Shaken, not stirred: Inefficient mixing of CM- and CI-like materials

Context. A recent study suggests that CM chondrite-like planetesimals formed in the vicinity of Saturn, in a pressure bump outside the gap carved by proto-Jupiter. While a fraction of these objects was implanted into the asteroid belt as a consequence of Saturn’s growth, it remains unclear whether the scattered remainder could reach the ice giant region and mix with more distant carbonaceous reservoirs. Aims. We test whether outward scattering during Saturn’s growth and migration can implant CM-like bodies onto long-lived orbits in the Uranus-Neptune region, where they could contaminate the CI reservoir. Methods. We performed N-body integrations of 100-km planetesimals launched from the outer edge of Jupiter’s gap, including gas drag and the gravitational perturbations of growing Jupiter and Saturn, with optional inclusion of a nearby ice giant embryo. We explored a range of gas surface-density profiles and growth timescales. Results. While Saturn’s growth efficiently scatters CM-like planetesimals, fewer than ∼2% are implanted beyond 15 au, even under gas-rich conditions, because gas drag damps their eccentricities and drives them back toward their pericenters rather than allowing them to circularize at larger distances. Adding an ice giant core modestly increases the outward reach (up to ∼4% in the most gas-rich case), but Type-I migration further lowers perihelia, making long-term retention at large distances difficult. For a CM mass budget of MCM,tot ~ 1M⊕, this implies that at most MCM ≲ 0.02-0.04 M⊕ reaches 15-25 au, corresponding to a diluted mass fraction of ≲ (1-2) × 10−3 in the outer ring, and hence negligible contamination of the CI reservoir. Conclusions. Combined with the distinct radial distributions of CM- and CI-like asteroids in the belt, these results imply limited mixing of carbonaceous reservoirs and isolation of the CI reservoir. This strongly suggests that Uranus and Neptune formed later than Jupiter and Saturn, under lower gas densities, supporting a sequential giant-planet formation model.