Mercury’s Iron Heart: How Simulations Reveal the Origins of the Solar System’s Most Metal-Rich Planet

Mercury’s unusually large iron core has long puzzled scientists. With a core mass fraction (CMF) of about 70%, it is more than twice as iron-rich as Earth, Venus, or Mars. Haniyeh Tajer and collaborators explore how such a planet could have formed using N-body simulations, numerical models that track the gravitational interactions and collisions of thousands of growing planetary bodies. Their work examines whether Mercury’s massive core is the result of violent impacts that stripped away its outer layers or whether it formed in an iron-rich region near the early Sun.

The Mystery of Mercury and the Rise of “Exo-Mercuries”

Mercury’s density is extreme even among the rocky planets, suggesting an unusually high metal content. Yet, observations of volatile elements on its surface argue against the idea that it lost its mantle in a single catastrophic collision. Tajer and colleagues also note that astronomers have begun to find “Exo-Mercuries”, dense exoplanets orbiting other stars that appear to share Mercury’s iron-rich composition. Understanding Mercury’s origin, therefore, could help explain the formation of these distant cousins as well.

Two main ideas dominate the debate. One is the giant impact hypothesis, where collisions during the late stages of planet formation could strip away a planet’s rocky mantle. The other is the iron-enriched disk hypothesis, in which the innermost parts of the solar nebula naturally produced metal-rich planetesimals because of conditions like strong magnetic fields or high temperatures. Tajer’s team tests both possibilities.

Simulating Planet Formation

Using the REBOUND code, the researchers modeled the chaotic endgame of planet formation. They included fragmentation physics, which allowed bodies to erode or break apart rather than merging perfectly. Their simulations began with disks of planetesimals and planetary embryos distributed between 0.3 and 2 astronomical units (AU) from the Sun. They varied both the surface density profiles (how mass is spread across the disk) and the initial CMF distributions, either uniform (Earth-like) or step-function patterns, where the inner region starts more iron-rich.

To track how planetary compositions evolved, the authors employed a mantle stripping model, which followed how iron and silicate material redistributed during collisions. They compared three sets of disk setups drawn from earlier studies, two from Chambers (2013) and one from Ueda et al. (2021), to test how different initial conditions affected Mercury-like outcomes.

Results: Uniform vs. Step-Function Disks

When all planetesimals began with uniform Earth-like compositions, the simulations failed to produce Mercury analogs. Even when impacts generated iron-rich fragments, these later merged with lower-CMF material, averaging out to moderate CMFs near 0.3–0.4. The results suggested that giant impacts alone cannot explain Mercury’s composition, echoing earlier findings that such events are too rare and inefficient.

However, when the team introduced step-function CMF distributions, iron-rich inner regions and silicate-rich outer regions, Mercury-like planets emerged naturally. These high-CMF worlds formed near the Sun, while more Earth-like planets developed farther out, closely mirroring the architecture of the inner solar system. The best results occurred when the inner iron-rich region extended to about 0.8 AU, and about 40% of the disk’s iron was concentrated inward.

Discussion: Limits and Implications

The study also explored key challenges. Collisions between smaller planetesimals are more effective at stripping mantles, but simulating these tiny bodies is computationally expensive. Additionally, the model lacks a mechanism to remove ejected mantle debris, a process that might have been helped by solar radiation or disk winds. Tajer and collaborators stress that including these physical effects and using higher-resolution simulations could change the outcome.

Interestingly, even when high-CMF planets formed, they were typically too massive compared to real Mercury, a common issue in solar system formation models. Nonetheless, these simulations demonstrate that a chemically varied early solar nebula, not just random impacts, could have been key to creating Mercury and similar exoplanets.

Conclusion

Tajer and her team conclude that Mercury’s “big heart” likely originated from an iron-rich inner disk, not from late-stage collisions alone. Their N-body models show that initial chemical gradients in the early solar system can naturally lead to both Mercury-like and Earth-like planets in the same system. This finding has broader implications for the growing population of dense exoplanets, suggesting that the processes that forged Mercury may be common throughout the galaxy.

Source: Tajer