An early giant planet instability recorded in asteroidal meteorites

The study addresses when and how the Solar System’s giant planets migrated, a process thought to be common in exoplanetary systems. Two principal mechanisms are considered: (1) dynamical instability among giant planets in the presence of an outer planetesimal disk (self- or disk-triggered), and (2) gas-driven (type II) migration within a still-present protoplanetary gas disk. Because these mechanisms operate on distinct timescales relative to gas-disk dissipation, constraining the timing of migration can diagnose its cause. Giant planet migration is expected to dynamically excite and scatter small bodies, producing an observable episode of enhanced collisions. The authors therefore use the thermochronologic record of asteroidal meteorites—specifically 40K-40Ar system ages—to infer the timing of any early bombardment in the asteroid belt and, by proxy, the timing of giant planet migration.

Past work invoked a Late Heavy Bombardment (~4.0 Ga) to explain lunar records, but subsequent analyses showed that lunar crater counts and thermochronology can be reconciled with a monotonic decline in impact flux without a sharp LHB spike. Independent dynamical and meteoritical constraints increasingly suggest any giant planet instability occurred within the first ~100 Myr of Solar System history. Early migration models include the Grand Tack (gas-driven, requiring a gas disk within ~1–5 Myr) and instability scenarios that occur during or after gas dissipation. Self-unstable orbital architectures can destabilize within ~5–15 Myr after gas dispersal, whereas planetesimal disk–triggered instabilities typically occur >15–100 Myr after gas loss. Observations of protoplanetary disks indicate dissipation usually within <5–10 Myr, setting tight windows to distinguish mechanisms.

- Data compilation: The authors compiled a database (n=203) of chondritic 40K-40Ar system cooling ages, including K-Ar (n=67) and 40Ar-39Ar (n=136) measurements, focusing on inner Solar System chondrites (ordinary H, L, LL; enstatite EH, EL; and Rumuruti). To avoid biases from low-temperature partial resetting in K-Ar data and young collisional events unrelated to giant planet migration, only >2 Ga 40Ar-39Ar ages (n=97) are used as priors in the inversion. - Calibration: Ages were recalibrated to a common framework using the SJ77 40K decay constants and consistent 40K/K values. Alternative calibration schemes were tested to evaluate sensitivity; results remained robust regarding the inferred timing. - Thermal model: An analytical solution simulates radiogenic heating and conductive cooling of a spherical asteroid with constant material properties and an accretion time relative to CAI-defined solar time-zero (4,567.3 Ma). Effective argon closure temperature is treated as a distributed parameter to encompass sample heterogeneity. - Impact reheating: Bombardments are represented as exponentially decaying fluxes characterized by onset time (t0), initial flux (F0), and e-folding time (τ). Reheating resets the 40K-40Ar system in fractional volumes across shells; impacts are parameterized probabilistically (cone-like reheating geometry) without explicit excavation/implantation. Up to three fluxes (α, β, γ) are allowed; α is treated as a primordial, slowly decaying background flux anchored at t0=0 Myr, while β and γ are post-accretion events with t0 explored. - Bayesian inversion: A Markov chain Monte Carlo framework (ImpactChron.jl, Julia) explores posterior distributions of environmental, cosmochemical, asteroidal, material, and bombardment parameters. Priors for non-bombardment parameters are compiled from literature (mostly log-normal), while bombardment parameters use uninformative uniform priors over the model time domain. The likelihood combines the fit to the observed age distribution and agreement with parameter priors. Model selection is guided by log-likelihood comparisons among 0-, 1-, 2-, and 3-bombardment scenarios. - Bias control: Petrologic-type weighting is applied to align simulated volumetric contributions with the observed sampling of types 3–7/melt in the age database, partially correcting for sampling biases.

- Necessity of bombardment: No-impact models fail to reproduce the early sharp peak, subsequent monotonic decline, and paucity of ages between 3.5–2.0 Ga. Log-likelihoods improve substantially when impacts are included (no impacts l ≈ −1,016 ± 2 vs. with impacts l ≳ −1,000). - Number and character of events: A two-bombardment model (one primordial background flux plus one post-accretion event) is sufficient and preferred, with performance comparable to three-flux models. - Post-accretion bombardment timing and intensity: The GPM-related bombardment is intense and brief, with a median onset at 11.3 Myr after CAIs. Reported uncertainty ranges include a 50% credible interval roughly spanning ~4.75–20.76 Myr, mean ≈ 15 ± 14 Myr; alternative calibrations yield similar or slightly later medians (>12 Myr). The event dissipates rapidly (τ on order of ~10–20 Myr) and at onset resets a large fraction (order ~50%) of near-surface asteroid volume in simulations. - Primordial/background flux: The background bombardment is mild and protracted with e-folding timescales of order ~200–400 Myr, consistent with secular leakage models for inner Solar System impact flux. - Rejection of a late heavy bombardment spike: The >2 Ga 40Ar-39Ar age distribution shows a predominantly monotonic decline from early times to ~3.0 Ga, inconsistent with a pronounced spike at ~4.0 Ga; models explicitly testing an LHB are disfavored. - Mechanism inference: Only ~20–26% of the onset-time posterior overlaps the ≤5 Myr window when gas could drive a Grand Tack–style migration, while >75% overlaps post-gas dissipation timescales. The favored interpretation is a dynamical instability shortly after gas dispersal, likely from self-unstable configurations and/or interaction with an outer planetesimal disk.

The study links the thermochronologic record of asteroidal meteorites to the dynamical history of the Solar System. The requirement of at least one early, intense, and brief bombardment to fit the >2 Ga 40Ar-39Ar age distribution indicates a giant planet migration episode occurred very early, with a median onset ~11 Myr after CAIs. The timing aligns poorly with gas-embedded, type II migration scenarios (Grand Tack), which must occur during the short-lived gas disk phase (≲5 Myr), and aligns better with a dynamical instability triggered after gas dispersal. The inferred intense/brief bombardment’s decay timescale matches expectations for dynamical cooling after planetary instability and the early, short-lived peak seen in impact-flux models. The background mild/protracted flux reproduces the secular long tail of impacts. Collectively, the findings support a single early GPM-induced instability that structured the asteroid belt and, by extension, the inner Solar System. The results reject a Late Heavy Bombardment spike at ~4 Ga and instead place major dynamical excitation in the first tens of Myr, consistent with independent meteoritic (e.g., Pd-Ag ages) and dynamical constraints.

This work provides a quantitative cosmochronological constraint on the timing of giant planet migration in the Solar System by inverting a curated database of chondritic 40Ar-39Ar ages with a physics-based asteroid thermal model. The preferred scenario features a mild, secular primordial flux and an intense, brief bombardment beginning near 11 Myr after CAIs, indicating a giant planet dynamical instability shortly after gas-disk dispersal. The analysis disfavors a Grand Tack-type gas-driven migration and a Late Heavy Bombardment spike at ~4 Ga. Future improvements should include expanded meteorite thermochronology, refined 40K decay constant calibrations for early Solar System timescales, better understanding of argon release behavior in chondrites, more physical impact-heating geometries, and integration with asteroid sample-return constraints. Observational and dynamical studies should emphasize young (≲20 Myr) planetary systems likely to exhibit or preserve signatures of recent instabilities.

- Model simplifications: Two-stage approach (analytical radiogenic cooling plus non-physical, probabilistic impact reheating), constant material properties, and simplified reheating geometry (cone to center) without explicit excavation or implantation. - Thermochronologic sensitivity: Insensitivity to impacts occurring before Ar closure (outer shells typically ≥5 Myr), limiting constraints on very early (<~5 Myr) bombardment onsets. - Data scope and biases: Focus on inner Solar System chondrites (excluding carbonaceous and achondrites) and non-random sampling of meteorites; petrologic-type weighting partially addresses but cannot eliminate bias. - Parameter uncertainties: Effective Ar closure temperature treated as a distribution; uncertainties in 40K decay constants and age recalibration persist despite tests indicating robustness of the main conclusions. - Event quantification: Impact flux parameters (F0, τ) are non-physical proxies for resetting intensity and duration; absolute fluxes should not be interpreted literally.