Two-source terrestrial planet formation with a sweeping secular resonance

TL;DR

Goldberg et al. tackle the mismatch between dynamical models that form terrestrial planets in a narrow ring and the inner Solar System’s chemical gradients. They propose a two-source framework with an inner reduced ring inside 1 au and an outer, oxidized disk that is swept inward by a secular resonance driven by a low-viscosity, wind-driven disk and Jupiter-Saturn in a 2:1 resonance. Using GPU-accelerated N-body simulations that include Type I migration, aerodynamic drag, disk gravity, and resonant perturbations, they show that sweeping resonances can concentrate outer-disk material into a Mars-like ring near 1.3–1.7 au and deliver oxidized material to proto-Earth, while a portion of outer material remains unaccreted and later implants into the asteroid belt as NC irons and aubrites. The model preserves primordial oxidation and isotopic gradients and makes testable predictions about meteoritic parent bodies and the timing of disk dissipation.

Abstract

The models that most successfully reproduce the orbital architecture of the Solar System terrestrial planets start from a narrow annulus of material that grows into embryos and then planets. However, it is not clear how this ring model can be made consistent with the chemical structure of the inner Solar System, which shows a reduced-to-oxidized gradient from Mercury to Mars and a parallel gradient in the asteroid belt. We propose that there were two primary reservoirs in the early inner Solar System: a narrow, refractory enriched ring inside of 1 au, and a less massive, extended planetesimal disk outside of 1 au with oxidation states ranging from enstatite chondrites to ordinary chondrites. We show through a suite of N-body simulations that an inwardly sweeping secular resonance, caused by aerodynamic drag and perturbations from a mean-motion resonant Jupiter and Saturn, gathers the outer planetesimal disk into a narrow ring that migrates radially, forms Mars, and contributes oxidized material to proto-Earth. Remaining unaccreted planetesimals can be implanted into the asteroid belt as the parent bodies of aubrites and non-carbonaceous iron meteorites, while the most reduced material is not implanted and thus unsampled in the meteorite collection. This model explains the oxidation and isotopic gradients within the inner Solar System within the context of a low-viscosity, magnetic wind-driven disk.

Figures (7)

• Figure 1: The locations of the secular resonances as a function of time during disk dissipation with the gas disk profile used in this work. Colors represent the three disk dissipation timescales $\tau_\textrm{diss}=\{1,2,3\}$ Myr and solid and dashed lines are the two Jupiter-Saturn configurations. The gray dotted and dash-dotted lines are a $1/r$ and broken-power law Woo2023 gas profile, respectively, but otherwise assume the same parameters as the solid green line.
• Figure 2: Typical evolution of one of our simulations, in this case using $M_\textrm{out}=\qty{0.8}{M_\oplus}$, $\tau_\textrm{diss}=\qty{2}{Myr}$ and REJSC5. Faint lines represent planetesimals and thick lines embryos (i.e., products of mergers). Lines are colored according to the planetesimal starting location. We propose that the orange and yellow particles have a reduced composition, while blue particles are oxidized, although the boundary between reduced and oxidized may not be sharp, nor does it necessarily coincide with the boundary between the swept and unswept planetesimal disks. The curved gray line marks the location of the secular resonance with Jupiter that sweeps over the inner Solar System during depletion of the gas, triggering inward migration of planetesimals and embryos. Dashed gray lines are the 3:1, 5:2, and 4:1 mean motion resonances with Jupiter.
• Figure 3: The same simulation as in Figure \ref{['fig:tracks']} but now in $a$-$e$ space. Each embryo is a pie chart representing its relative fraction of inner and outer disk material and sized according to its mass. The inner ring rapidly grows embryos that move outwards to 1 au by Type I migration. Later, the sweeping secular resonance with aerodynamic drag collects the spread-out population of outer disk planetesimals into an eccentric ring and deposits them near 1.5 au where they rapidly grow into Mars-sized embryos.
• Figure 4: Mass in each planet bin at the end of each simulation. Black points are from Nesvorny2025 at 5 Myr. Colored points are this work. Circles and crosses are $M_\textrm{out}=\qty{0.5}{M_\oplus}$ and $\qty{0.8}{M_\oplus}$ respectively. Green, blue, and red are $\tau_\textrm{diss}=$ 1, 2, and 3 Myr, respectively.
• Figure 5: Radial mass concentration ($S_c$) and fraction $p$ of initial particles from the three populations that were not accreted by the end of the integration among our simulation sets