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A proposed collision involving sulfur-rich, Mercury-like material could help explain both an isotope mismatch in Earth’s mantle and a source of heat for the planet’s magnetic field. Experiments support the chemistry behind the idea, but they do not show that the collision actually happened.
What are the two mysteries?
One puzzle concerns the composition of Earth’s silicate portion—the crust and mantle. Its reported samarium-to-neodymium ratio (Sm/Nd) is higher than the ratio in chondritic material, commonly used as a reference for the building blocks of Earth. The mismatch raises a question about how Earth acquired its present composition.
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The other concerns Earth’s geodynamo: the process in the liquid outer core that generates the magnetic field. A proposed explanation needs to account for a substantial heat supply to help power that process.
What kind of collision do the authors propose?
In a 2015 Nature paper, Oxford researchers Anke Wohlers and Bernard J. Wood considered whether early Earth could have incorporated a reduced, sulfur-rich component with a composition resembling Mercury. They also considered an alternative: material resembling an enstatite chondrite, a type of meteorite. “Mercury-like” describes a compositional analogy; the proposal is not that Mercury itself struck Earth.
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The idea is that this material joined Earth during accretion, while the planet was forming its core. The authors used high-pressure experiments and models of how elements divide between metal and silicate during core formation. They did not recreate a planetary collision.
How could the addition affect the isotope puzzle?
During core formation, elements do not all behave alike: some preferentially enter metal, while others remain in silicate. The authors argue that sulfur-rich, reduced material could change this partitioning and contribute to a silicate Earth with a higher Sm/Nd ratio than the chondritic reference.
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For the Mercury-like or enstatite-chondrite-like addition scenario, Wohlers and Wood proposed an approximately +14 parts per million (ppm) anomaly in the 142Nd/144Nd ratio relative to chondrite. That figure is a result of their proposed scenario, not a measured property of an identified impactor.
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How might it contribute heat for the geodynamo?
The same proposed sulfur-rich core composition would affect uranium and thorium, which produce heat through radioactive decay. The authors argue that uranium would partition strongly into the core and thorium slightly. In their paper’s words, this could supply “a substantial part” of the heat source they describe as missing from geodynamo explanations.
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This is a proposed consequence of the modeled material and its core-forming chemistry. It is not a direct measurement showing that Earth’s core contains the specific uranium inventory implied by the scenario.
Is this the established explanation?
No. The experiments and partitioning models show how a sulfur-rich addition could contribute to the isotope pattern and heat budget; they do not establish that an early collision occurred or settle either puzzle.
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The 2015 paper also discusses earlier explanations for the isotope discrepancy, including a hidden reservoir with low Sm/Nd or the loss of early crust. A later Nature Geoscience perspective discusses another missing-reservoir possibility: material removed to space by impact ablation. These are competing ideas, not a settled ranking. Evaluating them involves asking where complementary low-Sm/Nd material resides or went, how each scenario affects heat-producing potassium, uranium and thorium, and whether it fits both the isotope evidence and Earth’s heat budget.
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- A plausible mechanism: high-pressure experiments and models support the possibility that sulfur-rich core-forming material could alter element partitioning.
- A paired explanation: in the authors’ scenario, the same addition could affect mantle Sm/Nd and place uranium, and to a lesser extent thorium, in the core.
- A hypothesis, not a witnessed event: the results do not directly demonstrate that the proposed collision happened.
Wohlers and Wood’s paper, “A Mercury-like component of early Earth yields uranium in the core and high mantle 142Nd,” appeared online on 15 April 2015 and in the 16 April issue of Nature. Read the paper in Nature.
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