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

Abstract

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. 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. 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. While Saturn's growth efficiently scatters CM-like planetesimals, fewer than about 2 percent are implanted beyond 15 au, even under gas-rich conditions, because gas drag damps their eccentricities and drives them back toward their perihelia rather than allowing them to circularize at larger distances. Adding an ice-giant core modestly increases the outward reach (up to about 4 percent 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 M_CM,tot about 1 M_Earth, this implies at most M_CM < 0.02-0.04 M_Earth reaches 15-25 au, corresponding to a diluted mass fraction < (1-2) x 10^-3 in the outer ring, hence negligible contamination of the CI reservoir. 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.

Figures (6)

• Figure 1: Cumulative distributions of the semi-major axis $N({\leq}a)$ of ${>}100$ km asteroids classified as CM-like, CI/IDP-like (including B types with the geometric albedo of $p_V < 0.1$), and S-type. Vertical dotted lines mark major mean-motion resonances with Jupiter. CM-like objects exhibit an approximately symmetric distribution, whereas CI-like objects show a notably asymmetric distribution. Adapted from Anderson2025.
• Figure 2: Comparison of the gas profiles used in the various simulations. The initial orbits of Jupiter, Saturn's core, and the additional ice giant core are indicated by grey vertical lines. Beyond 10 au, the profiles can be seen as Flat Raymond2017a, Sloped Cresswell2008, and with a Sharp Drop-off Desch2018.
• Figure 3: Fraction (%) of planetesimals from each initial formation zone that end up within 1-au-wide semi-major-axis bins at $t = 300,\mathrm{kyr}$ for the two-planet model (log scale). We consider implanted objects as those with $e<0.4$. In all cases, most bodies formed between 9 and 10 au remain near their birth locations, reflecting a dynamically quiet zone largely unaffected by Saturn’s growth. The prominent blue feature near $\sim$7 au corresponds to particles trapped in Saturn’s co-orbital region.
• Figure 5: Same as Fig. \ref{['fig:2P_Imp']}, but for the three-planet model. We consider implanted objects as those with $e<0.4$. Faster planetary growth clears the local planetesimal population. Objects that formed beyond 10 au are less likely to interact with the forming planetesimals.
• Figure 7: Same as Fig. \ref{['fig:2P_AE']}, but for the five-planet models Anderson2025. In both simulations, Uranus and Neptune are initialized in the 'Wide' configuration. On the left, they are allowed to migrate naturally as they grow due to Type-I torque. On the right, the semi-major axes are fixed. Planetesimals are color-coded based on their formation location. In the simulations in which the ice giant cores are allowed to migrate, surviving planetesimals are circularized quickly.