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

The origin of Earth's water is a longstanding question in planetary science. While some water may have been inherited from the protosolar nebula, isotopic evidence strongly suggests a significant contribution from extraterrestrial sources. Carbonaceous chondrite (CC) meteorites, particularly those rich in volatiles, are prime candidates. However, the process of atmospheric entry significantly alters these meteorites, particularly the weaker, more hydrated types, leading to biases in our understanding of their original composition and contribution to Earth's water. The Hayabusa2 mission's successful return of samples from the C-type asteroid 162173 Ryugu offers a unique opportunity to circumvent this limitation. Ryugu's samples, characterized by high porosity and low density, are believed to represent relatively unaltered primitive materials not adequately represented in existing meteorite collections. This study focuses on high-precision oxygen isotope analysis of Ryugu samples to determine their relationship to known meteorite groups and assess their potential role in delivering water to early Earth. The study’s importance lies in its potential to refine our understanding of early Solar System processes and the delivery of volatiles to the inner Solar System, thereby providing crucial insights into the formation and evolution of Earth and other terrestrial planets.

Prior research has suggested a link between hydrated asteroids and the delivery of water to Earth, supported by isotopic measurements of carbonaceous chondrites (CCs). CC meteorites provide valuable information on volatile-rich asteroids, but the alteration they undergo during atmospheric entry introduces bias. Previous studies have shown that CI chondrites, the most hydrated CC type, are promising candidates for Earth's water source, but their rarity in meteorite collections has limited our understanding. Studies of the asteroid Ryugu, based on near-infrared spectroscopy, initially suggested a composition similar to CY chondrites, while later curation studies at JAXA indicated similarities to CI chondrites. These conflicting interpretations underscore the need for detailed characterization of the Ryugu particles, particularly through high-precision oxygen isotope analysis, which is widely recognized as a powerful technique for establishing relationships between samples and meteorite groups.

Subsamples from four distinct Ryugu particles (three from Chamber C and one from Chamber A) were analyzed for their bulk oxygen isotopic compositions using laser fluorination. A 'single-shot' technique was employed, ensuring no atmospheric contamination during transport, loading, and analysis. The Ryugu particles are predominantly composed of fine and coarse-grained phyllosilicates (64-88 vol%), with varying amounts of carbonate minerals (2-21 vol%), magnetite (3.6-6.8 vol%), and sulfide minerals (2.4-5.6 vol%). Seven individual analyses were conducted on material extracted from the four Ryugu particles, with analyzed masses ranging from 0.18 to 1.83 mg. For comparison, CI chondrites (Orgueil, Ivuna, Alais) and CY chondrites (Y-82162, B-7904) were also analyzed. A weighted average was calculated for the Ryugu data for comparison with the CY and CI chondrites. The oxygen isotope analyses were performed using an infrared laser-assisted fluorination system at the Open University, with procedures designed to minimize atmospheric contamination, including a modified single-shot mode and cryogenic separation to eliminate potential interference. Overall system precision in bellows mode was ±0.05% for δ¹⁸O; ±0.10% for δ¹⁷O; ±0.02% for Δ¹⁷O (2 s.d.), slightly lower precision was observed in micro-volume mode. Blank corrections were applied to account for the small sample size. Additional calculations were performed to model the effect of terrestrial contamination on the Δ¹⁷O composition of CI chondrites using two methods based on Orgueil's modal and chemical composition.

The weighted mean oxygen isotope composition of the seven Ryugu analyses (δ¹⁸O = 15.88 ± 4.85‰, Δ¹⁷O = 0.66 ± 0.09‰) closely overlaps with that of CI chondrites, but is significantly lighter than CY chondrites. Individual Ryugu analyses showed a large range in δ¹⁸O values (11.46 to 19.30‰), reflecting intrinsic isotopic heterogeneity at the sampling scale. The range is not unexpected given the mineralogical heterogeneity within the Ryugu particles. Analyses of individual mineral phases in Ryugu particles by other studies show a large variation in δ¹⁸O, with magnetite ranging from –5.3 to 7.4‰, dolomite 25.4 to 41.6‰ and Ca-carbonate 34.2 to 39‰. The single particle analyzed from chamber A had the lowest δ¹⁸O value (11.46 ± 0.12‰), likely due to sample heterogeneity. The Ryugu particles are isotopically distinct from the CY chondrites in terms of both δ¹⁸O and Δ¹⁷O. All three CI meteorites measured displayed lower Δ¹⁷O values than the weighted average for Ryugu, consistent with substantial terrestrial contamination. Calculations suggest that the Δ¹⁷O difference between Ryugu and CI chondrites can be attributed to terrestrial contamination of the latter, although primary differences may also play a role. The close match between δ¹⁸O compositions of Ryugu particles and CIs provides a firm basis for linking asteroid Ryugu to the CI chondrites.

The strong correlation between the oxygen isotope data of Ryugu particles and CI chondrites, supported by mineralogical and petrological studies, strongly suggests a close relationship. Differences between Ryugu and CI chondrites likely result from terrestrial alteration of the latter. Terrestrial weathering of CI chondrites, such as Orgueil, would lead to incorporation of atmospheric oxygen, shifting the bulk Δ¹⁷O value closer to the terrestrial fractionation line. This is consistent with the absence of ferrihydrite or sulfate in Ryugu particles, which are present in Orgueil. The lower Δ¹⁷O values in CI chondrites compared to Ryugu likely reflects terrestrial contamination, particularly of interlayer water. This raises important implications for using CI meteorite data as proxies for solar system values and highlights the importance of Ryugu data in providing a less-contaminated perspective on these values. The overall data suggests that CI-like material is more abundant in the asteroid belt than its rarity as meteorites might suggest, indicating that much CI material is too friable to survive atmospheric entry.

This study provides compelling evidence linking the oxygen isotope composition of Ryugu samples to CI chondrites. The observed differences between Ryugu and CI chondrites are primarily attributed to terrestrial contamination of the latter. This suggests that CI-related material may be a more significant source of Earth’s water than previously thought, given the low survival rate of friable CI-like materials during atmospheric entry. Future research should focus on further detailed analysis of Ryugu samples to refine our understanding of the delivery of volatiles to Earth and the early Solar System.

The study's analysis was based on a limited number of Ryugu particles (four) due to the small sample size returned by Hayabusa2. While the sample was deemed representative of the asteroid, the inherent heterogeneity of Ryugu particles could affect the interpretation. Further analyses on a larger number of samples would strengthen the conclusions. The calculations modeling terrestrial contamination do not definitively prove that this is the sole cause of the Δ¹⁷O difference between Ryugu and CI chondrites, although it's a highly probable explanation.