Fig. 4. Schematic of the possible source region of Cb-type asteroids and CI chondrites.

Planetesimals formed in different regions of the protoplanetary disk. Volatile-poor planetesimals (red circles) formed in the inner region, while volatile-rich planetesimals (blue circles) formed beyond Jupiter’s orbit. The growth and migration of the gas and ice giant planets implanted some of the planetesimals into the Main Belt (small arrows), while the majority of planetesimals were transported outward or ejected from the disk (large arrows) (39). A plausible explanation for the distinct Fe nucleosynthetic heritage and primitive chemical composition of CI chondrites and Ryugu is that they were implanted in the Main Belt by excitation from Uranus and Neptune (filled bright blue circles), while other CCs formed in more internal regions near Jupiter and Saturn (filled dark blue circles) (fig. S4). The icy planetesimals that were formed around Uranus and Neptune and were ejected outward went to populate the Oort cloud (44, 45). CI chondrites and Ryugu may thus share some parentage with long-period comets. Such a scenario could explain the trichotomy between NC, CC, and CI for nucleosynthetic anomalies (Fig. 3).