Carbonaceous Chondrite Meteorites Research Articles

CAI formation in the early Solar System

Calcium-aluminium-rich inclusions (CAIs) are the oldest dated solid materials in the Solar System, and are found as light-coloured crystalline ingredients in carbonaceous chondrite meteorites. Their formation time is commonly associated with age zero of the Solar System. Nevertheless, the physical and chemical processes that once led to the formation of these submillimetre- to centimetre-sized mineral particles in the early solar nebula are still a matter of debate. In this paper, we propose a pathway to form such inclusions during the earliest phases of disc evolution. We combine 1D viscous disc evolutionary models with 2D radiative transfer, equilibrium condensation, and new dust opacity calculations. We show that the viscous heating associated with the high accretion rates in the earliest evolutionary phases causes the midplane inside of about 0.5\,au to heat up to limiting temperatures of about 1500-1700\,K, but no further. These high temperatures force all refractory material components of the inherited interstellar dust grains to sublimate --- except for a few Al-Ca-Ti oxides, such as Al$_2$O$_3$, Ca$_2$Al$_2$SiO$_7$, and CaTiO$_3$. This is a recurring and very stable result in all our simulations, because these minerals form a natural thermostat. Once the Mg-Fe silicates are gone, the dust becomes more transparent and the heat is more efficiently transported to the disc surface, which prevents further warming. This thermostat mechanism keeps these minerals above their annealing temperature for hundreds of thousands of years, allowing them to form large pure crystalline particles. These particles are dragged out by the viscously spreading disc, and once they reach a distance of about 0.5\,au, the silicates recondense on the surface of the Ca-Al-rich particles, adding an amorphous silicate matrix. We estimate that this mechanism of CAI production works during the first 50\,000 years of disc evolution. These particles then continue to move outward and populate the entire disc up to radii of about 50\,au, before the accretion rate eventually subsides, the disc cools, and the particles start to drift inwards.

Infrared Spectroscopy of RNA Nucleosides in a Wide Range of Temperatures.

The RNA world hypothesis suggests that early cellular ancestors relied solely on RNA molecules for both genetic information storage and cellular functions. RNA, composed of four nucleosides-adenosine, guanosine, cytidine, and uridine-forms the basis of this theory. These nucleosides consist of purine nucleobases, adenine and guanine, and pyrimidine nucleobases, cytosine and uracil, bonded to ribose sugar. Notably, carbonaceous chondrite meteorites have revealed the presence of these bases and sugar, hinting at the potential existence of nucleosides in space. This study aims to present the infrared spectra of four RNA nucleosides commonly found in terrestrial biochemistry, facilitating their detection in space, especially in astrobiological and astrochemical contexts. Laboratory measurements involved obtaining mid- and far-IR spectra at three temperatures (-180 °C, room temperature, and +180 °C), followed by calculating molar extinction coefficients (ε) and integrated molar absorptivities (ψ) for corresponding bands. These spectral data, along with ε and ψ values, serve to provide quantitative insights into the presence and relative abundance of nucleosides in space and aid in their detection.

Raman–Infrared Spectral Correlation of an Artificially Space-Weathered Carbonaceous Chondrite Meteorite

Raman and infrared measurements of the same locations were conducted on a northwest African (NWA) 10580 CO3 meteorite sample, before and after three proton irradiations (1 keV ion energy using 1011, 1014, and 1017 ion/cm2 fluent values), to simulate space weathering effects. In the case of Raman spectroscopy, both FWHM and peak positions of the major olivine and pyroxene bands changed after the irradiation, and the minor bands disappeared. In the FTIR spectra, the minor bands of olivine and pyroxene also disappeared; meanwhile, major IR bands of pyroxene remained visible, demonstrating both positive and negative peak shifts, and the olivines were characterised only by negative peak shifts. The olivines were characterised by negative FWHM changes for major bands, but positive FWHM changes for minor bands. The pyroxenes were characterised by elevated FWHM changes for minor bands after the irradiation. The disappearance of minor bands both of IR and Raman spectra indicates the amorphization of minerals. The negative peak shift in IR spectra indicates Mg loss for olivine and pyroxene, in agreement with the literature. The Raman spectra are characterised by positive peak shift and positive FWHM changes; the IR spectra are characterised by a negative peak shift. The Mg loss, which was detected by negative peak shifts of FTIR bands, may be caused by distortion of the crystal structure, which could be detected by a positive peak shift in Raman spectra. This joint observation and interpretation has not been formulated in the literature, but indicates further possibilities in the confirmation of mineral changes by different instruments. Shock alteration-based observations by other researchers could be used as a reference for irradiation experiments as irradiation makes a similar structural alteration, like a low-grade shock metamorphism.

The nature of insoluble organic matter in Sutter's Mill and Murchison carbonaceous chondrites: Testing the effect of x‐ray computed tomography and exploring parent body organic molecular evolution

AbstractThis study analyzed samples of the Murchison and Sutter's Mill carbonaceous chondrite meteorites in support of the future analysis of samples returned from the asteroid (10155) Bennu by the OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission. Focusing specifically on the insoluble organic matter (IOM), this study establishes that a total of 1.3 g of bulk sample from a single chondritic meteorite are sufficient to obtain a wide range of cosmochemical information, including light element analysis (H, C, and N), isotopic analysis (D/H, 13C/12C, and 15N/14N), and x‐ray fluorescence spectroscopy for major elemental abundances. IOM isolated from the bulk meteorite samples was analyzed by light element and isotopic analysis as described above, 1H and 13C solid‐state nuclear magnetic resonance spectroscopy, Raman spectroscopy, and complete noble gas analyses (abundances and isotopes). The samples studied included a pair from Murchison (CM2), one of which had been irradiated with high‐energy x‐rays in the course of computed tomographic imaging. No differences between the irradiated and non‐irradiated Murchison samples were observed in the many different chemical and spectroscopic analyses, indicating that any x‐ray–derived sample damage is below levels of detection. Elemental, isotopic, and molecular spectroscopic data derived from IOM isolated from the Sutter's Mill sample reveals evidence that this meteorite falls into the class of heated CM chondrites.

Grain Size Effects on UV–MIR (0.2–14 μm) Spectra of Carbonaceous Chondrite Groups

Carbonaceous chondrites are among the most important meteorite types and have played a vital role in deciphering the origin and evolution of our solar system. They have been linked to low-albedo C-type asteroids, but due to subdued absorption bands, definitive asteroid–meteorite linkages remain elusive. A majority of these existing linkages rely on fine-grained (typically < 45 μm) powders across a limited wavelength range in the visible to near-infrared (0.35–2.5 μm). While this is useful in interpreting the fine-grained regolith of larger main-belt objects like Ceres, recent spacecraft missions to smaller near-Earth asteroids (NEAs), such as Bennu and Ryugu, have shown that their surfaces are dominated by larger grain size material. To better interpret the surfaces of these smaller, carbonaceous NEAs, we obtained laboratory reflectance spectra of seven carbonaceous chondrite meteorite groups (CI, CM, CO, CV, CR, CK, C2-ungrouped) over the ultraviolet to mid-infrared range (0.2–14 μm). Each meteorite contained five grain size bins (45–1000 μm) to help constrain spectral grain size effects. We find a correlation between grain size and absolute reflectance, spectral slope, band depth, and the Christiansen feature band center. Principal component analysis of grain size variation illustrates a similar trend to lunar-style space weathering. We also show that the Bus–DeMeo asteroid taxonomic classification of our samples is affected by grain size, specifically shifting CM2 Aguas Zarcas from a Ch-type to B-type with increasing grain size. This has implications for the parent body of the OSIRIS-REx target, Bennu. With Aguas Zarcas, we present results from Hapke modeling.

Nuclear and Orbital Characterization of the Transition Object (4015) 107P/Wilson–Harrington

Comet 107P/Wilson–Harrington, cross-listed as asteroid 4015, is one of the original transition objects whose properties do not neatly fit into a cometary or asteroidal origin. Discovered in a period of apparently gas-dominated activity in 1949, it was subsequently lost and recovered as the inactive asteroid 1979 VA. We obtained new and reanalyzed archival observations of the object, compared to meteorites, and conducted new orbital integrations in order to understand the nature of this object and to understand where it falls on the asteroid–comet continuum. Wilson–Harrington’s reflectance spectrum is approximately neutral from visible to near-infrared wavelengths, but has a reflectance maximum near 0.8–0.9 μm. The object’s spectrum is well matched by laboratory spectra of carbonaceous chondrite meteorites like the CM Murchison or the C i Ivuna. The object’s phase curve slope is compatible with either an asteroidal or cometary origin, and its recent orbital history has no periods with high enough temperatures to have altered its surface. While it is possible that some unknown process has acted to change the surface from an originally cometary one, we instead prefer a fundamentally asteroidal origin for Wilson–Harrington, which can explain its surface and orbital properties. However, this would require a way to maintain significant (hyper)volatile supplies on the near-Earth objects beyond what is currently expected. Wilson–Harrington’s similar meteorite affinity and possible orbital link to sample return targets (162173) Ryugu and (101955) Bennu suggest that the returned samples from the Hayabusa-2 and OSIRIS-REx missions might hold the key to understanding this object.

Infrared absorption spectra from organic matter in the asteroid Ryugu samples: Some unique properties compared to unheated carbonaceous chondrites

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

The Winchcombe fireball—That lucky survivor

AbstractOn February 28, 2021, a fireball dropped ∼0.6 kg of recovered CM2 carbonaceous chondrite meteorites in South‐West England near the town of Winchcombe. We reconstruct the fireball's atmospheric trajectory, light curve, fragmentation behavior, and pre‐atmospheric orbit from optical records contributed by five networks. The progenitor meteoroid was three orders of magnitude less massive (∼13 kg) than any previously observed carbonaceous fall. The Winchcombe meteorite survived entry because it was exposed to a very low peak atmospheric dynamic pressure (∼0.6 MPa) due to a fortuitous combination of entry parameters, notably low velocity (13.9 km s−1). A near‐catastrophic fragmentation at ∼0.07 MPa points to the body's fragility. Low entry speeds which cause low peak dynamic pressures are likely necessary conditions for a small carbonaceous meteoroid to survive atmospheric entry, strongly constraining the radiant direction to the general antapex direction. Orbital integrations show that the meteoroid was injected into the near‐Earth region ∼0.08 Myr ago and it never had a perihelion distance smaller than ∼0.7 AU, while other CM2 meteorites with known orbits approached the Sun closer (∼0.5 AU) and were heated to at least 100 K higher temperatures.

Insights into the formation and evolution of extraterrestrial amino acids from the asteroid Ryugu

All life on Earth contains amino acids and carbonaceous chondrite meteorites have been suggested as their source at the origin of life on Earth. While many meteoritic amino acids are considered indigenous, deciphering the extent of terrestrial contamination remains an issue. The Ryugu asteroid fragments (JAXA Hayabusa2 mission), represent the most uncontaminated primitive extraterrestrial material available. Here, the concentrations of amino acids from two particles from different touchdown sites (TD1 and TD2) are reported. The concentrations show that N,N-dimethylglycine (DMG) is the most abundant amino acid in the TD1 particle, but below detection limit in the other. The TD1 particle mineral components indicate it experienced more aqueous alteration. Furthermore, the relationships between the amino acids and the geochemistry suggest that DMG formed on the Ryugu progenitor body during aqueous alteration. The findings highlight the importance of aqueous chemistry for defining the ultimate concentrations of amino acids in primitive extraterrestrial samples.

Macromolecular organic matter in samples of the asteroid (162173) Ryugu.

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

Meteorites have inherited nucleosynthetic anomalies of potassium-40 produced in supernovae.

Meteorites record processes that occurred before and during the formation of the Solar System in the form of nucleosynthetic anomalies: isotopic compositions that differ from the Solar System patterns. Nucleosynthetic anomalies are rarely seen in volatile elements such as potassium at bulk meteorite scale. We measured potassium isotope ratios in 32 meteorites and identified nucleosynthetic anomalies in the isotope potassium-40. The anomalies are larger and more variable in carbonaceous chondrite (CC) meteorites than in noncarbonaceous (NC) meteorites, indicating that CCs inherited more material produced in supernova nucleosynthesis. The potassium-40 anomaly of Earth is close to that of the NCs, implying that Earth's potassium was mostly delivered by NCs.

Early fluid activity on Ryugu inferred by isotopic analyses of carbonates and magnetite

Samples from asteroid Ryugu returned by the Hayabusa2 mission contain evidence of extensive alteration by aqueous fluids and appear related to the CI chondrites. To understand the sources of the fluid and the timing of chemical reactions occurring during the alteration processes, we investigated the oxygen, carbon and 53Mn–53Cr systematics of carbonate and magnetite in two Ryugu particles. We find that the fluid was initially between 0 and 20 °C and enriched in 13C, 17O and 18O, and subsequently evolved towards lighter carbon and oxygen isotopic compositions as alteration proceeded. Carbonate ages show that this fluid–rock interaction took place within approximately the first 1.8 million years of Solar System history, requiring early accretion either in a planetesimal less than ∼20 km in diameter or within a larger body that was disrupted and reassembled. The aqueous activity responsible for carbonate formation on Ryugu happened much earlier—less than 1.8 million years after CAI formation—than estimates (4–6 Myr) from carbonaceous chondrite meteorites. Ryugu’s parent body either was smaller than ∼20 km in diameter or was disrupted before reaching the high temperatures required.

Oxygen isotope evidence from Ryugu samples for early water delivery to Earth by CI chondrites

The delivery of water to the inner Solar System, including Earth, is still a debated topic. A preferential role for hydrated asteroids in this process is supported by isotopic measurements. Carbonaceous chondrite (CC) meteorites represent our main source of information about these volatile-rich asteroids. However, the destruction of weaker materials during atmospheric entry creates a bias in our CC data. The return of surface materials from the C-type asteroid 162173 Ryugu by the Hayabusa2 spacecraft provides a unique opportunity to study high-porosity, low-density, primitive materials, unrepresented in the meteorite record. We measured the bulk oxygen isotope composition from four Ryugu particles and show that they most closely resemble the rare CI (CC Ivuna-type) chondrites, but with some differences that we attribute to the terrestrial contamination of the CI meteorites. We suggest that CI-related material is widespread among carbonaceous asteroids and a more important source of Earth’s water and other volatiles than its limited presence in our meteoritic collection indicates.

Lithium and boron isotopic compositions of olivine in chondrules from carbonaceous and ordinary chondrite meteorites: Implications for the origin of solar 11B/10B ratio

The origin of solar 11B/10B ratio ∼4 remains an open question. It has been thought that a significant portion of boron in the Solar System was derived from continuous spallation nucleosynthesis during interactions between Galactic Cosmic Rays and CNO nuclei in the interstellar medium. However, because GCR-produced boron is characterized by 11B/10B ∼2.5, an endmember with 11B/10B > 4 is required to account for the solar 11B/10B ratio. Two leading hypotheses for the sources of 11B-rich components include low energy spallation in the Sun’s parental molecular cloud and the neutrino process during the supernova explosions. In this study, lithium and boron elemental and isotopic compositions of seven porphyritic olivine chondrules and one isolated olivine crystal from four meteorites (Allende, Yamato 81020, Asuka 12236, and QUE 97008) were determined in-situ to help constrain which of the two nucleosynthetic mechanisms has most likely supplied the forming Solar System with extra 11B. Apparent isotopic variations in Li/Si, B/Si, δ7Li and δ11B were found in chondrule olivine crystals, but only three chondrules exhibit statistically resolved δ11B heterogeneities. Using these three chondrules as a basis for discussion, we evaluated the processes that could potentially cause elemental and isotopic variations of Li and B. We argue that the data can be best understood in the context of condensation of lithium-boron-rich material onto chondrule precursors in (an) initially heterogeneous gaseous reservoir(s), which could be realized if the Solar System derived 11B-rich components from a supernova or supernovae.

Noble gases and nitrogen in samples of asteroid Ryugu record its volatile sources and recent surface evolution.

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 measured noble gas and nitrogen isotopes in Ryugu samples and found that they are dominated by presolar 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 different abundances among the samples. Our measurements support a close relationship between Ryugu and CI chondrites. Noble gases produced by galactic cosmic rays, indicating a ~5 million year exposure, and from implanted solar wind record the recent irradiation history of Ryugu after it migrated to its current orbit.

Composition of inner main-belt planetesimals

Aims. We carried out a spectroscopic survey in order to investigate the composition of 64 asteroids of the inner main belt, which are leftovers of the original planetesimals of our Solar System (we call them inner main belt planetesimals or IMBPs). Following published methods, we identified IMBPs in the inverse size versus semimajor axis (α) space, after the removal of all asteroids belonging to collisional families. Methods. We conducted several ground-based observational campaigns of these IMBPs in the visible range at the 1.82 m Asiago telescope, and in the near-infrared range at the Telescopio Nationale Galileo, the Lowell Discovery Telescope, and the NASA InfraRed Telescope Facility telescopes. As several of the identified planetesimals already have spectra published in the literature, we collected all the available data and focused the telescope time to investigate those never observed before, or to complete the 0.45–2.5 μm range spectrum for those for which there is only partial spectral coverage or data with poor signal-to-noise ratio. In this way, we obtained new spectra for 24 IMBPs. Combining new and literature observations, we present spectra for 60 IMBPs in both the visible and near-infrared range, and 4 IMBPs in the visible only. All spectra were classified following well-established taxonomies. We also characterized their spectral absorption bands – when present –, their spectral slopes, and their mineralogy. In addition, we performed curve matching between astronomical and laboratory spectra in order to identify the closest meteorite analog using the RELAB database. Results. The majority of the IMBPs belong to the S-complex; the latter are best matched with ordinary chondrite meteorites, and their olivine/(olivine and pyroxene) abundance ratio is not correlated with the semi-major axis. This result does not support the hypothesis that this ratio increases with heliocentric distance. Furthermore, ~27% of the IMBPs belong to the C-complex, where Ch/Cgh types dominate, meaning that most of the carbonaceous-rich planetesimals were aqueously altered. These are best fitted by CM2 carbonaceous chondrite meteorites. Finally, the remaining IMBPs (~20%) belong to the X-complex, and have various mineralogies and meteorite matches, while a few are end-member classes, including L-, K-, V-, and D- or T-types. Conclusions. Our spectroscopic investigation of IMBPs confirms that silicate-rich bodies dominated the inner main belt where temperature has permitted the condensation of silicate rocks. However, almost all the spectral types are found, with the notable exception of olivine-rich A-types and Q-type asteroids. Their absence, as well as the absence of the R- and O-types among planetesimals, might be due to the rarity of these types among large asteroids. However, the absence of Q-types among primordial planetesimals is expected, as they have undergone surface rejuvenating processes. Therefore, Q-types have relatively young and less weathered surfaces compared to other types. Our results support the hypothesis of compositional mixing in the early Solar System. In particular, the fact that most of the C-complex planetesimals are aqueous altered, and the presence of three D- or T-type asteroids among them indicate that these bodies migrated from beyond 3 au to their current position.

A pristine record of outer Solar System materials from asteroid Ryugu’s returned sample

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.

Presolar Stardust in Asteroid Ryugu

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 ppm for O-anomalous grains, ppm for SiC grains, and 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: ppm SiC and 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.

The Nature of Low-albedo Small Bodies from 3 μm Spectroscopy: One Group that Formed within the Ammonia Snow Line and One that Formed beyond It

We present evidence, via a large survey of 191 new spectra along with previously published spectra, of a divide in the 3 μm spectral properties of the low-albedo asteroid population. One group (“sharp types,” or STs, with band centers <3 μm) has a spectral shape consistent with carbonaceous chondrite meteorites, while the other group (“not sharp types,” or NSTs, with bands centered >3 μm) is not represented in the meteorite literature but is as abundant as the STs among large objects. Both groups are present in most low-albedo asteroid taxonomic classes, and, except in limited cases, taxonomic classifications based on 0.5–2.5 μm data alone cannot predict whether an asteroid is an ST or NST. Statistical tests show that the STs and NSTs differ in average band depth, semimajor axis, and perihelion at confidence levels ≥98% while not showing significant differences in albedo. We also show that many NSTs have a 3 μm absorption band shape like comet 67P and likely represent an important small-body composition throughout the solar system. A simple explanation for the origin of these groups is formation on opposite sides of the ammonia snow line, with the NST group accreting H2O and NH3 and the ST group only accreting H2O, with subsequent thermal and chemical evolution resulting in the minerals seen today. Such an explanation is consistent with recent dynamical modeling of planetesimal formation and delivery and suggests that much more outer solar system material was delivered to the main asteroid belt than would be thought based on the number of D-class asteroids found today.

Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites.

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 measured the mineralogy and bulk chemical and isotopic compositions of Ryugu samples. The samples are mainly composed of materials similar to those of 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, about [Formula: see text] million (statistical) or [Formula: see text] million (systematic) years after the 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 that of the Sun's photosphere than other natural samples do.