Katherine I. Dale | Searcharxiv

arXiv2026

Oxidation Constraints on Terrestrial Planet Formation from a Ring

The present-day solar system comprises meteorites with varying oxidation levels, derived from different parent bodies. Previous studies (e.g. Rubie et al., 2011) of the partitioning of siderophile elements between mantle and core during planetary growth and differentiation showed that Earth must accrete reduced bodies first and oxidised bodies later. Here we show that, if the terrestrial planets formed from a narrow ring of planetesimals, this condition is not fulfilled, whatever heliocentric gradient of oxidation is assumed in the ring. The reason is that planetary embryos quickly accrete planetesimals from the whole width of the ring, incorporating both reduced and oxidised material. The partially oxidised state of all planetary embryos leads to mismatches with the composition of the bulk silicate Earth (BSE) because oxygen fugacity strongly affects the partitioning of siderophile elements. We demonstrate that reproducing the BSE composition requires reduced and oxidised reservoirs to remain segregated until embryo formation is almost complete. The delivery of oxidised material to the terrestrial planet-forming ring towards the end of the disc's lifetime is therefore a key requirement of any successful dynamical model of terrestrial planet formation.

astro-ph.EP↗

arXiv2025

Compositional Outcomes of Earth Formation from a Narrow Ring

We address Earth formation from an elemental perspective, using a method similar to Rubie et al. (2015) but with updates from Dale et al. (2023) to simulate the chemical evolution of Earth's mantle during metal-silicate equilibration events from accretional collisions. Our model introduces two key differences: (1) Earth forms from a dense ring of planetesimals and planetary embryos near 1 AU, extending into the asteroid belt, and (2) we divide this population into four zones. The innermost zone contains planetesimals enriched in refractory elements relative to Si and depleted in volatiles. The remaining zones represent enstatite, ordinary, and CI chondrites. We fit the Earth's bulk silicate composition by adjusting the boundaries of these zones and the refractory enrichment in the inner zone, giving us four compositional free parameters. A fifth parameter relates to the depth of planetesimal equilibration after a giant impact. We examined twenty-two ring model simulations, expanded to forty-eight based on hot or cold targets during collisions. Seventeen simulations resulted in a mantle chemistry resembling the bulk silicate Earth (BSE), despite differences in growth sequences. These variations lead to different fitting parameter values, altering the proportions of different meteorite types required to match the BSE. However, findings show Earth must accrete 60-80% of material from the innermost refractory-enriched zone. This indicates that, with the right growth sequence, multiple ring model structures can yield an Earth-anaologue composition consistent with the BSE.

astro-ph.EP↗