The origin of Earth's water remains a topic of significant scientific debate. One leading hypothesis suggests that carbonaceous chondritic meteorites, specifically the CI (Ivuna-type) chondrites, could be a major source. CI chondrites are valuable because they are considered a proxy for the bulk composition of the Solar System. However, the meteorite record is inherently biased. The process of atmospheric entry and subsequent terrestrial weathering significantly alters the composition of meteorites, making it challenging to obtain a pristine sample representative of their original composition. This bias limits our understanding of the true chemical makeup of these crucial early Solar System bodies and their role in delivering water to Earth. The Hayabusa2 mission, launched by the Japan Aerospace Exploration Agency (JAXA), presented a unique opportunity to circumvent this limitation. Hayabusa2 successfully collected samples from the C-type asteroid Ryugu and returned them to Earth. These samples, having bypassed atmospheric entry and terrestrial contamination, offer an unprecedented opportunity to study pristine extraterrestrial materials. Prior remote sensing observations, including data from the Near-IR Spectrometer (NIRS3) on board Hayabusa2, suggested that Ryugu's composition might be akin to CY (Yamato-type) chondrites, a type of carbonaceous chondrite exhibiting thermal or shock metamorphism. However, these initial classifications remained inconclusive and necessitated a comprehensive in-situ analysis of the returned samples. This study aims to conduct a thorough mineralogical, isotopic, and elemental characterization of the Ryugu samples to resolve these ambiguities and provide a more accurate representation of asteroid Ryugu's bulk composition and its potential significance in the context of early Solar System processes and water delivery to Earth.

Several studies have focused on carbonaceous chondrites as potential sources of Earth's water. CI chondrites, in particular, have been extensively studied due to their chemical resemblance to the Sun's composition, suggesting they could represent a primitive, unaltered sample of Solar System material. However, studies such as King et al. (2020) have highlighted the extent of terrestrial modification in CI chondrites, emphasizing the need for pristine samples. The Yamato-type (CY) carbonaceous chondrite group has also been considered as a potential source material for Ryugu, based on remote sensing data (King et al., 2019). These studies underscore the importance of analyzing extraterrestrial materials that have not undergone significant terrestrial alteration. The use of advanced analytical techniques, including those employed in this study, has become crucial for refining our understanding of the processes and materials involved in the formation of the early Solar System. The MicrOmega hyperspectral microscope's initial compositional analysis of Ryugu samples (Pilorget et al., 2021) and preliminary analysis of Hayabusa2 samples (Yada et al., 2021) further motivated the need for detailed isotopic, elemental and mineralogical characterization, particularly to differentiate between CI and CY chondrite interpretations.

This research employed a multi-faceted approach involving both bulk and microanalytical techniques to characterize Ryugu samples. The study focused on eight Ryugu particles (totaling ~60 mg), four from the spacecraft's Chamber A (surface material) and four from Chamber C (material from near an artificial crater). These samples were meticulously handled to prevent terrestrial contamination, using a dedicated, pure-nitrogen-filled chamber at the JAXA curation facility. The detailed methodology incorporated the following steps: 1. **Mineralogical Analysis:** Five Ryugu particles (A0029, A0037, C0009, C0014, and C0068) underwent detailed mineralogical analysis utilizing various techniques, including optical microscopy, scanning electron microscopy (SEM)-energy-dispersive spectroscopy (EDS), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). This provided information on the mineral composition, abundances, and spatial relationships within the particles. High-resolution transmission electron microscopy (TEM) was employed to study the fine-scale textures and structures of the minerals. 2. **Oxygen Isotope Analysis:** Bulk oxygen isotope analysis was performed using a laser fluorination system on a sample extracted from particle C0014. This was compared with analyses of Orgueil (CI chondrite) and Y-82162 (CY chondrite) to determine the relationship between Ryugu and known carbonaceous chondrite types. 3. **Organic Matter Characterization:** Coordinated microanalysis techniques, including focused-ion-beam (FIB) sectioning, scanning transmission X-ray microscopy (STXM), near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy, and nanoscale secondary-ion mass spectrometry (NanoSIMS), were used to study the spatial distribution and chemical properties of organic carbon within the particles. This provided insights into the types of organic molecules present and their relationship with other minerals. 4. **Elemental Abundance Analysis:** Instrumental neutron activation analysis (INAA) was performed on Ryugu particles A0098 and C0068 to determine the bulk elemental abundances. This helped to compare the overall elemental composition of Ryugu particles with the known compositions of CI and CM chondrites. 5. **Light Element Isotope Analysis:** NanoSIMS was used to analyze the hydrogen, carbon, and nitrogen isotopic compositions of several Ryugu particles. The results helped to shed light on the origin and evolutionary history of the samples. Extreme care was taken throughout the sample handling and analytical process to minimize contamination and ensure the integrity of the pristine samples. The use of inert atmospheres (pure, dry N2), specialized sample holders, and dedicated transfer vessels for transport between instruments ensured the reliability of the data obtained.

The key findings of this study reveal several crucial aspects of the Ryugu samples: 1. **Compositional Similarity to CI Chondrites:** Mineralogical, elemental, and oxygen isotopic analyses strongly suggest that Ryugu particles share a close compositional match with CI (Ivuna-type) chondrites. The presence of abundant fine- and coarse-grained phyllosilicates (serpentine-saponite intergrowth), carbonates (mainly dolomite), sulfides (pyrrhotite and pentlandite), and magnetite is consistent with CI chondrite compositions. The bulk elemental abundances also closely align with CI chondrites, notably lacking ferrihydrite and sulfate minerals indicative of terrestrial weathering in CI meteorites. The oxygen isotope data (Δ¹⁷O and δ¹⁸O) of Ryugu particle C0014-4 show a clear separation from CY chondrites, further supporting a CI chondrite affinity. 2. **Low-Temperature Aqueous Alteration:** The intricate spatial relationship observed between aliphatic-rich organics and phyllosilicates suggests that aqueous alteration of Ryugu's parent body occurred at low temperatures (maximum -30 °C). This is consistent with the presence of cubanite, a mineral that is not stable at high temperatures. 3. **Outer Solar System Origin:** The heavy hydrogen (δD) and nitrogen (δ¹⁵N) isotopic compositions of Ryugu particles point to an origin in the outer Solar System, potentially aligning with the isotopic composition of some comets and interplanetary dust particles (IDPs). This isotopic signature, notably heavier than in CI chondrites, hints at a less processed component potentially linked to outer Solar System sources. The heterogeneity observed in δD and δ¹⁵N values may also suggest variations in initial isotopic signatures from the early Solar System. 4. **Aliphatic-Rich Organics:** The Ryugu particles exhibit abundant sub-micrometer-sized, aliphatic-rich organic carbon associated with phyllosilicates. This type of organic matter differs from that found in previously studied carbonaceous chondrites, showing more similarity to IDPs and cometary particles. The spatial distribution of these organics and the low-temperature alteration indicate that they may have been relatively well preserved from significant thermal alteration. A presolar graphite grain was identified within the organic-rich matrix which exhibits an extreme ¹³C enrichment. This hints at pre-solar components in the Ryugu sample. 5. **Pristine Nature:** The Ryugu particles are deemed the most uncontaminated and unfractionated extraterrestrial materials studied to date, providing the best available proxy for the bulk composition of the Solar System. The absence of terrestrial alteration products makes these samples exceptionally valuable for unraveling early Solar System processes. This underlines the significance of sample-return missions in providing pristine, uncontaminated samples for detailed analysis.

This study's findings significantly advance our understanding of the composition and origin of C-type asteroids and their potential role in delivering volatiles to the early Earth. The close match between Ryugu particles and CI chondrites refines the understanding of CI chondrites as a proxy for bulk solar system composition. The observed low-temperature aqueous alteration is key to understanding the conditions present on the Ryugu parent body, adding valuable insights into the processes that shaped early Solar System objects. The detection of aliphatic-rich organics and their relationship with minerals contributes to the broader understanding of organic matter delivery to the early Earth, but further research is required to ascertain their precise pre-solar or outer solar system contribution. The isotopic data point towards an outer Solar System origin for at least some components of Ryugu, raising the possibility that these asteroids could have contributed to the delivery of volatiles from the outer Solar System, albeit possibly mixed with materials from other sources. This study demonstrates that the currently available CI meteorites are demonstrably terrestrially contaminated and therefore that sample return missions are essential for understanding the pristine components of the early solar system.

This study provides compelling evidence that Ryugu particles are remarkably pristine representatives of the bulk Solar System composition, closely resembling aqueously altered CI chondrites. The findings indicate low-temperature aqueous alteration and suggest a contribution from the outer Solar System. The discovery of abundant aliphatic-rich organics associated with phyllosilicates adds to our knowledge of organic matter distribution and preservation in early Solar System objects. Future research should focus on a more comprehensive isotopic analysis to further explore the origin and distribution of volatiles and organics. Comparative studies of other returned samples from different asteroids could offer further insights into the diversity and evolutionary pathways of early Solar System bodies.

The relatively small sample size (eight particles, 60 mg total) and the heterogeneity inherent in these samples might introduce some sampling bias. Although extreme care was taken to avoid contamination, the possibility of trace terrestrial contamination cannot be completely ruled out. Further, the complex fine-scale mixture of various components in the FIB sections might complicate the interpretation of isotopic variations. More comprehensive analyses of a larger number of samples would enhance the statistical power of these findings and provide a more robust understanding of the overall composition of Ryugu.