An impact-driven dynamo for the early Moon



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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

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.


  1. 1

    Purucker, M. E. & Nicholas, J. B. Global spherical harmonic models of the internal magnetic field strength of the Moon based on sequential and coestimation approaches. J. Geophys. Res. 115, E12007 (2010)

    Article  ADS  Google Scholar 

  2. 2

    Fuller, M. & Cisowski, S. M. in Geomagnetism (ed. Jacobs, J. A. ) 307–455 (Academic, 1987)

    Google Scholar 

  3. 3

    Stegman, D. R., Jellinek, A. M., Zatman, S. A., Baumgardner, J. R. & Richards, M. A. An early lunar core dynamo driven by thermochemical mantle convection. Nature 421, 143–146 (2003)

    CAS  Article  ADS  Google Scholar 

  4. 4

    Lawrence, K., Johnson, C., Tauxe, L. & Gee, J. Lunar paleointensity measurements: implications for lunar magnetic evolution. Phys. Earth Planet. Inter. 168, 71–87 (2008)

    Article  ADS  Google Scholar 

  5. 5

    Garrick-Bethell, I., Weiss, B. P., Shuster, D. L. & Buz, J. Early lunar magnetism. Science 323, 356–359 (2009)

    CAS  Article  ADS  Google Scholar 

  6. 6

    Wieczorek, M. A. et al. The constitution and structure of the lunar interior. Rev. Mineral. Geochem. 60, 221–364 (2006)

    CAS  Article  Google Scholar 

  7. 7

    Hood, L. L. & Artemieva, N. A. Antipodal effects of lunar basin-forming impacts: initial 3D simulations and comparisons with observations. Icarus 193, 485–502 (2008)

    Article  ADS  Google Scholar 

  8. 8

    Wieczorek, M. A. & Le Feuvre, M. Did a large impact reorient the Moon? Icarus 200, 358–366 (2009)

    Article  ADS  Google Scholar 

  9. 9

    Le Bars, M., Lacaze, L., Le Dizès, S., Le Gal, P. & Rieutord, M. Tidal instability in stellar and planetary binary systems. Phys. Earth Planet. Inter. 178, 48–55 (2010)

    Article  ADS  Google Scholar 

  10. 10

    Halekas, J. S., Lin, R. P. & Mitchell, D. L. Magnetic fields of lunar multi-ring impact basins. Meteorit. Planet. Sci. 38, 565–578 (2003)

    CAS  Article  ADS  Google Scholar 

  11. 11

    Hood, L. L. Central magnetic anomalies of Nectarian-aged lunar impact basins: Probable evidence for an early core dynamo. Icarus 211, 1109–1128 (2011)

    Article  ADS  Google Scholar 

  12. 12

    Cintala, M. J. & Grieve, R. A. F. Scaling impact melting and crater dimensions: implications for the lunar cratering record. Meteorit. Planet. Sci. 33, 889–912 (1998)

    CAS  Article  ADS  Google Scholar 

  13. 13

    Williams, J. G., Boggs, D. H., Yoder, C. F., Ratcliff, J. T. & Dickey, J. O. Lunar rotational dissipation in solid body and molten core. J. Geophys. Res. 106, 27933–27968 (2001)

    Article  ADS  Google Scholar 

  14. 14

    Malkus, W. V. R. Precession of the Earth as the cause of geomagnetism. Science 160, 259–264 (1968)

    CAS  Article  ADS  Google Scholar 

  15. 15

    Kerswell, R. R. Upper bounds on the energy dissipation in turbulent precession. J. Fluid Mech. 321, 335–370 (1996)

    MathSciNet  Article  ADS  Google Scholar 

  16. 16

    Kerswell, R. R. & Malkus, W. V. R. Tidal instability as the source for Io’s magnetic signature. Geophys. Res. Lett. 25, 603–606 (1998)

    Article  ADS  Google Scholar 

  17. 17

    Rochester, M. G., Jacobs, J. A., Smylie, D. E. & Chong, K. F. Can precession power the geomagnetic dynamo? Geophys. J. R. Astron. Soc. 43, 661–678 (1975)

    Article  ADS  Google Scholar 

  18. 18

    Loper, D. E. Torque balance and energy budget for the precessionally driven dynamo. Phys. Earth Planet. Inter. 11, 43–60 (1975)

    Article  ADS  Google Scholar 

  19. 19

    Tilgner, A. Precession driven dynamo. Phys. Fluids 17, 034104 (2005)

    MathSciNet  Article  ADS  Google Scholar 

  20. 20

    Wu, C.-C. & Roberts, P. H. On a dynamo driven by topographic precession. Geophys. Astrophys. Fluid Dyn. 103, 467–501 (2009)

    Article  ADS  Google Scholar 

  21. 21

    Kerswell, R. R. Elliptical instability. Annu. Rev. Fluid Mech. 34, 83–113 (2002)

    MathSciNet  Article  ADS  Google Scholar 

  22. 22

    Cébron, D., Le Bars, M., Leontini, J., Maubert, P. & Le Gal, P. A systematic numerical study of the tidal instability in a rotating triaxial ellipsoid. Phys. Earth Planet. Inter. 182, 119–128 (2010)

    Article  ADS  Google Scholar 

  23. 23

    Christensen, U. R. & Tilgner, A. Power requirement of the geodynamo from ohmic losses in numerical and laboratory dynamos. Nature 429, 169–171 (2004)

    CAS  Article  ADS  Google Scholar 

  24. 24

    Christensen, U. R. & Aubert, J. Scaling properties of convection-driven dynamos in rotating spherical shells and application to planetary magnetic fields. Geophys. J. Int. 166, 97–114 (2006)

    Article  ADS  Google Scholar 

  25. 25

    Christensen, U. R., Holzwarth, V. & Reiners, A. Energy flux determines magnetic field strength of planets and stars. Nature 457, 167–169 (2009)

    CAS  Article  ADS  Google Scholar 

  26. 26

    Yoder, F. & Hutchison, R. The free librations of a dissipative Moon. Phil. Trans. R. Soc. Lond. A 303, 327–338 (1981)

    Article  ADS  Google Scholar 

  27. 27

    Meyer, J. & Wisdom, J. Precession of the lunar core. Icarus 211, 921–924 (2011)

    Article  ADS  Google Scholar 

  28. 28

    Gattacceca, J. et al. Unraveling the simultaneous shock magnetization and demagnetization of rocks. Earth Planet. Sci. Lett. 299, 42–53 (2010)

    CAS  Article  ADS  Google Scholar 

  29. 29

    Arkani-Hamed, J. Did tidal deformation power the core dynamo of Mars? Icarus 201, 31–43 (2009)

    Article  ADS  Google Scholar 

  30. 30

    Smith, D. E. et al. Initial observations from the Lunar Orbiter Laser Altimeter (LOLA). Geophys. Res. Lett. 37, L18204 (2010)

    ADS  Google Scholar 

  31. 31

    Lavorel, G. & Le Bars, M. Experimental study of the interaction between convective and elliptical instabilities. Phys. Fluids 22, 114101 (2010)

    Article  ADS  Google Scholar 

  32. 32

    Cébron, D., Maubert, P. & Le Bars, M. Tidal instability in a rotating and differentially heated ellipsoidal shell. Geophys. J. Int. 182, 1311–1318 (2010)

    Article  ADS  Google Scholar 

  33. 33

    Greenspan, H. P. The Theory of Rotating Fluids (Cambridge Univ. Press, 1968)

    Google Scholar 

  34. 34

    Cébron, D., Le Bars, M. & Meunier, P. Tilt-over mode in a precessing triaxial ellipsoid. Phys. Fluids 22, 116601 (2010)

    Article  ADS  Google Scholar 

  35. 35

    Peale, S. J. in Planetary Satellites (ed. Burns, J. A. ) 87–112 (Univ. Arizona Press, 1977)

    Google Scholar 

  36. 36

    Lacaze, L., Le Gal, P. & Le Dizès, S. Elliptical instability in a rotating spheroid. J. Fluid Mech. 505, 1–22 (2004)

    MathSciNet  Article  ADS  Google Scholar 

  37. 37

    Correia, A. C. M. & Laskar, J. Mercury’s capture into the 3/2 spin–orbit resonance including the effect of core–mantle friction. Icarus 201, 1–11 (2009)

    Article  ADS  Google Scholar 

Download references


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.

Author information




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.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

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

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Le Bars, M., Wieczorek, M., Karatekin, Ö. et al. An impact-driven dynamo for the early Moon. Nature 479, 215–218 (2011).

Download citation

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.