A groundbreaking study challenges long-standing views on the origin of CI chondrites, some of the most primitive meteorites linked to the outer Solar System. Scientists now propose these meteorites, including samples from asteroids Ryugu and Bennu, may have formed much closer to the Sun—near the water ice line inside Jupiter’s orbit—rather than in the distant, cold reaches of the protoplanetary disk.
The key to this paradigm shift lies in isotopic analyses, particularly of chromium-54 (⁵⁴Cr). Researchers identified thermal processing effects on ices that altered isotopic signatures in chondrule precursors, suggesting these materials experienced warming events such as vertical mixing within the disk or shock heating during chondrule formation. This thermal alteration led to a progressive loss of interstellar water ice, implying that the volatile components of CI chondrites—a class traditionally associated with the outer disk—actually underwent significant processing near the inner disk’s water ice boundary.
This revised model explains several enigmatic characteristics of CI chondrites. These meteorites lack abundant high-temperature refractory inclusions like chondrules and CAIs, likely filtered by Jupiter’s pressure barrier. The environment near the water ice line favors sublimation and recondensation of water ice, enabling the loss of nucleosynthetic isotopes trapped in interstellar ices while preserving ice-rich compositions. Moreover, the rapid formation of CI-like bodies, evidenced by Ryugu and Bennu’s presence in the asteroid belt, fits naturally with accretion at this dynamic ice line.
The study also reinterprets the inner Solar System’s volatile inventory, suggesting that terrestrial planets, including Earth, accreted from a mix of ureilite-like rocky materials and CI-like volatile-rich compounds. This insight opens new avenues for understanding the delivery of prebiotic molecules and water essential for planetary habitability.
Concurrently, the research delineates the outer Solar System’s composition as a blend of late-infalling molecular cloud materials with isotopically anomalous organics and ices. Meter-scale clasts found in meteorites such as Zag and Isheyevo, exhibiting extreme nitrogen and hydrogen isotope anomalies akin to those of comets, are presented as remnants of freshly supplied outer disk material. These findings highlight a complex interplay where the outer disk is polluted by late-stage infall from the molecular cloud, contrasting with the more thermally processed inner disk.
This nuanced view also addresses the isotopic and organic heterogeneity in Ryugu and Bennu samples, which record diverse parent-body processing and reservoirs. The data challenge the assumption that these carbonaceous asteroids are homogeneous representatives of true outer Solar System material, emphasizing the need to reevaluate their provenance.
By integrating isotopic systematics, mineralogy, and organic chemistry, the authors offer a schematic that redefines how materials migrate and blend across the evolving protoplanetary disk. Outward transport appears constrained to refractory inclusions like CAIs, whereas fine-grained, isotopically CI-like dust can migrate inward past Jupiter’s orbit. This refined model bridges the chondrite dichotomy traditionally ascribed to formation location, proposing a more interconnected disk evolution with significant implications for planet formation theories.
Overall, this innovative research compels the scientific community to rethink the birthplace of some of the Solar System’s most pristine materials, reshaping our understanding of early Solar System dynamics and the origins of water and organics on Earth.
Subject of Research: Origin and isotopic evolution of CI chondrites and early Solar System disk dynamics
Article Title: Isotopic imprints of late molecular cloud infall in the outer Solar System
Article References:
van Kooten, E., Lodal, S.S., Onyett, I. et al. Nat Astron (2026). https://doi.org/10.1038/s41550-026-02927-1
