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The formation of Mercury remains an unsolved mystery. The planet closest to the sun has a disproportionately large metallic core—accounting for about 70% of its mass—and a relatively small rocky mantle. Until now, the most widely accepted explanation was that Mercury lost much of its crust and mantle after colliding catastrophically with a large celestial body. However, dynamic simulations show that this type of impact involving bodies of very different masses is extremely rare.

A new study proposes an alternative explanation based on a type of event that was much more common in the early solar system—a near-collision between bodies of similar masses. The findings are in the journal Nature Astronomy.

The study's first author was Patrick Franco, an astronomer with a Ph.D. from the National Observatory in Brazil and postdoctoral researcher at the Institut de Â鶹ÒùÔºique du Globe de Paris in France.

"Through simulation, we show that the formation of Mercury doesn't 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 dynamic point of view," says Franco.

"Our work is based on the finding, 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 in Mercury."

This possible collision would have occurred at a relatively late stage in the formation of the solar system when rocky bodies of similar sizes competed for space in the inner regions, closer to the sun.

"They were evolving objects, within a nursery of planetary embryos, interacting gravitationally, disturbing each other's orbits, and even colliding, until only the well-defined and stable orbital configurations we know today remained," explains Franco.

To recreate this hypothetical scenario, the researchers used a computational numerical method called "smoothed particle hydrodynamics" (SPH). SPH can simulate gases, liquids, and solid materials in motion, especially in contexts involving large deformations, collisions, or fragmentations.

Widely used in cosmology, astrophysics, and planetary dynamics, as well as engineering and computer graphics, this method employs the Lagrangian function, which was developed by Joseph Louis Lagrange (1736–1813). The function describes the evolution of a system by considering how each constituent point or particle moves individually in space over time.

Unlike the Eulerian formalism (developed by Leonhard Paul Euler, 1707–1783), which observes what happens at fixed points in space, the Lagrangian function follows the "point of view" of the moving particle.

"Through detailed simulations in smoothed particle hydrodynamics, we found that it's possible to reproduce both Mercury's total mass and its unusual metal-to-silicate ratio with high precision. The model's margin of error was less than 5%," Franco says.

The proposal helps explain why Mercury has a low total mass despite its large metallic core and why it retains only a thin layer of rocky material. =

"We assumed that Mercury would initially have a composition similar to that of the other terrestrial planets. The collision would have stripped away up to 60% of its original mantle, which would explain its heightened metallicity," the researcher explains.

Where is the debris?

In addition, the new model avoids a limitation of previous scenarios.

"In these scenarios, the material torn away during the collision is reincorporated by the planet itself. If this were the case, Mercury wouldn't exhibit its current disproportion between core and mantle. But in the model we're proposing, depending on the initial conditions, part of the material torn away may be ejected and never return, which preserves the disproportion between core and mantle," Franco argues.

The obvious question in this case is where the ejected material went.

"If the impact occurred in nearby orbits, one possibility is that this material was incorporated by another planet in formation, perhaps Venus. It's a hypothesis that still needs to be investigated in greater depth," the researcher says.

According to Franco, the proposed model can be extended to investigate the formation of other rocky planets and contribute to our understanding of differentiation processes 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 that have studied Mercury, such as BepiColombo, a joint initiative of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA).

"Mercury remains the least explored planet in our system. But that's changing. There's a new generation of research and missions underway, and many interesting things are yet to come," says Franco.