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

TL;DR

This work investigates why Mercury exhibits a disproportionately large iron core by testing two formation pathways: late-stage mantle stripping via giant impacts and early iron enrichment of inner-disk material. It employs N-body simulations of late-stage planet formation with a fragmentation-based collision model implemented in REBOUND to capture realistic collision outcomes, including erosive events. By contrasting a uniform initial CMF with a step-function inner-disk CMF and employing different surface-density prescriptions, the study finds that mantle-stripping alone does not reliably produce a high $CMF$, whereas an inner-disk enhanced CMF can yield a Mercury-like planet alongside Earth-like planets, aligning with solar system architecture. The results provide a framework to extend these analyses to exoplanet analogs, offering insight into the origin of high-CMF rocky worlds such as Exo-Mercuries.

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. 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 particles, without mechanisms that deplete stripped mantle material, these particles 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.

Figures (1)

• Figure 1: Mass-density relationships for a sample of exoplanets, including exo-Mercuries. We derive this sample from the NASA exoplanet archive christiansen_nasa_2025, only keeping planets that have mass, radius, and stellar effective temperature measurements. The blue, red, and black curves are mass-density relationships for a planet with Earth-like, Mercury-like, and pure iron composition. These curves are derived from planetary interior models made with ExoPlexUnterborn_2023. We colored planets whose measured bulk densities were higher than the predicted Earth-like density given their mass. Colors represent the stellar effective temperature. The gray points are all the other planets.