New study suggests Mercury’s giant core came from a collision of twin planets

Researchers used smoothed particle hydrodynamics simulations to test how different types of collisions might reproduce Mercury’s unusual structure. They found that near-grazing impacts between protoplanets of similar mass could yield a Mercury-like planet within less than 5 percent of its actual mass and composition.

These models show that Mercury’s current mass, about 0.055 Earth masses, and its extreme silicate to iron ratio of roughly 30 to 70, could be reproduced without invoking rare or exceptional events. The collision would have stripped away as much as 60 percent of Mercury’s original mantle, leaving a planet dominated by its metallic core.

Most earlier theories suggested Mercury formed after a collision with a much larger body that tore away its mantle. But dynamical models of planetary accretion show such unequal impacts are relatively uncommon. Encounters between bodies of similar size were far more frequent in the crowded inner Solar System, where planetary embryos disturbed each other’s orbits and collided while competing for space near the Sun.

Because grazing collisions naturally eject significant amounts of mantle material into heliocentric orbit, they provide a more plausible explanation for Mercury’s unusual proportions than the re-accretion-prone catastrophic impact model.

One question left open by the simulations is where the stripped mantle material went. The study suggests that depending on the collision’s geometry, the debris may have escaped into the Sun, remained in orbit before being removed, or been captured by neighboring planets such as Venus. This hypothesis has yet to be tested, but it provides a framework for understanding Mercury within the context of Solar System evolution as a whole.

Smoothed particle hydrodynamics is a method that models solids, liquids, and gases as interacting particles tracked over time. Unlike fixed-grid approaches, SPH follows the motion of each particle individually, making it ideal for simulating large deformations and fragmenting collisions.

The study allowed researchers to track both metallic and silicate components during impact, showing how a grazing collision could leave behind a Mercury-like planet.

The results move Mercury’s formation from a rare special case to a scenario consistent with the statistics of early Solar System collisions. This aligns theoretical models of orbital dynamics with geophysical observations and expands the scope of plausible outcomes for planet formation.

It also shows that Mercury’s history is not an isolated anomaly but part of a continuum of diverse pathways that shaped the terrestrial planets. Just as Earth’s Moon likely formed from a giant impact and Mars stopped growing prematurely, Mercury may be the product of a grazing collision between equals.

Future work will compare the predictions of this model with geochemical data from meteorites and direct measurements from space missions. The BepiColombo spacecraft, a joint project of the European Space Agency and the Japan Aerospace Exploration Agency, is currently en route to Mercury and will provide unprecedented data on the planet’s surface chemistry, internal structure, and magnetic field.

Such evidence could confirm whether the grazing impact scenario is consistent with Mercury’s composition. Researchers also plan to refine their simulations to include processes such as partial melting and long-term orbital evolution.

Mercury remains the least explored planet in the Solar System, but ongoing missions and new generations of numerical models are starting to close that gap.

References:

¹ Formation of Mercury by a grazing giant collision involving similar-mass bodies– Franco, P. et al. – Nature Astronomy – June 27, 2025 – https://doi.org/10.1038/s41550-025-02582-y

² Collision between two bodies of similar mass may explain the formation of Mercury – EurekAlert! – September 22, 2025

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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