Preliminary constraints on Solar System formation scenarios via models of isotopic fractionation during terrestrial core formation

Modern simulations of planet formation can successfully build the Solar System, matching astrophysical properties such as the masses and orbits of the terrestrial planets. Problematically, there are now multiple and conflicting hypothesized planet formation scenarios that match these astrophysical including the Grand Tack Scenario (Jacobson and Morbidelli, 2014), the Early giant planet instability (Clement et al., 2018), pebble accretion (Johansen et al., 2021), inside-out growth (Walsh and Levison, 2019), and narrow planetesimal disk-driven models (Morbidelli et al., 2022). A major difference between these proposed models of Solar System formation scenarios is the degree of radial mixing required to form the planets. To further distinguish between these astrophysically-successful models of Solar System formation, we must use geochemical constraints.

Observed chemical and isotopic heterogeneity between Solar System bodies implies that these bodies are sourced from different building blocks or were fractionated by formation processes or both. Metal-silicate equilibration during core formation is frequently proposed as a mechanism to account for observed isotopic heterogeneities, particularly in Fe and other siderophile elements that segregate to the core of a growing planet (e.g., Poitrasson et al., 2004; Shahar et al., 2016; Ni et al., 2022). However, experimental data of isotopic fractionation at core formation conditions is limited, and existing literature relies on models that use oversimplified single-stage core formation models for interpretation. For computational convenience, single-stage models assert that the entire mantle and core of a planet chemically equilibrate with each other-this is geophysically unlikely but becomes physically impossible at the mid-mantle pressures and temperatures that are often required to match the geochemistry of the mantle (Righter, 2003; Rubie et al., 2003). Instead, we use a model based on Rubie et al. (2011, 2015) that combines astrophysical N-body simulations of Solar System formation and a mass-balanced approach to tracking both the elemental and isotopic composition during multiple stages of core-mantle differentiation and re-equilibration. Here, we present modeling results of the effects of dynamical planetary accretion histories, radial mixing, and planetary core formation on notable stable and radiogenic isotopic systems during the formation of the Earth. This approach leverages geochemical constraints to determine the viability of various astrophysically-successful Solar System formation scenarios.