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An impact-driven dynamo for the early Moon

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Abstract

The origin of lunar magnetic anomalies1,2,3,4,5 remains unresolved after their discovery more than four decades ago. A commonly invoked hypothesis is that the Moon might once have possessed a thermally driven core dynamo3, but this theory is problematical given the small size of the core and the required surface magnetic field strengths6. An alternative hypothesis is that impact events might have amplified ambient fields near the antipodes of the largest basins7, but many magnetic anomalies exist that are not associated with basin antipodes. Here we propose a new model for magnetic field generation, in which dynamo action comes from impact-induced changes in the Moon’s rotation rate. Basin-forming impact events are energetic enough to have unlocked the Moon from synchronous rotation8, and we demonstrate that the subsequent large-scale fluid flows in the core, excited by the tidal distortion of the core–mantle boundary9, could have powered a lunar dynamo. Predicted surface magnetic field strengths are on the order of several microteslas, consistent with palaeomagnetic measurements5, and the duration of these fields is sufficient to explain the central magnetic anomalies associated with several large impact basins.

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Figure 1: Surface topography and total magnetic field strength of the 550-km-diameter Crisium impact basin.
Figure 2: Estimated magnetic field strength at the Moon’s surface as a function of Earth–Moon separation and post-impact rotational period.
Figure 3: Time evolution of the surface magnetic field strength (red) and depth to the Curie temperature in an impact melt sheet (blue) following an impact event.

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Acknowledgements

M.L.B. and D.C. acknowledge discussions with P. Le Gal and S. Le Dizès. Ö.K. acknowledges the support of the Belgian PRODEX programme.

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Authors and Affiliations

Authors

Contributions

M.L.B. and D.C. performed the fluid mechanics analysis, M.A.W. contributed to the analysis of the basin magnetic anomalies and post-impact rotation rates, Ö.K. contributed to analysis of the post-impact rotational evolution of the Moon and M.L. performed the impact melt thermal evolution analysis. All authors contributed to the conclusions presented in the manuscript.

Corresponding author

Correspondence to M. Le Bars.

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The authors declare no competing financial interests.

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This file contains Supplementary Text and Data, Supplementary Figures 1-10 with legends, Supplementary Tables 1-2 and additional references. (PDF 3320 kb)

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Le Bars, M., Wieczorek, M., Karatekin, Ö. et al. An impact-driven dynamo for the early Moon. Nature 479, 215–218 (2011). https://doi.org/10.1038/nature10565

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