Preliminary analysis of the Hayabusa2 samples returned from asteroid Ryugu

Preliminary analysis of the Hayabusa2 samples returned from asteroid Ryugu


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 sensing2,3,4,5,6,7 and on-asteroid measurements8,9 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 measurements5,9. 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 Ryugu3,4 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.


On 6 December 2020, samples from the C-type asteroid 162173 Ryugu were returned to Earth by Hayabusa2 in a hermetically sealed container within the re-entry capsule (Tachibana, S. et al., manuscript in preparation), and transported from South Australia to the Extraterrestrial Sample Curation Center (ESCuC) in Sagamihara, Japan. Samples were recovered in a non-destructive manner and under a strict contamination-controlled conditions to perform initial descriptions before delivery for in-depth investigations by the nominated analytical teams and for future research worldwide, as detailed in the Methods and Extended Data Fig. 1. The asteroid Ryugu is the fourth extraterrestrial body of which samples have been returned to the Earth by spacecraft, following past sample return missions by Apollo13, Luna14 and Chang’e-515 from the Moon, Stardust from comet 81 P/Wild216 and Hayabusa from near-Earth S-type asteroid Itokawa17,18. The Ryugu sample has sizes ranging from ~8 mm, the largest average diameter, down to fine submillimetre dusts, with millimetre-scale particles being the most common (Extended Data Fig. 2).

A total of 5.424 ± 0.217 g was collected from Ryugu (Extended Data Fig. 2), and has been kept as physically and chemically pristine as possible, handled only in a vacuum or in purified nitrogen without exposure to Earth’s atmosphere. From Chamber A, 3.237 ± 0.002 g of samples was recovered, collected during the first touch-down sampling (TD1) at the equatorial ridge region of Ryugu10. We assume that these samples represent the surface materials of Ryugu at the uppermost centimetre-scale layer, and that this layer is influenced by insolation, radiation, temperature cycling and micrometeoritic impacts. From Chamber C, 2.025 ± 0.003 g of samples was recovered, collected during the second touch-down sampling (TD2) at a site10 proximal to the artificial crater excavated by the Small Carry-on Impactor (SCI)6,19. We assume that some of the samples in Chamber C represent subsurface materials excavated by the impact experiments, and that these samples have not experienced long-term exposure to space.

The size–frequency distributions for particles larger than 1 mm hand-picked from the bulk samples of chambers A and C were reconstructed from individual particle measurements (Fig. 1). The sample size distribution has a slope of −3.88 ± 0.25 in the power-index. This power index of Chamber A + C particles is steeper than the global average index (−2.65 ± 0.05) obtained for boulders (5–140 m in size) on Ryugu or the power-index (approximately −2) for gravels (0.02 m to several metres in size) at the local touch-down sites11 observed by the telescopic Optical Navigation Camera (ONC-T)20. The steeper power-index in the returned particles implies a higher relative abundance of the smaller particles but there are several possible interpretations for the steep power-index arise, including: the fragile nature of samples from Ryugu that may have undergone further fragmentations during impact sampling using a bullet and the cone-shaped collector21, the shock and vibration experienced during Earth entry in the sample container mounted inside the re-entry capsule22, possible artificial fractionation effects from the better transference of smaller particles through the sampler horn21 and/or a sampling bias caused by hand-picking of particles with vacuum tweezers by several personnel (as mentioned in the Methods). The power-index of Chamber A particles, −4.59 ± 0.44, is steeper than those of Chamber C, −3.15 ± 0.20, which shows a much shallower power-index in the size range larger than 3 mm. This larger size enrichment in Chamber C could indicate that such larger particles might have been excavated from regolith below Ryugu’s surface by the SCI close to site TD210,19.

Fig. 1: Size distributions of Ryugu particles from chambers A and C.

A dashed red and blue line is a fitting line to the size distribution of Ryugu particles in the chamber A and C, respectively, and a solid black line is a fitting line to the size distribution of those in chambers A and C. The power-index of those in the chambers A and C (shown as Chamber A + C) is −3.88, which is much steeper than of the global average of Ryugu boulders of >5 m, −2.65 (ref. 11). This might indicate that further fragmentation occurred for smaller Ryugu grains before and/or after their recovery.

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From the micrographs of Ryugu particles and their weights measured using a balance, the bulk densities of Ryugu particles could be estimated on the basis of assumptions mentioned in the Methods. The average bulk density of Ryugu particles in both chambers A and C is 1,282 ± 231 kg m−3 (Fig. 2). This is lower than the average bulk density of CI chondrites23 of 2,110 kg m−3, that of the Tagish Lake meteorite24 (1,660 ± 80 kg m−3, the most porous meteorites ever found on Earth). Assuming that millimetre-sized sample grains have the same grain density as CI chondrites (Orgueil; 2,380 ± 80 kg m−3)23, we estimate the microporosity of Ryugu samples to be 46%. Our value is consistent with the porosity determined by remote thermal imaging by the Thermal Infrared Imager (TIR)25 and on-site thermal measurements9 with the radiometer (MARA) on the Mobile Asteroid Surface Scout (MASCOT)26; this value is lower than that of meteorites, suggesting that thermal measurements made remotely at the centimetre scale can be confirmed by laboratory sample measurements made at the millimetre scale. Thus the microscopic observations and weight measurements for the Ryugu samples imply low density and/or high microporosity. The calculated bulk density of the Ryugu samples is comparable to that of Ryugu rock estimated from the bulk density of Ryugu and linear mixture packing theory: 1,380 ± 70 kg m−3 within the range of variation27.

Fig. 2: Distributions of the bulk densities of Ryugu particles from chambers A and C.

The average sample bulk density (1,282 ± 231 kg m−3) is slightly larger than that of Ryugu (1,190 kg m−3, vertical dashed purple line)2 but much smaller than those of the Tagish Lake meteorite (vertical dashed green line)24 and CI chondrites (vertical dashed orange line)23, indicating the porous nature of Ryugu samples compared with known primitive chondrites.

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Such high-microporosity materials have not been discovered in any meteorites found on Earth, probably due to break-up owing to their fragile nature during entry into the Earth’s atmosphere or a higher abundance of lower-density components such as carbonaceous materials (1,300–1,400 kg m−3)28 compared with other carbonaceous chondrites (CCs). The global average density (bulk density) of Ryugu is 1,190 ± 20 kg m−3, indicating a macroporosity of 7%, which is inconsistent with the large macroporosities required for primitive asteroids when typical meteoritic density is assumed23, provided that the returned samples collected from the two sampling sites on the surface of Ryugu are representative of bulk materials on Ryugu. The low macroporosity of Ryugu is probably consistent with the packing model using the size–frequency distribution of Ryugu29. No substantial difference in density distribution was found between chambers A and C, consistent with the same thermal properties inside and outside the artificial crater30. There are particles denser than 1,800 kg m−3 (>2σ) in Chamber A, which is within the density range of typical meteorites found on Earth12 and indicates that Ryugu might consist of a mixture of particles from different origins30 or varying degrees of alteration in the parent bodies5,30.

Optical and near-infrared reflectance profiles of the samples measured using optical microscopy, Fourier transform infrared (FTIR) spectroscopy and the infrared hyperspectral microscope (MicrOmega)31,32 show very dark features with an albedo of ~0.02 from 0.4 µm to 4 µm (Figs. 3 and 4), which is in good agreement with the global average of albedo3,4 observed by ONC-T and the Near Infrared Spectrometer (NIRS3)33. The surface compositions and inclusions of each sample show some variety but most of them are considered representative of the typical surface materials of Ryugu as they have spectroscopically homogeneous and featureless characteristics without apparent high-temperature components (such as chondrules or Ca–Al-rich inclusions (CAI)), but have many bright and patchy fine inclusions (Extended Data Fig. 2). Although full photometric measurements are needed to elucidate the optical properties of the Ryugu samples, the apparent rarity of chondrules in Ryugu samples is consistent with predictions by a previous study27. The surface morphology of the samples is mainly classified into two patterns of rugged and smooth surfaces even at the millimetre to submillimetre scale, which is similar to the patterns found for centimetre- to metre-scale surfaces3,8 observed by ONC-T and the imager on MASCOT (MasCAM)34. The presence of different types of surface morphology may indicate past mixing processes of materials of different origin or different degrees of alteration5,7,30. The shape distribution of the particles, which has been studied in a separate paper (Tachibana, S. et al., manuscript in preparation), shows variations in aspect ratios, including elongated and flattened particles that are consistent with the ejecta observed during the sampling operations (Tachibana, S. et al., manuscript in preparation).

Fig. 3: Infrared reflectance spectra of Ryugu bulk samples from chambers A and C.

a, Spectra are normalized to each of continua between 2.0 µm to 4.0 µm. Both spectra show features at 2.72 µm (a vertical dashed black line in the left), corresponding to hydroxyl (OH) absorption, and 3.4 µm (a vertical dashed black line in the right), corresponding to organic molecule or carbonate adsorption. A faint absorption at 3.1 µm (a vertical dashed black line in the centre) is also confirmed in spectra of bulk samples analysed by MicrOmega, indicating the presence of a nitrogen-rich phase32. b, The same raw spectra are compared with remote-sensing data for Ryugu taken by NIRS3 with its error bars 4. The absorption feature at 2.72 µm (a vertical dashed green line) observed with NIRS3 is confirmed by the Ryugu samples. The continuum of Ryugu samples is reddenned compared with that of NIRS3, which might reflect the space weathering effect being clearer in the samples than remote-sensing data.

Source data

Fig. 4: Comparison of visible spectroscopic data for chambers A and C with that of ONC-T for Ryugu and other CCs.

The red lines represent typical types of CC from RELAB data51 and the blue lines are unusual types of CC from RELAB and Sugita et al.3. Note that the data for each of the CCs are from powder samples within sizes ranging from <63 µm to <155 µm. Ryugu particles obtained from chambers A and C show an albedo of ~0.02 (reflectance factor at 30°, 0°, 30°), which is comparable to remote-sensing data of Ryugu’s surface taken by ONC-T3,39.

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