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A proposed early-Earth addition of sulfur-rich, Mercury-like material could help explain both an isotope mismatch in Earth’s silicate portion and a source of heat for the geodynamo. Experiments support the chemistry behind the idea, but they do not show that the collision actually occurred.
What are the two mysteries?
One puzzle is chemical: the reported samarium-to-neodymium ratio (Sm/Nd) of the silicate Earth—the crust and mantle—is higher than the ratio in chondritic material commonly used as a reference for Earth’s building blocks. The second concerns Earth’s magnetic field. The geodynamo, which generates that field, needs a heat budget, and researchers have considered whether the available sources fully account for it.
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What kind of collision did the study propose?
In a 2015 Nature paper, Anke Wohlers and Bernard J. Wood proposed that early Earth may have incorporated a reduced, sulfur-rich component with a composition resembling Mercury. They also considered enstatite-chondrite-like material as an alternative. “Mercury-like” describes a compositional analogy; the proposal does not identify Mercury itself as the impactor.
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How could the addition explain the isotope mismatch?
When a planet differentiates, elements divide between metallic material that forms the core and silicate material that forms the mantle and crust. The paper argues that sulfur-rich metal could change how samarium and neodymium partition during this process. That change could leave the silicate Earth with a higher Sm/Nd ratio than the chondritic reference.
For the proposed Mercury-like or enstatite-chondrite-like addition, Wohlers and Wood calculated an approximately +14 parts per million (ppm) anomaly in the 142Nd/144Nd ratio relative to chondrite. This is the study’s modeled result for the scenario, not a measured property of an identified impactor.
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How might the same material affect the geodynamo’s heat budget?
The authors also propose that a sulfur-rich core would take up uranium strongly and thorium slightly. Uranium and thorium produce heat through radioactive decay; placing some of these heat-producing elements in the core could provide a substantial part of the heat source that the authors described as “missing” from geodynamo explanations.
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This is a proposed consequence of the modeled composition and partitioning. It is not a direct measurement establishing that Earth’s core contains the specific uranium inventory implied by the scenario.
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Does the proposal settle the isotope question?
No. Wohlers and Wood presented the impactor scenario as one possible explanation among competing accounts. Earlier ideas included a hidden reservoir with low Sm/Nd or the loss of early crust. A later 2015 Nature Geoscience perspective discussed another missing-reservoir possibility: material lost to space through impact ablation.
These explanations differ in where complementary low-Sm/Nd material might be—or whether it was lost—and in how they treat heat-producing potassium, uranium, and thorium. The cited work does not establish a final winner. The experiments support a plausible geochemical mechanism; they do not directly establish that the proposed ancient collision happened.
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Publication details
Wohlers and Wood’s paper, “A Mercury-like component of early Earth yields uranium in the core and high mantle 142Nd,” appeared in Nature, volume 520, pages 337–340. It was published online on 15 April 2015 and in the 16 April 2015 issue. The related Chemistry World listing was dated 16 April 2015.
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