Mercury may have been formed by the collision of two protoplanets

Space

With information from Agência Fapesp - 08/08/2025

Snapshots of the collision captured in the computer simulation. [Image: Patrick Franco et al. - 10.1038/s41550-025-02582-y]

How was Mercury formed?

The formation of the planet Mercury is still an unsolved problem: The planet closest to the Sun has a disproportionately large metallic core, with about 70% of its mass, and a relatively small rocky mantle.

The most widely accepted explanation until now was that Mercury lost much of its crust and mantle after a catastrophic collision with a large celestial body. But dynamical simulations show that this type of impact, involving bodies of very different masses, is extremely rare.

Therefore, astronomers are proposing an alternative explanation, based on a type of event that was much more common in the early Solar System: The collision between celestial bodies of similar masses.

"Through simulation, we showed that the formation of Mercury does not require exceptional collisions. A grazing impact between two protoplanets of similar masses can explain its composition. This is a much more plausible scenario from a statistical and dynamical point of view," explains Patrick Franco of the National Observatory.

A grazing impact is a type of collision between celestial bodies that occurs at a very low angle (less than 20 degrees) in relation to the surface of the struck object, with one nearly grazing the other, rather than a head-on collision.

"Our work is based on the observation, made in previous simulations, that collisions between very unequal bodies are extremely rare events. Collisions between objects of similar masses are more common, and the objective of the study was precisely to verify whether these collisions would be capable of producing a planet with the characteristics observed on Mercury," added the researcher.

Summary of simulation results for the configuration with a target core-to-mass ratio of 0.5. [Image: Patrick Franco et al. - 10.1038/s41550-025-02582-y]

Smoothed particle hydrodynamics

This possible collision would have occurred relatively late in the formation of the Solar System, when rocky bodies of similar sizes competed for space in the inner regions, closest to the Sun. "They were evolving objects, within a nursery of planetary embryos, interacting gravitationally, perturbing each other's orbits, and even colliding, until only the well-defined and stable orbital configurations we know today remained," Patrick describes.

To recreate this hypothetical scenario, the researchers used a computational numerical method called "smoothed particle hydrodynamics," which makes it possible to simulate gases, liquids, and solid materials in motion, especially in contexts where large deformations, collisions, or fragmentation occur.

This method, widely used in cosmology, astrophysics, planetary dynamics, engineering, and computer graphics, employs the Lagrangian function (Joseph Louis Lagrange, 1736–1813) as a mathematical resource. Lagrangian function describes the evolution of a system by considering how each constituent point or particle moves individually in space over time. In contrast to the Eulerian formalism (Leonhard Paul Euler, 1707–1783), which observes what happens at fixed points in space, the Lagrangian function follows, so to speak, the "point of view" of the moving particle.

The resulting model can explain with great precision why Mercury has a low total mass despite having a large metallic core, and why it retains only a thin layer of rocky material.

"Through detailed smoothed particle hydrodynamics simulations, we found that we can accurately reproduce both Mercury's total mass and its unusual metal-to-silicate ratio. The model's error margin was less than 5 percent," Patrick said. "The collision would have stripped away up to 60 percent of Mercury's original mantle, which would explain its increased metallicity."

The BepiColombro spacecraft is a joint European and Japanese mission to study the composition, geophysics, atmosphere, magnetosphere, and history of Mercury. [Image: ESA/ATG/JAXA]

Where is the wreckage?

The new model also avoids a limitation of the previous ones. "In these scenarios, the material torn off during the collision is reincorporated by the planet itself. If this were the case, Mercury would not exhibit its current core-to-mantle disproportion. However, in the model we are proposing, depending on the initial conditions, some of the torn off material may be ejected and never return, which preserves the core-to-mantle disproportion," argues Patrick.

In this case, the obvious question is where the ejected material went. "If the impact occurred in close orbits, one possibility is that this material was incorporated by another planet in formation, perhaps Venus. This hypothesis still needs to be investigated further," said the researcher.

According to Patrick, the proposed model can be extended to investigate the formation of other rocky planets and contribute to understanding the processes of differentiation and material loss in the early Solar System. The next steps in the research should include comparisons with geochemical data from meteorites and samples from space missions, such as the BepiColombo spacecraft, which will study Mercury starting next year.

"Mercury remains the least explored planet in our system. But that is changing. There is a new generation of research and missions underway, and many interesting things are yet to emerge," Patrick said.

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