Dynamical avenues for Mercury's origin II: in-situ formation in the inner terrestrial disk

TL;DR

This study explores in-situ formation of Mercury in a depleted inner terrestrial disk, identifying initial conditions that produce Mercury analogs consistent with observed planetary characteristics.

Contribution

It demonstrates that Mercury could have formed locally in a mass-depleted inner disk, using specific disk profiles and mass distributions often neglected due to computational challenges.

Findings

Moderate initial masses (0.1-0.25 Earth masses) best reproduce Mercury.

Shallow surface density profiles improve Mercury formation likelihood.

Larger initial masses tend to produce overly massive Mercury analogs.

Abstract

Modern terrestrial planet formation models are highly successful at consistently generating planets with masses and orbits analogous to those of Earth and Venus. In stark contrast to classic theoretical predictions and inferred demographics of multi-planet systems of rocky exoplanets, the mass (>10) and orbital period (>2) ratios between Venus and Earth and the neighboring Mercury and Mars are not common outcomes in numerically generated systems. While viable solutions to the small-Mars problem are abundant in the literature, Mercury's peculiar origin remains rather mysterious. In this paper, we investigate the possibility that Mercury formed in a mass-depleted, inner region of the terrestrial disk (a < 0.5 au). This regime is often neglected in terrestrial planet formation models because of the high computational cost of resolving hundreds of short-period objects over ~100 Myr timescales. By testing multiple disk profiles and mass distributions, we identify several promising sets of initial conditions that lead to remarkably successful analog systems. In particular, our most successful simulations consider moderate total masses of Mercury-forming material (0.1-0.25 Earth masses). While larger initial masses tend to yield disproportionate Mercury analogs, smaller values often inhibit the planets' formation as the entire region of material is easily accreted by Venus. Additionally, we find that shallow surface density profiles and larger inventories of small planetesimals moderately improve the likelihood of adequately reproducing Mercury.