Forming Mercury from excited initial conditions

TL;DR

This study uses simulations to explore Mercury's formation, finding that Mercury-like bodies can form with certain initial conditions, but their composition and stability depend on debris dynamics and initial disk configurations.

Contribution

It introduces a new simulation approach including an inner embryo disk and analyzes Mercury analog formation and stability in this context.

Findings

Mercury analogues with high core-mass fractions form in about 10% of simulations.

Most Mercury analogues have lower core-mass fractions (~0.7) due to debris accretion.

Formation rates are similar with or without an inner embryo disk.

Abstract

Mercury is notoriously difficult to form in solar system simulations, due to its small mass and iron-rich composition. Smooth particle hydrodynamics simulations of collisions have found that a Mercury-like body could be formed by one or multiple giant impacts, but due to the chaotic nature of collisions it is difficult to create a scenario where such impacts will take place. Recent work has found more success forming Mercury analogues by adding additional embryos near Mercury's orbit. In this work, we aim to form Mercury by simulating the formation of the solar system in the presence of the giant planets Jupiter and Saturn. We test out the effect of an inner disk of embryos added on to the commonly-used narrow annulus of initial material. We form Mercury analogues with core-mass fractions (CMF) $> 0.4$ in $\sim 10\%$ of our simulations, and twice that number of Mercury analogues form during the formation process, but are unstable and do not last to the end of the simulations. Mercury analogues form at similar rates for both disks with and without an inner component, and most of our Mercury analogues have lower CMF than that of Mercury, $\sim 0.7$, due to significant accretion of debris material. We suggest that a more in-depth understanding of the fraction of debris mass that is lost to collisional grinding is necessary to understand Mercury's formation, or some additional mechanism is required to stop this debris from accreting.