth particles of millimeter to submillimeter size, preserving physical and chemical properties as they were on the asteroid. The particle size distribution is found steeper than that of surface boulders11. Estimated grain densities of the samples have a peak around 1350 kg m-3, which is lower than that of meteorites suggests a high micro-porosity down to millimeter-scale, as estimated at centimeter-scale by thermal measurements. The extremely dark optical to near-infrared reflectance and the spectral profile with weak absorptions at 2.7 and 3.4 microns implying carbonaceous composition with indigenous aqueous alteration, respectively, match the global average of Ryugu, confirming the sample’s representativeness. Together with the absence of chondrule and Ca-Al-rich inclusion of larger than sub-mm, these features indicate Ryugu is most similar to CI chondrites but with darker, more porous and fragile characteristics.

The asteroid explorer Hayabusa2 was launched by Japan Aerospace Exploration Agency (JAXA) on December 3rd, 2014. The primary mission of the spacecraft is to sample pieces of the asteroid and return it to Earth for more advanced scientific analysis on the Earth. After three-year cruise phase, Hayabusa2 finally arrived at the asteroid Ryugu on June 28, 2018, and mission operations started. Hayabusa2 carries multiple rovers, separates them to land on the asteroid surface. One of these rovers, called MASCOT, was developed under the international cooperation between Deutsches Zentrum für Luft-und Raumfahrt (DLR) and Center National d'études Spatiales (CNES). This rover was planned to be separated to land on the surface of the asteroid and planned to perform several missions on the asteroid surface. In order to support these missions, the mother ship Hayabusa2 was requested to separate this rover at a very low altitude about 50 ​m, and to hover about 3 ​km after separation to achieve a reliable communication link with MASCOT. On October 2–5, 2018, we performed the operation for MASCOT release. In this paper, we introduce the flight results of the entire operation for the MASCOT release.