An early giant planet instability recorded in asteroidal meteorites

Planetary migration, a widespread phenomenon observed in numerous planetary systems, including our own, involves the movement of planets from their initial formation locations. The timescales of this migration are crucial for understanding the underlying dynamical processes. In our Solar System, evidence suggests giant planet migration (GPM) significantly altered the asteroid belt's structure and composition through collisions. This study seeks to determine the precise timescale of GPM and the mechanism responsible by analyzing the thermochronological records preserved in asteroidal meteorites. These records, specifically the ⁴⁰K-⁴⁰Ar system ages, capture the thermal imprints of energetic collisions associated with GPM, providing a unique window into the timing and intensity of past bombardment events.

Several lines of evidence point toward at least one episode of GPM in the Solar System's history. The mixing of inner and outer Solar System materials in asteroids necessitates dynamical mixing of protoplanetary reservoirs. The current orbital architecture of giant planets and the Kuiper Belt, along with the low masses of Mars and the asteroid belt, strongly suggest a history of orbital excitation and migration. Two main mechanisms are proposed for GPM: dynamical instability triggered by gravitational interactions between planets, and inward migration driven by tidal interactions with a gaseous protoplanetary disk (Type II migration). The presence or absence of a gaseous disk distinguishes these mechanisms, and constraining the timescale of migration can help to identify which mechanism best explains our Solar System's evolution. The Late Heavy Bombardment (LHB) hypothesis, initially proposed to explain enhanced lunar bombardment, has been debated extensively. While not universally accepted, the LHB highlights the potential for significant bombardment events following GPM. However, recent studies suggest a more monotonic decline in bombardment flux, questioning the existence of a distinct LHB.

This study focuses on the ⁴⁰K-⁴⁰Ar system in chondrite meteorites (ordinary chondrites, enstatite chondrites, and Rumuruti-type chondrites) from the inner Solar System. This system is sensitive to resetting at relatively low temperatures, making it ideal for capturing the subtle thermal signals of impact heating. A comprehensive database of ⁴⁰K-⁴⁰Ar system ages was compiled, with a preference for higher-resolution ⁴⁰Ar-³⁹Ar ages over K-Ar ages due to the latter's susceptibility to partial resetting. Ages younger than 2 Ga were excluded to focus on the timescales of potential GPM. To disentangle endogenic and exogenic heating effects, a Bayesian statistical approach was employed. A Markov chain Monte Carlo (MCMC) algorithm was used to explore the parameter space of an asteroid-scale thermal code. The code models radiogenic heating and conductive cooling, with superimposed impact reheating from exponentially decaying bombardment fluxes. The model is constrained by two sets of priors: the database of ⁴⁰K-⁴⁰Ar system ages and published constraints on thermochronologic model parameters. The MCMC algorithm yields posterior estimates of the parameters, including the timescales of GPM and the intensity of bombardment events.

Analysis of the >2 Ga ⁴⁰Ar-³⁹Ar age distribution revealed a need for one or more bombardment events to reproduce the observed data. Simulations without impact reheating failed to match the age distribution's shape. Models incorporating impact reheating yielded posterior distributions consistent with the prior distributions of ⁴⁰Ar-³⁹Ar ages and model parameters. The preferred model involved two bombardment events: a primordial, mild/protracted bombardment reflecting long-term collisions, and a post-accretion, intense/brief bombardment event associated with GPM. The post-accretion bombardment had a median onset date of 11.3^+4.4_−11.0 Myr (50% credible interval), with a mean of 15 ± 14 Myr (1σ). The intense/brief bombardment event is consistent with theoretical predictions of GPM-associated bombardment in the inner Solar System, with an e-folding timescale similar to other models' most intense and short-lived components. The timing of the GPM-induced bombardment, post-dating the likely timeframe of gas disk dissipation (3-5 Myr), points towards a dynamical instability triggered after gas disk dissipation as the most likely mechanism for GPM in the Solar System. This is supported by the overlap of the 50% CI of the bombardment onset date (4.75–20.76 Myr) with the timescales of instability from self-unstable orbital architectures left behind after gas disk dissipation.

The findings provide strong evidence for a single GPM event in the Solar System's early history, significantly predating any canonical LHB. The timing strongly suggests a dynamical instability rather than a gas-driven migration, as the majority of the posterior distribution overlaps timescales of instability after gas dissipation. The study refutes the hypothesis of an intense bombardment occurring before 100 Myr. While the precise timing within the first 100 Myr isn't uniquely constrained, the most probable scenario is instability triggered by unstable giant planet orbital configurations, possibly exacerbated by interaction with a massive outer planetesimal disk. Gas dissipation is implicated as a critical factor in triggering or predisposing the system to instability.

This study offers a precise cosmochronological constraint on the timescale of GPM in our Solar System, supported by a substantial portion of the meteorite record. The results strongly suggest that GPM in our Solar System resulted from a dynamical instability shortly after the dissipation of the gaseous protoplanetary disk, likely due to inherent orbital instability without the gas disk's stabilizing influence. Future research should refine these findings by expanding meteorite thermochronometry, improving thermal models, and providing greater astronomical context. The results also motivate observational focus on young exoplanetary systems to potentially observe similar migration and instability events.

The model simplifies meteorite thermal histories, particularly by ignoring impact heating before thermochronologic closure. This limits the sensitivity of the model to early, high-flux bombardment scenarios. The model assumes constant material parameters, while these are known to vary with temperature and shock histories. The assumption that parent bodies were not catastrophically disrupted before widespread cooling also needs further investigation. However, the model's ability to accurately reproduce the broad ⁴⁰Ar-³⁹Ar thermochronologic record and its concordance with existing constraints on chondritic planetesimals support its overall validity.