A new study suggests that Mercury's unusual structure may have resulted from a grazing collision between two similar sized protoplanets, rather than a rare catastrophic impact with a much larger body. Mercury's dense metallic core makes up about 70% of its mass, while its rocky mantle remains unusually thin compared to other terrestrial planets.
The research, led by Patrick Franco of the N by Clarence Oxford
Sao Paulo, Brazil (SPX) Sep 25, 2025
A new study suggests that Mercury's unusual structure may have resulted from a grazing collision between two similar sized protoplanets, rather than a rare catastrophic impact with a much larger body. Mercury's dense metallic core makes up about 70% of its mass, while its rocky mantle remains unusually thin compared to other terrestrial planets.
The research, led by Patrick Franco of the National Observatory in Brazil and postdoctoral researcher at the Institut de Physique du Globe de Paris, used smoothed particle hydrodynamics simulations to replicate the conditions of the early Solar System. These models demonstrated that a late stage near collision between planetary embryos could have stripped Mercury of up to 60% of its original mantle, leaving the core dominant.
"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," said Franco.
The study also resolves a limitation of earlier theories. In prior models, ejected mantle material would fall back onto the planet, negating the core-mantle imbalance observed today. Franco's team showed that under specific initial conditions, debris from the collision could have been permanently expelled, possibly contributing to the growth of other planets, such as Venus.
The method used, smoothed particle hydrodynamics (SPH), follows particles individually through time and space, capturing the effects of large deformations, collisions, and fragmentations with high accuracy. In this case, the simulations reproduced Mercury's mass and its high metal-to-silicate ratio with a margin of error below 5%.
According to Franco, the model provides a statistically realistic explanation for Mercury's present form and opens possibilities for understanding material redistribution during planet formation. The findings could also help explain differentiation and mantle loss in other rocky planets.
Future work will involve comparing these models with geochemical signatures from meteorites and spacecraft observations, including data from ESA and JAXA's joint BepiColombo mission now en route to Mercury.
"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," Franco said.
Research Report:Formation of Mercury by a grazing giant collision involving similar-mass bodies
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