The unusual composition of Mercury, particularly its disproportionately large metallic core, may be the result of a more common cosmic event than previously believed: a collision between two protoplanets of similar mass. This revised theory, published in Nature Astronomy, challenges the long-standing hypothesis that a single, massive impact was responsible for stripping away much of Mercury's original mantle.
Astronomer Patrick Franco led the research, employing advanced smoothed-particle hydrodynamics simulations. These models suggest that impacts between similarly sized bodies could indeed produce a planet with Mercury's distinctive mass and iron-to-silicate ratio. Franco's findings indicate that Mercury's formation may not require extraordinary collisions but rather a more statistically probable, gentler impact between similarly sized protoplanets. This aligns with earlier research from 2017, which indicated that Mercury-like planets were formed in only a small fraction of high-resolution N-body simulations, underscoring the rarity of specific impact types.
The new model proposes that up to 60% of Mercury's mantle could have been ejected during such an impact, a scenario that successfully replicates Mercury's total mass and its metal-to-silicate ratio with an error margin of less than five percent. An intriguing aspect of this new hypothesis is the potential fate of the ejected material. If the collision occurred in closely aligned orbits, the debris might have been incorporated into another nascent planet, possibly Venus. This intriguing possibility warrants further investigation.
The findings are expected to be further validated by comparing the model's predictions with geochemical data from meteorites and the ongoing BepiColombo mission. This collaborative European Space Agency and Japan Aerospace Exploration Agency endeavor aims to provide unprecedented insights into Mercury's composition, geophysics, atmosphere, and history, contributing significantly to our understanding of rocky planet formation in the early solar system. The BepiColombo mission, launched in 2018, is scheduled to reach Mercury in late 2026, utilizing a series of gravity-assist flybys to achieve its orbit. Its scientific objectives include mapping the planet's surface and interior, investigating its magnetosphere, and understanding the planet's formation and evolution.
This research offers a more nuanced perspective on planetary formation, suggesting that the diverse characteristics of planets like Mercury can arise from a broader range of early solar system dynamics than previously assumed. The study contributes to the ongoing effort to unravel the complex processes that shaped our solar system, emphasizing how even seemingly common events can lead to unique planetary outcomes.