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Moon-forming impactor as a source of Earth’s basal mantle anomalies

Nature volume 623, pages 95–99 (2023)Cite this article

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Abstract

Seismic images of Earth’s interior have revealed two continent-sized anomalies with low seismic velocities, known as the large low-velocity provinces (LLVPs), in the lowermost mantle1. The LLVPs are often interpreted as intrinsically dense heterogeneities that are compositionally distinct from the surrounding mantle2. Here we show that LLVPs may represent buried relics of Theia mantle material (TMM) that was preserved in proto-Earth’s mantle after the Moon-forming giant impact3. Our canonical giant-impact simulations show that a fraction of Theia’s mantle could have been delivered to proto-Earth’s solid lower mantle. We find that TMM is intrinsically 2.0–3.5% denser than proto-Earth’s mantle based on models of Theia’s mantle and the observed higher FeO content of the Moon. Our mantle convection models show that dense TMM blobs with a size of tens of kilometres after the impact can later sink and accumulate into LLVP-like thermochemical piles atop Earth’s core and survive to the present day. The LLVPs may, thus, be a natural consequence of the Moon-forming giant impact. Because giant impacts are common at the end stages of planet accretion, similar mantle heterogeneities caused by impacts may also exist in the interiors of other planetary bodies.

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Fig. 1: Schematic diagram illustrating the giant-impact origin of the LLVPs.
Fig. 2: Density profiles of the TMM and the BSE as a function of pressure.
Fig. 3: The formation of LLVP-like thermochemical piles from intrinsically dense TMM.

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Data availability

All data and parameters are available in the main text or the supplementary materials. The data that support the findings of this study are also available at https://doi.org/10.6084/m9.figshare.24013776.v1. Source data are provided with this paper.

Code availability

The author’s modified 2D Citcom code used in this study is available from https://figshare.com/projects/Yuan_Li_2022_NG/129185. The GIZMO code is made available at http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html. SWIFT is publicly available at http://swiftsim.com. WoMa is publicly available at https://github.com/srbonilla/WoMa, or the Python module can be installed directly with pip (https://pypi.org/project/woma/).

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Acknowledgements

We thank M. Gurnis, D. Stevenson, R. Canup, P. Olson, S. Stewart, M. Zolotov, T. Becker, M. Jackson, S.-H. Shim, D. Grady, R. Shi and S. Yuan for their support, discussions and insights. The numerical models were performed on the Agave cluster at Arizona State University. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. This work is supported by National Science Foundation grants EAR-1849949, EAR-1855624 and EAR-2216564. Q.Y. acknowledges support from the O. K. Earl Postdoctoral Fellowship at Caltech. T.S.J.G. recognizes support from the U.S. Geological Survey, Astrogeology Science Center. J.A.K. acknowledges support from a NASA Postdoctoral Program Fellowship, administered by Oak Ridge Associated Universities. Y.M. acknowledges a Stanback Postdoctoral Fellowship from the Caltech Center for Comparative Planetary Evolution. V.R.E. is supported by Science and Technology Facilities Council (STFC) grant ST/T000244/1. The MFM giant-impact simulations were performed on the Piz Daint supercomputer of the Swiss Nation Supercomputing Centre and the local cluster of the Shanghai Astronomical Observatory. The research in this paper made use of the SWIFT open-source simulation code70,85, v.0.9.0. This work used the DiRAC@Durham facility managed by the Institute for Computational Cosmology on behalf of the STFC DiRAC High-Performance Computing Facility (www.dirac.ac.uk). The equipment was funded by capital funding from the Department for Business, Energy and Industrial Strategy via STFC capital grants ST/K00042X/1, ST/P002293/1, ST/R002371/1 and ST/S002502/1, Durham University and STFC operations grant ST/R000832/1. DiRAC is part of the National e-Infrastructure.

Author information

Authors and Affiliations

  1. School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA

    Qian Yuan, Mingming Li, Steven J. Desch, Byeongkwan Ko & Edward J. Garnero

  2. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA

    Qian Yuan, Yoshinori Miyazaki & Paul D. Asimow

  3. Department of Earth and Environmental Sciences, Michigan State University, East Lansing, MI, USA

    Byeongkwan Ko

  4. Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai, China

    Hongping Deng

  5. U.S. Geological Survey, Astrogeology Science Center, Flagstaff, AZ, USA

    Travis S. J. Gabriel

  6. NASA Ames Research Center, Moffett Field, CA, USA

    Jacob A. Kegerreis

  7. Institute for Computational Cosmology, Department of Physics, Durham University, Durham, UK

    Vincent Eke

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  1. Qian Yuan
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Contributions

Q.Y. and E.J.G. conceptualized the initial idea. Q.Y., M.M.L. and E.J.G. designed the study. Q.Y. performed and analysed the geodynamic models with supervision from M.M.L. S.J.D. constrained the impact scenario and provided the composition of Theia. B.K. and Q.Y. computed the thermodynamic and seismic calculations. H.P.D., J.A.K. and V.R.E. performed the impact simulations and analysed the results. T.S.J.G. performed independent verifications of the SPH results and consulted on the SPH numerics. Y.M. developed the thermal evolution model. P.D.A. examined the fragmentation, dilution effect and magma mixing associated with the impact. All authors contributed to the writing and editing of the manuscript.

Corresponding author

Correspondence to Qian Yuan.

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Nature thanks Stéphane Labrosse and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Entropy profile (blue, in Jkg−1K−1) of mantle material in the post-impact Earth for our impact model using the meshless finite mass (MFM) method18.

The red curve shows the liquidus of forsterite16 and the rheological transition of the mantle was marked by the orange curve where the melt fraction above that depth becomes larger than 40%42,86.

Source data

Extended Data Fig. 2 Phase diagrams of the bulk silicate Earth (a), Theia_1 (b), Theia_2 (c) and Theia_3 (d) with geotherm from ref. 57.

The FeO contents of Theia are 13 wt% (Theia_1), 15 wt% (Theia_2), and 17 wt% (Theia_3), respectively. Phase equilibria were calculated using Perple_X27,28 with thermodynamic data from ref. 29. St: stishovite, Fp: ferropericlase, Ring: ringwoodite, Wad: wadsleyite, Ol: olivine, Cpx: clinopyroxene, Brg: bridgmanite, Gt: garnet, CaPv: davemaoite.

Source data

Extended Data Fig. 3 Phase diagrams of the bulk silicate Earth (a), Theia_1 (b), Theia_2 (c) and Theia_3 (d) with geotherm from ref. 58.

The FeO contents of Theia are 13 wt% (Theia_1), 15 wt% (Theia_2), and 17 wt% (Theia_3), respectively. Phase equilibria were calculated using Perple_X27,28 with thermodynamic data from ref. 29. Fp: ferropericlase, Wad: wadsleyite, Ol: olivine, Cpx: clinopyroxene, Brg: bridgmanite, Gt: garnet, CaPv: davemaoite.

Source data

Extended Data Fig. 4 Density difference between Theia mantle material and the bulk silicate Earth as a function of pressure.

Phase equilibria were calculated using Perple_X27,28 with thermodynamic data from ref. 29.

Source data

Extended Data Fig. 5 One numerical experiment showing that dense TMM sinks to the CMB before upper mantle materials mix with lower mantle materials.

a-d, Snapshots of the temperature fields (a, c) and compositional fields (b, d) at 0.00 Myr (a-b) and 27.36 Myr (c-d). At t = 0.00, random TMM blobs are placed in the lower mantle (b). After 27.36 Myr, the TMM blobs reach the CMB (d) whereas there is little mixing between the upper mantle and lower mantle (c).

Source data

Extended Data Table 1 The depth of layer boundary and the enclosing mass of Theia mantle material below the boundary for the canonical Moon-forming giant impact simulations from ref. 19
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Extended Data Table 2 Results from the MFM simulations with updated versions of the ANEOS equations of state16
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Extended Data Table 3 Results from the SPH simulations
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Extended Data Table 4 Major elemental compositions of the mantle of Theia and proto-Earth, and the bulk silicate Earth (BSE) (wt%) used in our thermodynamic modeling
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Extended Data Table 5 Physical parameters for mantle convection models
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Extended Data Table 6 Full list of performed mantle convection models
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Supplementary information

Supplementary Video 1

A canonical giant-impact simulation using the MFM method shows the preservation of a mostly solid lower layer in Earth’s mantle after the impact. Model evolution spans 13.1 h after the giant impact, and the entropy unit is MJ K−1 kg−1.

Supplementary Video 2

A canonical giant-impact simulation using the SPH method, highlighting the preservation of a mostly solid lower layer in Earth’s mantle after the impact event. Entropy unit is kJ K−1 kg−1.

Supplementary Video 3

Reference case of a successful mantle convection model showing that the random spheres of solid TMM in the lower layer of Earth’s mantle quickly descend to the lowermost mantle and are later shaped into isolated thermochemical piles (large low-velocity provinces in the models) by mantle convection after Earth’s history.

Supplementary Video 4

Mantle convection in case 2 showing that a less dense TMM will be mostly entrained away in the background mantle.

Supplementary Video 5

Mantle convection in case 3 showing that the 3.5% denser TMM can sink and survive Earth’s 4.5 Gyr convective history.

Supplementary Video 6

Mantle convection in case 4 showing that a TMM with an end-member density of 5% can still sink and survive Earth’s 4.5 Gyr convective history as isolated thermochemical piles.

Supplementary Video 7

Mantle convection in case 5 showing that a half-sized TMM will not be able to survive Earth’s 4.5 Gyr convective history.

Supplementary Video 8

Mantle convection in case 6 showing that a TMM enriched in radioactive elements can sink and survive Earth’s 4.5 Gyr convective history.

Supplementary Video 9

Mantle convection in case 7 showing that a higher temperature-dependent viscosity does not affect our convection results.

Supplementary Video 10

Mantle convection in case 8 showing that a periodic side-boundary condition does not affect our numerical results.

Supplementary Video 11

Mantle convection in case 9 showing that a different initial temperature does not affect our convection results.

Source data

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Yuan, Q., Li, M., Desch, S.J. et al. Moon-forming impactor as a source of Earth’s basal mantle anomalies. Nature 623, 95–99 (2023). https://doi.org/10.1038/s41586-023-06589-1

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