Mercury as the relic of Earth and Venus' outward migration

TL;DR

This paper explores how early inward and outward migration of planetary embryos in the inner solar system can explain Mercury's unique characteristics, including its small mass, orbit, and composition, through advanced numerical simulations.

Contribution

It develops a refined model of planetary migration that accounts for Mercury's properties, integrating recent high-resolution accretion simulations and compositional predictions.

Findings

Mercury-like planets can form from migrating embryos in simulations.

Earth and Venus likely accreted mostly dry material during migration.

Venus' composition should be similar to Earth's, with Mercury showing diverse compositions.

Abstract

In spite of substantial advancements in simulating planet formation, the planet Mercury's diminutive mass, isolated orbit, and the absence of planets with shorter orbital periods in the solar system continue to befuddle numerical accretion models. Recent studies have shown that, if massive embryos (or even giant planet cores) formed early in the innermost parts of the Sun's gaseous disk, they would have migrated outward. This migration may have reshaped the surface density profile of terrestrial planet-forming material and generated conditions favorable to the formation of Mercury-like planets. Here, we continue to develop this model with an updated suite of numerical simulations. We favor a scenario where Earth and Venus' progenitor nuclei form closer to the Sun and subsequently sculpt the Mercury-forming region by migrating towards their modern orbits. This rapid formation of ~0.5 Earth-mass cores at ~0.1-0.5 au is consistent with modern high-resolution simulations of planetesimal accretion. In successful realizations, Earth and Venus accrete mostly dry, Enstatite Chondrite-like material as they migrate; thus providing a simple explanation for the masses of all four terrestrial planets, inferred isotopic differences between Earth and Mars, and Mercury's isolated orbit. Furthermore, our models predict that Venus' composition should be similar to the Earth's, and possibly derived from a larger fraction of dry material. Conversely, Mercury analogs in our simulations attain a range of final compositions.