Compositional Outcomes of Earth Formation from a Narrow Ring

Abstract

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.