Search

Introduction: The Hayabusa 2 mission successfully returned with samples of C-type near-Earth asteroid (162173) Ryugu on December of 2020. Ryugu samples contain large quantities of volatiles such as water and noble gases [1, 2]. Studying Ryugu samples is a great opportunity to understand the shock metamorphism on water-bearing rubblepile asteroids. In this study, we have examined shock metamorphism on sixteen coarse mm-sized particles recovered by the Hayabusa 2 mission. Results: FE-SEM observation of polished surfaces of Ryugu samples shows they consist mainly of fine-grained phyllosilicates, carbonates, Fe sulfides, and magnetite. Chondrules and CAIs are absent from most of the samples. Heavy shock features such as shock melting or high-pressure mineral formation were not observed in all samples. However, some samples show weak to moderate shock effects. Among them, C0055 exhibits distinct shock deformation features. It shows strong uniaxial compression and high-density subparallel fractures. Synchrotron CT analysis revealed that the subparallel fractures extend across the entire particle in the direction perpendicular to the shock compression. C0055 is broken along subparallel fractures, showing a plate-like morphology. The average width between the fractures is 4.8 µm. Synchrotron X-ray diffraction (XRD) analysis indicated that phyllosilicates in this sample consist of serpentine and saponite, similar to other Ryugu samples. Anhydrous silicates such as olivine and pyroxene are absent in this sample. Many dolomites (5 ~10 µm in size) show planar fractures with an average width of 0.6 ± 0.3 µm (n=13). TEM observation reveals the presence of straight dislocations in the vicinity of the fractures of C0055 dolomite, while dislocation lines in other Ryugu dolomites are generally curly-like. Discussion: Shock stages of meteorites are defined based on shock features in anhydrous silicates such as olivine, pyroxene, and plagioclase [3, 4]. However, such minerals are very rare and small (< ~10 µm) in Ryugu samples [5]. Hence in this study, we used hydrous silicates and carbonates which are abundant in Ryugu. Unlike anhydrous silicates, the shock stage classification of hydrous silicates has not yet been established, but the subparallel fractures in phyllosilicate-rich areas of C0055 can be used for shock intensity characterization. Phyllosilicate-rich matrix in the Murchison CM chondrite that was shocked to 4 and 10 GPa did not show such subparallel fractures, while that of a 21 GPs sample shows fractures [6]. Spacings between fractures are ~5 µm for both C0055 and the Murchison 21GPa sample. Subparallel fractures were produced upon pressure release when shock-heated phyllosilicates were partially dehydrated and expanded by heating. Therefore, production of the subparallel fractures is more dependent on heat than on pressure. The experimental shock compression requires higher pressure to produce subparallel fractures due to lower shock-induced heat than the natural impact, because shock pressure in experiment is attained by multiple reflection shock wave while it in natural events is attained by single shock. The heat generated during high pressure regime of experimental 21GPa-impact (145 J/g) is equivalent to that of 3.8 GPa-impact (170J/g) [6]. Therefore, in the natural impact events, the subparallel fractures start to form at pressures as low as 3.8 GPa. This suggests that C0055 might have experienced an impact exceeding 3.8 GPa. The presence of straight dislocations in dolomite in C0055 accompanied with planar fractures is compatible with mild shock. Ryugu is a rubble pile asteroid that experienced a catastrophic impact [7]. Therefore, we expected the Ryugu sample to exhibit evidence of catastrophic impact, but our research showed most of the Ryugu samples do not. Even the most highly shocked C0055 does not show heavy shock features. The survival of phyllosilicates in C0055 indicates that the phyllosilicates are only partially dehydrated and decomposed. If they were completely dehydrated upon impact, then explosive expansion must have occurred and such materials could not be left behind in the regolith. Therefore, it is likely that there is not much material remaining from the catastrophic shock in present-day Ryugu. [ABSTRACT FROM AUTHOR]

Introduction: The mineralogy, petrology and chemical properties of the samples collected from the asteroid 162173 Ryugu by the Hayabusa2 spacecraft show that the samples correspond to CI meteorites not affected by terrestrial weathering (e.g., [1,2]). Our analysis using SR-based X-ray nanotomography (XnCT) is also consistent CI meteorites [3]. Here we report the morphologies of major mineral phases in 3D to discuss their crystallization conditions in aqueous alteration together with their crystallization sequence based on their textures. Methods: We examined 75 particles (10 ~ 180 μm in apparent size) by XnCT at BL47XU of SPring-8 [3]. Most of them were imaged by DET-SIXM method combined with absorption and phase-shift tomography to discriminate mineral phases with the voxel size of 50 ~ 110 nm, their Mg# and densities. Some samples were imaged by absorption tomography with higher resolution (voxel size: 15 ~ 18 nm). Results and Discussion: The matrix of analyzed particles is mainly composed of Mg-rich phyllosilicates (serpentine and saponite, 82 vol.%) with different features (fine to coarse aggregates, Mg# = 0.75 ~ 0.92 (mean: 0.83± 0.04), density = 1.2 ~ 2.1 (mean: 1.7±0.26) g/cm³), IOMs (0.7 vol.%) and voids (12 vol.%). The major minerals in the matrix are magnetite (2.5 vol.%), Fe(-Ni)S (1.4 vol.%; pyrrhotite >> pentlandite), dolomite (1.2 vol.%), breunnerite and apatite (0.2 vol.%). The data here should show the representative feature of Ryugu samples through the 3D analysis of abundant particles without selection bias but may have large uncertainties for the modal abundance in particular IOMs due to insufficient spatial resolution and thresholding of the XnCT analysis. 3D morphologies. Magnetite has a variety of morphology (typically a few ~ 10 μm); spherulite and its aggregate, framboid composed of mono-dispersed grains of a few 100s nm ~ a few μm, plaquette, large equant grains composed of {311} and elongated grains. Transitional morphologies were also observed. Whiskers (~0.5 μm thick and 20 μm long) and cube (~1 μm) are rarely present as well. In contrast, pyrrhotite is mostly separated hexagonal plates ({100} and {001} in the hexagonal setting) with different size (longest length: <1 ~ 60 μm). The width/length ratio ranges from 0.06 to 0.98 (mean: 0.43±0.20) and is independent of the size. Pentlandite (<~5 μm) is embedded in pyrrhotite or present as fragmented grains in the matrix. Nano-sized pyrrhotite and pentlandite grains were also observed in some matrix by TEM [4] although their morphologies were not clearly known. Dolomite is usually present as aggregates of subhedral ~ euhedral grains (<1 ~ 50 μm). Some are euhedral grains of flattened rhombohedron probably composed of {101}. Separated breunnerite grains (30 ~ 100 μm) are thin rhombohedron of possibly {012}. Apatite is usually present as aggregates of subhedral ~ euhedral grains (<1 ~ 50 μm) and one hexagonal prism of {100} (or {110}) and {001} was observed. Crystallization sequence. The crystallization sequence of the major minerals estimated based on inclusions and their relative enclosing relations, which can be easily recognized in 3D, is follows; magnetite → Fe(-Ni)S (pentlandite → pyrrhotite) → apatite → dolomite/breunnerite. For magnetite, the sequence is likely spherulitic → plaquette/framboidal (from fine to coarse) → equant/elongated. This indicates that magnetite crystallization condition changed from high to low supersaturations. The overall sequence can be explained by aqueous alteration of primitive solar materials such as GEMS-like materials. The high supersaturation for magnetite was probably caused by abrupt dissolution of relatively Fe0 or FeO-rich amorphous silicates followed by precipitation of magnetite and Mg-rich phyllosilicates. In contrast, pyrrhotite hexagonal plates with different size might be formed by dissolution and reprecipitation from original Fe(-Ni)S nanograins at relatively low supersaturation. Dolomite and apatite having equilibrium shapes [5,6] indicate that they precipitated at relatively low supersaturations in the late stage. [ABSTRACT FROM AUTHOR]

Introduction: Asa part of the Hayabusa2 initial analysis [1], we analyzed eighteen Ryugu "stone" samples of 1 ~ 8 mm in size: seven stones from the 1st touch down site and eleven from the 2nd touch down site. Ryugu stones were analyzed first using X-rays. UV, visible, near-infrared mid-infrared, and mutes probes for surface reflectance spectra, internal 3D structure and element distribution [2], crystal structures, and bulk composition of light elements such as C, N, and 0. Measurements for magnetic, thermal, and physical properties mine some stones were also performed. TOF-SDIS analysis of fluid inclusions is being continued [3]. Small particles separated from some stones were ana-lyzed by IR-CT [4]. Based on 3D-CT images of individual stones, objects of interests were identified, separated by pFIE cutting, and exposed by polishing on the surface of epoxy disks. FE.EPMAJFE-SEM analysis was made on ~40 polished epoxy disks for chemical composition and textural observation [5-9]. FIB sections and small (~15 μ in diameter) fragments were separated from the polished surface and analyzed by rod, S170.1, XRF, and nano-CT. Results and discussion: Eighteen stone 2D and 3D images indicate that Ryon samples are breccias of small fragments (< ~1mm in size) of basically CI chondrite mineralogy [10]. Most of the fragments are heavily altered and consists mainly of phyllosilicates. carbonates (dolomite and brannerite), Fe sulfides and oxides, and phosphates (F-bearing hydroxyapatite); this mineralogy we call the major lithology. The mineralogy of the fragments of the major lithology shows some diversity. the abundance and shape of carbonates and phosphates differ among fragments. On the other band, some fragments in many stones are less altered and contain anhydrous silicates such as olivine and low-Ca pyroxene of typically Mr#>97 but down to Mg#~50, which we call the less-altered litbology. In the less-altered lithology, the abundance of olivine and pyroxene differs between fragments and some fragments contain them up to - 10 volti. Therefore, fragments with the less-altered lithology can be classified as CL-type thou-drite material. Most, but not all. fragments with the less-altered lithology contain only calcite as carbonate, phosphides (FeNi)3P and (Fe, Ni)2P instead of apatite. and poorly-crystallized amorphous silicates. Small (<20μm in size) type-I barred and porphyritic chondrules and CAI with spinel+hibonite-tennskite assemblages also occur. GaIS-ble ob-jects are abundant in the least-altered fragments. In summary, mineralogical and physical properties of Ryugu samples reveal incipient to advanced aqueous alteration in its parent asteroid. [ABSTRACT FROM AUTHOR]

Introduction: Initial analysis of asteroid Ryugu samples has revealed that they were mm-μm scale breccia mostly composed of aqueously altered phases (e.g., Mg-Fe phyllosilicates, carbonates) and showed remarkable similarities to CI chondrites in mineralogy and chemistry [e.g., 1,2]. Our detailed analysis of Ryugu samples has detected small amounts (<<1 vol.%) of unaltered silicates that would escape severe aqueous alteration [3]. Olivine is the most abundant (>90 vol%) phase among such unaltered silicates. Although most Ryugu olivine grains are smaller than 5 μm, they bear important information about the precursors of the parent Ryugu asteroid. In this abstract we report additional results of olivine mineralogical study of Ryugu samples, and especially compare with olivine grains in CI chondrites (Ivuna and Orgueil) to further discuss their relationship and possible common origin. Samples and Methods: Polished sections of A0026, A0055, A0063, A0106, C0002, C0023, C0025, C0033, C0040, C0046, C0055, C0076 and C0103 were analyzed using JEOL JXA-8530F FE-EPMA at Univ. of Tokyo. The quantitative analysis was performed at 12 kV accelerating voltage / 30 nA beam current (counting times at peaks: 30 sec), allowing us to obtain accurate compositions even from ~1 μm olivine grain. For comparison, polished sections of Ivuna and Orgueil with similar size to Ryugu samples (a few mm in size) were analyzed by the same method. Results: Olivine grains are present only in weakly-altered lithologies in Ryugu samples [e.g., 2]. So far we have not found olivine from Chamber A samples. Although we acquired compositions from ~200 additional olivine grains this time, the obtained compositions are basically identical to what we have already obtained [3]. Olivine compositions are clustered at Fo99 and extends to Fo44. Most olivine has 0.0-0.7 wt% MnO, and rare (<5%) LIME olivine (MnO/FeO>1 in wt%) grains are present. There is a positive correlation between Mn and Cr in olivine (MnO: 0.0-1.5 wt% vs. Cr2O3: 0.0-0.7 wt%) (Fig. 1). Most olivine grains are Ca-poor (CaO: <0.1 wt%). The comparative analysis of Ivuna and Orgueil has shown that both samples are similarly brecciated to Ryugu samples in size and mineralogy, and we found that weakly-altered lithologies contain tiny (mostly <5 μm) olivine grains. The size, texture, abundance and mineral compositions of olivine in Ivuna and Orgueil are strikingly similar to those of Ryugu olivines both in major and minor elemental compositions (Fig. 1). Discussion and Conclusion: We found identical olivine mineralogy between Ryugu samples and CI chondrites, further supporting that Ryugu samples are similar to CI chondrites [e.g., 1,2]. Although olivine has been reported in CI chondrites, the reported size is larger than 50 μm in many cases [e.g., 4]. Smaller olivine grains (<5 μm) as found in this study have not been well documented except few cases [e.g., 5]. Because the reported CI olivine composition is slightly different from our CI chondrite data [e.g., 4], there would be multiple origins for olivine in CI chondrites and Ryugu samples. In any case, the presence of small olivine grains (<5 μm) found in Ryugu and CI chondrites is important because such tiny olivine could be common precursors of these chondritic materials. [ABSTRACT FROM AUTHOR]

Introduction: The spacecraft Hayabusa2 explored C-type asteroid Ryugu and brought back representative samples of Ryugu to the Earth in 2020 [1]. We performed reflectance spectroscopy of Ryugu samples to investigate (1) spectral diversity among Ryugu samples and (2) similarities and differences between Ryugu samples and carbonaceous chondrites. These results can also be applied to spectroscopic observations performed by the spacecraft [2,3]. Sample and Analysis: We measured Visible-infrared (Vis-IR, 0.4-18 μm in wavelength) reflectance spectra of 6 and 8 coarse grains (> 1mm in size) collected from the first and second touchdown sites, respectively, and some carbonaceous chondrites. Samples were put into an air-tight cell sealed with a CaF2 or ZnSe window together with reflectance standards. All the sample handling was performed in a pure-N2 -purged glove-box to avoid atmospheric alteration of the samples. Petrological and mineralogical characterization of the samples including synchrotron X-ray computed tomography and X-ray diffraction analysis was done by other analysis groups in the Hayabusa2 initial analysis "Stone" team [e.g., 4]. IR signature of Ryugu samples compared to hydrated chondrites: All the Ryugu coarse grains have an absorption band at ~2.71 μm attributed to Mg-rich phyllosilicates and a reflectance maximum (Reststrahlen band) at ~9.8 μm suggesting saponite-bearing phyllosilicate composition. These signatures of phyllosilicates of Ryugu are similar to those of CI chondrites, which is consistent with mineralogical observations of Ryugu samples [e.g., 4]. Ryugu sample spectra show absorption features due to aliphatic compounds (3.4 μm), carbonates (e.g., ~3.4, 3.95 μm), and some samples show an absorption feature at 5.8 μm probably due to carbonyl compounds [5]. Ryugu samples show a sharper 2.7 μm band than CIs, suggesting that they contain little molecular water compared to CIs. Sulfate features are not observed in Mid-IR reflectance spectra of Ryugu unlike CIs [e.g., 6]. Tagish Lake C2-ungrouped chondrite also shows an absorption band at ~2.7 μm and Reststrahlen band at ~9.8 μm. However, IR spectral features due to carbonates are more significant in the spectra of Tagish Lake than those of Ryugu as seen in absorption bands around ~3.4 μm and a reflectance maximum at 11.2 μm [7]. CM chondrites also consist mainly of hydrated minerals, however, their spectral shapes around ~3 μm and ~10 μm are different from those of Ryugu most likely due to the different phyllosilicate composition. Vis reflectance spectra of Ryugu samples: Ryugu coarse grains have ~2-4% of reflectance in the v-band (0.55 μm) at the phase angle of 30 °, which is roughly consistent with those of hydrated carbonaceous chondrites (CI, CM, Tagish Lake, and Tarda). The reflectance spectra of CO and CV chondrites, which are dominated by anhydrous minerals and more chondrule-rich, are brighter than those of Ryugu samples. The IR spectra suggest the compositional link between Ryugu and CI chondrites, however, Vis reflectance spectra of Ryugu are not similar to CI chondrites. Low reflectance Vis spectra without a UV drop (a sudden reflectance decrease towards UV wavelengths) of Ryugu samples are not similar to unheated Orgueil but similar to Orgueil samples heated at 300-400 °C for 50 hours (or longer) in reduced conditions [8]. The causes of differences between Vis spectra of Ryugu and unheated CIs may be explained by: (1) atmospheric alteration of CIs (e.g., oxidation and water adsorption of CIs [e.g., 9]), (2) different organic composition, and (3) gentle heating experienced by Ryugu. Vis reflectance spectra of Ryugu particles show some diversity of properties (reflectance, spectral slope, and overall shapes), while IR spectra are consistent among samples. This suggests that the spectral diversity of Ryugu samples are originated from physical properties or orientations of sample surfaces rather than the compositional differences among individual particles. [ABSTRACT FROM AUTHOR]

Introduction: The successful probing, return and curation of asteroid Ryugu provides pristine sample material from the surface and subsurface of this C-type asteroid. The sampled material of the Hayabusa2 mission includes many coarse-grained fragments [1, 2]. In order to obtain the highest level of scientific output the initial study included several non-invasive and non-destructive analytical techniques. Synchrotron X-ray fluorescence computed tomography (SXRF-CT) [e.g. 3] is an ideal non-invasive analytical technique to achieve detailed in depth structural and chemical information and was applied to several mm-sized rock fragments of the asteroid Ryugu from both, surface and sub-surface samples. Here we present the first dataset on coarse grain C0033, collected at the second touchdown site. Methods: X-ray fluorescence spectroscopy experiments were performed at the P06 beamline of PETRA III (DESY, Hamburg, Germany) and at the ID15A beamline of the ESRF (Grenoble, France). The excitation energy E0 was 20.5 keV and 90 keV respectively, using Si(111) monochromator optics. The Kirkpatrick-Baez (KB) mirrorbased X-ray beam focusing provided a 0.2×0.24 µm² beam spot size at sample position during the P06 experiments, and a 0.5×0.5 µm² beam spot at sample position for the ID15A experiments. High-Energy SXRF: Every detailed SXRF analysis was started with a chemical 2D overview projection of the full volume of the mm-sized rock fragment. In case of sample C0033 an Fe-rich vein was identified. Several crosssection CT scans were performed during our subsequent study, one exactly at the height where the Fe-rich vein was observed. The cross section indicates the presence of REE-rich phases throughout the vein and confirms the vein-like nature of the Fe-enriched area. Lower-Energy SXRF: For further studies a polished section (~100 µm thick) of grain C0033 was prepared exactly at the height of the magnetite vein and was then further analysed at lower X-ray energy (20.5 keV) at PETRA III beamline P06. The Fe distribution found confirms the existence of a magnetite-rich vein. Ca and Y element maps indicate the presence of multiple apatite grains closely related to the vein. Quantification: The SXRF measurements not only provide element distribution maps but can also be quantified. In order to get better statistics small area maps and point analyses were performed. The resulting CI normalized trace element concentrations of magnetite and apatite grains located within the magnetite vein observed in C0033 show values close to CI bulk composition. All quantified measurements were acquired at PETRA III P06 beamline applying a primary beam energy of 20.5 keV. Discussion and Conclusion: The detailed SXRF study of mm-sized Ryugu rock fragments enabled the identification and localization of unique sample characteristics like the magnetite-apatite vein found in grain C0033. Veins crosscutting the sample material are compelling evidence for intense hydrous alteration on the parent body. The widespread occurrence of sulphate veins found in most CI chondrites was first interpreted as evidence for late stage lowtemperature alteration on the asteroid [e.g. 5], but later clear proof was found for the terrestrial formation of these features in almost all samples [6]. The study of rock samples sampled on the sub-surface of asteroid Ryugu now for the first time enables to study hydrous alteration processes without any terrestrial alteration. Our study shows compelling evidence for the existence of low temperature hydrous alteration veins on asteroid Ryugu. Surprisingly, the vein consists of a mixture of mostly magnetite with apatite, which is in contrast to the observation of carbonate veins found on the surface of asteroid Bennu [7]. [ABSTRACT FROM AUTHOR]

Introduction: In December 2020, the Hayabusa2 reentry capsule returned 5.4 g of material from the asteroid Ryugu. A fraction of the collected material was then examined by Initial Analysis Teams. As a part of the Mineralogy / Petrology STONE team, we received ~30 small fragments to study the 3D heterogeneity and the assembly of the different phases. Material and methods: In this study, we will present the results of a muti-analytical sequence applied to nine micrometric Ryugu fragments (ranging from 30 to 80 µm in size), coming from 3 bigger millimetric grains (A0064, C0046, and C0002). These fragments were welded with platinum on metallic needles using FIB (Focus Ion Beam) microscopes in Saclay and Lille [1]. Then, we analyzed these fragments using an IR micro-tomography setup available at the SMIS beamline of the SOLEIL synchrotron (France). Multiple configurations were used: analyses in transmission by Infrared Computed Tomography [2] (IR-CT) and analyses in reflectance by Infrared Surface Imaging [3] (IRSI). Regions of interest with peculiar spectral signatures were identified on the 3D volume and 3-µm thin-slices (~ 15 x 20 x 3 µm³) around these regions were extracted using FIB. They were finally analyzed in 2D with high-resolution IR imaging and SEM-EDX. Average composition: We will present the average IR spectra in the 2.5 - 12.5 µm range both in reflectance and transmission of the 9 small studied fragments. Several grains showed IR signatures at ~ 2.7 µm, 3.0-3.1 µm, 3.4 µm, and 3.9 µm, in a good agreement with the bands identified by NIRS3 or by MicrOmega in the JAXA curation facility [4,5], plus several mid-IR signatures of great interest, such as bands attributed to Si-O stretching in phyllosilicates, and C=O stretching in organics and carbonates. In general, the 2.7 µm feature observed in our data exhibits similar maximum position but is deeper to the one observed with NIRS3. However, we notice a variability of the composition between the 9 fragments, for instance in the intensity of the 2.7 and 3.4 µm features. Study of heterogeneity: The small fragment C0002-FO019, exhibits two regions with very different IR signatures. The difference in the position and the shape of the SiO feature could sign different lithologies affected by various degrees of aqueous alteration. In particular, some spectra show a shoulder at ~ 998 cm-1 which could be due to anhydrous silicate related to a less-altered lithology [6]. The origin of the variation of the 10 µm band (chemical composition, or structural diversity of the mineral) will be discussed. A correlation with the 2.7 µm band will also be presented. Complementary measurements performed with SEM-EDX to better constrain the mineralogy of the surface of the extracted thin-slices will be presented and we will discuss the structural diversity at the scale of ~100 nm. [ABSTRACT FROM AUTHOR]

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., (© 2024. The Author(s).)

The Hayabusa2 spacecraft delivered samples of the carbonaceous asteroid Ryugu to Earth. Some of the sample particles show evidence of micrometeoroid impacts, which occurred on the asteroid surface. Among those, particles A0067 and A0094 have flat surfaces on which a large number of microcraters and impact melt splashes are observed. Two impact melt splashes and one microcrater were analyzed to unveil the nature of the objects that impacted the asteroid surface. The melt splashes consist mainly of Mg-Fe-rich glassy silicates and Fe-Ni sulfides. The microcrater trapped an impact melt consisting mainly of Mg-Fe-rich glassy silicate, Fe-Ni sulfides, and minor silica-rich glass. These impact melts show a single compositional trend indicating mixing of Ryugu surface materials and impactors having chondritic chemical compositions. The relict impactor in one of the melt splashes shows mineralogical similarity with anhydrous chondritic interplanetary dust particles having a probable cometary origin. The chondritic micrometeoroids probably impacted the Ryugu surface during its residence in a near-Earth orbit.

The carbonaceous asteroid Ryugu has been explored by the Hayabusa2 spacecraft to elucidate the actual nature of hydrous asteroids. Laboratory analyses revealed that the samples from Ryugu are comparable to unheated CI carbonaceous chondrites; however, reflectance spectra of Ryugu samples and CIs do not coincide. Here, we demonstrate that Ryugu sample spectra are reproduced by heating Orgueil CI chondrite at 300°C under reducing conditions, which caused dehydration of terrestrial weathering products and reduction of iron in phyllosilicates. Terrestrial weathering of CIs accounts for the spectral differences between Ryugu sample and CIs, which is more severe than space weathering that likely explains those between asteroid Ryugu and the collected samples. Previous assignments of CI chondrite parent bodies, i.e., chemically most primitive objects in the solar system, are based on the spectra of CI chondrites. This study indicates that actual spectra of CI parent bodies are much darker and flatter at ultraviolet to visible wavelengths than the spectra of CI chondrites.

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., (© 2023. Springer Nature Limited.)

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 16 O-rich and -poor with Δ 17 O (=δ 17 O - 0.52 × δ 18 O) values of ~ -23‰ and ~ -3‰, resembling what has been proposed as early generations of chondrules. The 16 O-rich objects are likely to be melted amoeboid olivine aggregates that escaped from incorporation into 16 O-poor chondrule precursor dust. Two CAIs composed of refractory minerals are 16 O-rich with Δ 17 O of ~ -23‰ and possibly as old as the oldest CAIs. The discovered objects (<30 µm) are as small as those from comets, suggesting radial transport favoring smaller objects from the inner solar nebula to the formation location of the Ryugu original parent body, which is farther from the Sun and scarce in chondrules. The transported objects may have been mostly destroyed during aqueous alteration in the Ryugu parent body., (© 2023. The Author(s).)