Extended Data Fig. 1: Workflow of asteroid-scale thermochronologic model.

My_ss denotes My (Myr) after solar system formation. (a) We use an analytical solution to the heat equation in a radiogenically heated, conductively cooling spherical body. Each curve traces the time-temperature history at a depth in the simulated asteroid after instantaneous accretion (solid grey line). As the temperature of a given depth passes below the effective closure temperature of Ar (dashed black line), we assign that timestep as the Ar-Ar age (panel b). Panels a and b share color scales and a logarithmic timescale. The parameters used in these simulations are the priors’ central tendencies in Table 1. (c) We calculate a distribution of Ar-Ar cooling ages from the calculated ages and volumetric proportions of each simulated radial shell (black curve, labeled “Unweighted”). We assign a petrologic type to each depth in the body based on the peak temperature of its time-temperature history (a) and recalculate a petrologic type-weighted distribution of ages ("Weighted”, red curve). This step increases the proportion of early ages from shallower depths. The blue curve ("Impact Reheated”) depicts the effect of impact reheating by the primordial impact flux depicted in panel d. (d) For simulations with bombardment histories, we “reheat” the body at a range of depths with one or more exponentially decaying fluxes of impacts. Panel d depicts two such fluxes: a “primordial” flux anchored to the solar age (0 My_ss) and a “post-accretion” flux beginning 300 My_ss. The primordial flux has a lower initial flux (20 My⁻¹) and longer e-folding timescale (200 My), resulting in a mild/protracted bombardment. The post-accretion flux has a higher initial flux (100 My⁻¹) and shorter e-folding timescale (20 My), resulting in an intense/brief bombardment.