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Mercury’s oversized core: a long-standing puzzle
Mercury presents one of the Solar System’s most persistent enigmas: a core disproportionately large compared with its silicate mantle and crust. Ground-based radio observations in the 1960s and 1970s first indicated a high bulk density for Mercury, and subsequent flyby and orbital missions — most notably Mariner 10 in 1975 and NASA's MESSENGER orbiter (2011–2015) — confirmed that Mercury’s iron-rich core makes up an unusually large fraction of the planet's mass. Where Earth’s core accounts for roughly 30% of planetary mass and Mars’ about 25%, Mercury’s core constitutes roughly 70% of its mass, producing a metal-to-silicate ratio that is difficult to reconcile with standard models of planetary growth.
Striking regions of chemical diversity on Mercury, mapped out by MESSENGER's XRS instrument
Researchers have long proposed that a catastrophic giant impact removed much of Mercury’s original mantle, leaving behind a small silicate shell above a dominant metallic interior. But conventional giant-impact models typically require an impact between bodies with very different masses — a proto-Mercury struck by a much smaller projectile — and detailed N-body simulations suggest such strongly mismatched collisions were statistically rare in the early Solar System.
A new hypothesis: grazing collisions between similar-mass bodies
A 2025 study in Nature Astronomy (Franco et al.) offers an alternative that addresses both the physics and the statistical likelihood of the event. Using high-resolution smoothed particle hydrodynamics (SPH) simulations, the team demonstrates that a low-velocity, grazing collision between two protoplanets of comparable mass can reproduce Mercury’s present-day mass and high metal-to-silicate ratio with remarkable fidelity — their models reproduce Mercury’s properties to within about 5%.
The grazing-impact model differs from classic “hit-and-strip” scenarios in one key respect: the impactor need not be much smaller than proto-Mercury. Instead, two similar-mass planetary embryos — each evolving in the crowded, chaotic inner Solar System — can engage in a shallow-angle collision that preferentially strips silicate mantle material while leaving a dense iron core largely intact. Because collisions between similar-mass bodies were far more common in the inner Solar System’s early assembly phase, this mechanism is dynamically and statistically more plausible.
How the simulations work and what they show
Smoothed particle hydrodynamics is a well-established numerical method for modeling fluid and solid behavior during high-energy interactions such as planetary collisions. SPH breaks each body into thousands to millions of discrete “particles” with assigned thermodynamic and material properties; their trajectories and interactions are integrated to follow shock propagation, melting, vaporization, and gravitational reaccumulation.
These screenshots from the simulations show how the impact event played out. "The proto-Mercury (0.13 M⊕) is represented by a pink mantle and a turquoise core. The target is represented by a red mantle and a yellow core," the authors explain. The impact velocity is relatively low and the impact angle is 32.5 degrees. (b) and (c) show the impact and material being blasted away. (d) shows the Mercury candidate with 0.056 Earth masses, very close to the measured 0.055 Earth masses. (Franco et al., NatAstr., 2025)
Franco and colleagues ran dozens of SPH experiments varying impact angle, velocity, and initial composition. Their preferred configuration involved a grazing encounter at an impact angle near 30–35 degrees and relatively low relative velocity. Such a collision can strip up to ~60% of a protoplanet’s mantle, boosting the residual planet’s metal fraction without completely disrupting the iron core. Crucially, the simulations show scenarios in which a significant fraction of mantle debris acquires escape trajectories and does not re-accrete onto the survivor, allowing the metal-to-silicate imbalance to persist.
Where did the lost mantle go?
A core challenge for any mass-stripping model is explaining why stripped mantle material did not simply fall back onto the surviving planet. The new study argues that several mechanisms in the early Solar System could prevent efficient reaccretion. These include gravitational scattering by nearby planetesimals and planetary embryos, dynamical interactions with forming neighboring planets, and transfer of debris to adjacent orbits. In some modeled outcomes, a portion of the ejected silicate material ends up incorporated into nearby bodies — Venus is a plausible recipient under certain orbital configurations, though that specific pathway requires further modeling and geochemical tests.
Scientific context and implications for planetary formation
If Mercury resulted from a grazing collision between similar-mass embryos, the event has broad implications for models of inner-planet assembly. It reinforces a view of the early inner Solar System as a dynamically violent environment where protoplanets evolved through repeated near-misses and mergers, rather than through a sequence dominated exclusively by rare hyper-asymmetric impacts.
The study also highlights the interplay between dynamical evolution and geochemical signatures: a planet’s bulk composition can be strongly influenced by a single stochastic event, yet that event must be consistent with the statistical distribution of collisions predicted by N-body models. Franco et al. address both requirements by demonstrating that the grazing collision scenario is both geophysically plausible and dynamically likely.
Mission data and future tests
Missions like MESSENGER provided the geophysical and compositional constraints that make Mercury such a compelling case study. Looking forward, the ESA/JAXA BepiColombo mission — arriving at Mercury in 2026 — will carry more than 20 science instruments designed to refine measurements of the planet’s interior structure, magnetic field, and surface composition. High-precision gravity and magnetic data may better constrain core size and state (solid inner core versus liquid outer core) and offer improved estimates of bulk density and moment of inertia.
Geochemical tests could further validate or falsify grazing-impact models. Detailed abundance patterns of refractory and volatile elements, isotopic ratios in Mercury-like meteorites (if identified), and, in a best-case future scenario, returned samples from Mercury itself would provide direct compositional fingerprints of any past large-scale mantle stripping.
Expert Insight
Dr. Lena Ortiz, planetary scientist at the Institute for Planetary Physics, comments: "The grazing twin-impact model elegantly reconciles two hard constraints: Mercury’s extreme metal-rich composition and the statistical rarity of highly unequal collisions. It moves the narrative from a highly exceptional event to a rather natural consequence of planet formation dynamics. The next step is to combine detailed geochemistry with improved dynamical models to see whether the predicted debris pathways match plausible sinks, such as Venus or the inner asteroid population."
Conclusion
Mercury’s oversized core no longer requires invoking an unusually rare type of impact. High-resolution SPH simulations indicate that a grazing collision between similarly sized protoplanets can strip mantle material efficiently while leaving a dense iron core largely intact. This scenario is both dynamically plausible in the crowded early inner Solar System and capable of reproducing Mercury’s metal-to-silicate ratio to within a few percent. Ongoing and upcoming observations — notably from BepiColombo — plus future geochemical analyses will be essential to test this hypothesis and refine our understanding of how terrestrial planets acquire their internal structure.
Source: sciencealert
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