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Core merging and stratification following giant impact

Nature Geoscience volume 9, pages 786–789 (2016)Cite this article

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Abstract

A stratified layer below the core–mantle boundary has long been suspected on the basis of geomagnetic and seismic observations1,2,3. It has been suggested that the outermost core has a stratified layer about 100 km thick3,4,5,6 that could be due to the diffusion of light elements7,8. Recent seismological evidence, however, supports a layer exceeding 300 km in thickness of enigmatic origin9,10,11. Here we show from turbulent mixing experiments that merging between projectile and planetary core following a giant impact can lead to a stratified layer at the top of the core. Scaling relationships between post-impact core structure and projectile properties suggest that merging between Earth’s protocore and a projectile core that is enriched in light elements and 20 times less massive can produce the thick stratification inferred from seismic data. Our experiments favour Moon-forming impact scenarios involving a projectile smaller than the proto-Earth12,13 and suggest that entrainment of mantle silicates into the protocore led to metal–silicate equilibration under extreme pressure–temperature conditions. We conclude that the thick stratified layer detected at the top of Earth’s core9,10 can be explained as a vestige of the Moon-forming giant impact during the late stages of planetary accretion.

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Figure 1: Turbulent core merging after a giant impact.
Figure 2: Collapsing fountain during the first steps of the merging process.
Figure 3: Long-term dynamics after fountain collapse.
Figure 4: Core stratification after merging with a projectile core enriched in light elements.

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Acknowledgements

We gratefully acknowledge support from grant EAR-110371 and Frontiers in Earth System Dynamics grant EAR-1135382 from the National Science Foundation.

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Authors and Affiliations

  1. Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA

    Maylis Landeau, Peter Olson & Benjamin H. Hirsh

  2. Univ. Lyon, Université Claude Bernard Lyon 1, Ens de Lyon, CNRS, UMR 5276 LGL-TPE, F-69622 Villeurbanne, France

    Renaud Deguen

Authors
  1. Maylis Landeau
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  2. Peter Olson
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  3. Renaud Deguen
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  4. Benjamin H. Hirsh
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Contributions

M.L., R.D., P.O. and B.H.H. designed and conducted the experiments; M.L. analysed the experimental data; M.L. and P.O. wrote the paper.

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Correspondence to Maylis Landeau.

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The authors declare no competing financial interests.

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Landeau, M., Olson, P., Deguen, R. et al. Core merging and stratification following giant impact. Nature Geosci 9, 786–789 (2016). https://doi.org/10.1038/ngeo2808

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