Origins of Mercury’s Big Heart of Iron: Exploring Pathways to Form High Core Mass Fraction Planets via N -body Simulations

Abstract Mercury’s core mass fraction (CMF) is 0.7, more than double that of the other rocky planets in the solar system, which have CMFs of 0.3 or lower. The origin of Mercury’s large, iron-rich core remains unknown. Adding to this mystery, an elusive population of “exo-Mercuries” with high densities is emerging. Therefore, understanding the formation of Mercury and its exoplanetary analogs is essential to developing a comprehensive planet formation theory. Two hypotheses have been proposed to explain the high CMF of Mercury: (1) giant impacts during the latest stages of planet formation strip away mantle layers, leaving Mercury with a large core; and (2) earlier-stage iron enrichment of planetesimals closer to the Sun leads to the formation of an iron-rich planet. In this work, we conduct N -body simulations to test these two possibilities. Our simulations are focused on the solar system, however, we aim to provide a framework that can later be applied to the formation of high-CMF exoplanets. To investigate the giant impact scenario, we employ uniform initial CMF distributions. To address the other hypothesis, we use a step function with higher CMFs in the inner region. For a uniform initial CMF distribution, our results indicate that although erosive impacts produce iron-rich planetesimals, without mechanisms that deplete stripped mantle material, these planetesimals merge with lower-CMF objects and do not lead to Mercury’s elevated CMF. However, a step-function initial CMF distribution leads to the formation of a high-CMF planet alongside Earth-like planets, resembling the architecture of the terrestrial solar system.