Bridging the Gap: How Carbonaceous Chondrites Connect the Inner and Outer Solar System

Asteroids preserve some of the earliest solid materials formed in the Solar System, but their narrow concentration in the asteroid belt belies the wide chemical and isotopic diversity of meteorites, suggesting they were implanted from a broad range of heliocentric distances [1, 2]. Isotopic analyses reveal distinct reservoirs for non-carbonaceous and carbonaceous chondrites, with further divisions among CC types, notably between CM and CI chondrites [3, 4, 5]. Spectroscopic surveys [6, 7, 3] show that large (D > 100 km) CM-like planetesimals display a symmetrical distribution across the asteroid belt, while CI-like asteroids follow a more asymmetric profile [8]. Our previous work [8] demonstrated that this dichotomy reflects two separate implantation events: CM-like bodies likely formed in a pressure bump just outside Jupiter’s early gap [4] and were implanted during Saturn’s growth, while CI-like material formed farther out, beyond the ice giants, and was implanted later during Uranus and Neptune’s migration [9]. This interpretation, consistent with the chondrule-poor nature of CIs, supports a chronology in which asteroids record distinct stages of giant planet formation and migration, providing a key to unlocking the early dynamical evolution of the Solar System. However, the final fate of the remaining population, which was scattered outwards, remains poorly constrained.

Here, we use an orbital model to investigate the injection of planetesimals into the ice-giant formation zone belt following giant planet growth in the protoplanetary disk (PSD) to determine whether CM-like material plausibly became incorporated into the building blocks of Uranus and Neptune (the proto-ice giants) and their satellites, to better constrain the compositional building blocks in the outer Solar System as a whole. We conduct simulations using REBOUND, including 100-km-sized planetesimals evolving under gas drag and the gravitational influence of growing giant planets within various gas disk profiles and planetary configurations, considering two, three, or five planets at a time.

 

Fig.1:  Final eccentricity and semi-major axis of our remaining particles after 1 Myr of simulation time in the three-planet model, for a moderately sloping gas profile following Σ_g ∝ r^−0.5. While some objects remain at a_i > a_Sat, they are not easily implanted there.

We find that only a small fraction of CM-like bodies can be circularized and implanted beyond Saturn’s orbit, even in gas-rich disks (Fig. 1). Most are either ejected or remain on eccentric orbits, reinforcing the notion that mixing between planetesimal reservoirs was minimal. This supports recent models of the outer Solar System as a series of compositionally distinct, radially confined planetesimal rings [10], potentially separated by gas gaps that further inhibited material exchange.

Our results provide further evidence that CI chondrites originated in a spatially and temporally isolated reservoir, likely the trans-Uranian region, and were implanted much later. As such, the CI-rich asteroids in the main belt may represent the most distant material ever delivered to the inner Solar System, now made accessible by recent sample return missions [11, 12, 13, 14]. These findings position carbonaceous chondrites as a crucial link between the inner and outer Solar System and offer a powerful record of early giant planet dynamics and compositional evolution.

References

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