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Long-lived magnetism from solidification-driven convection on the pallasite parent body

Abstract

Palaeomagnetic measurements of meteorites1,2,3,4,5 suggest that, shortly after the birth of the Solar System, the molten metallic cores of many small planetary bodies convected vigorously and were capable of generating magnetic fields6. Convection on these bodies is currently thought to have been thermally driven7,8, implying that magnetic activity would have been short-lived9. Here we report a time-series palaeomagnetic record derived from nanomagnetic imaging10 of the Imilac and Esquel pallasite meteorites, a group of meteorites consisting of centimetre-sized metallic and silicate phases. We find a history of long-lived magnetic activity on the pallasite parent body, capturing the decay and eventual shutdown of the magnetic field as core solidification completed. We demonstrate that magnetic activity driven by progressive solidification of an inner core11,12,13 is consistent with our measured magnetic field characteristics and cooling rates14. Solidification-driven convection was probably common among small body cores15, and, in contrast to thermally driven convection, will have led to a relatively late (hundreds of millions of years after accretion), long-lasting, intense and widespread epoch of magnetic activity among these bodies in the early Solar System.

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Figure 1: Representative XPEEM images of the kamacite, tetrataenite rim and CZ in the Imilac and Esquel pallasites.
Figure 2: The five best-fitting and average field components for each CZ region of the Imilac and Esquel pallasites.
Figure 3: Measured and simulated dynamo field intensity trends.
Figure 4: Cooling of a 200-km-radius body, consisting of a core, mantle and regolith.

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Acknowledgements

We acknowledge the Helmholtz-Zentrum Berlin for the use of the synchrotron radiation beamtime at beamline UE49 of BESSY II. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC grant agreement numbers 320750 and 312284, the Natural Environment Research Council, Fundación ARAID and the Spanish MINECO MAT2011-23791. We thank the Natural History Museum, London, and the Sedgwick Museum of Earth Sciences, University of Cambridge, for samples. We also thank J. Tarduno for discussions concerning magnetic shielding.

Author information

Authors and Affiliations

Authors

Contributions

J.F.J.B., C.I.O.N., J.H.-A., F.K., G.v.d.L. and R.J.H. collected the XPEEM images. T.K. and H.A. collected the SEM images. J.F.J.B. and C.I.O.N. analysed the XPEEM images. J.F.J.B. and F.N. performed the planetary cooling and dynamo generation simulations. J.F.J.B., R.J.H. and F.N. wrote the paper with contributions from all other authors.

Corresponding author

Correspondence to James F. J. Bryson.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 A 15-µm field-of-view XPEEM image of the kamacite, tetrataenite rim, CZ and aligned CZ in the Imilac meteorite.

The boundaries between the kamacite and tetrataenite rim and the tetrataenite rim and CZ are marked as solid black lines, and the boundary between the CZ and aligned CZ is marked as a dashed line. Note that the interface between the CZ and fine aligned CZ is abrupt and parallel to the tetrataenite rim. The in-plane orientation of the X-ray beam is included as an arrow; the beam was 16° out of plane. This orientation applies to all images.

Extended Data Figure 2 Representative SEM images of the CZ in the pallasites.

Coarser tetrataenite islands in the Imilac (a) and Esquel (b) meteorites. The regions corresponding to Fig. 1 are included (red boxes). The CZ is unclear and island sizes could not be extracted beyond four and six regions of the Imilac and Esquel meteorites, respectively. The average island size of each region is included in Extended Data Table 1.

Extended Data Figure 3 Additional XPEEM images and pixel intensity histograms for the Imilac meteorite.

ac, 5-µm field-of-view XPEEM images from three areas of the Imilac meteorite. These areas, along with Fig. 1a, passed the selection criteria outlined in the ‘Experimental analysis’ section of the Methods. Note that the magnetic patterns vary between images. d, Average pixel intensity histograms calculated from the four images of the Imilac meteorite. The dashed line is the optimized fit curve for the best fit for region 1; the fits for other regions were not included (to reduce clutter), but all displayed similar agreement to their experimental counterpart.

Extended Data Figure 4 As for Extended Data Fig. 3 but for the Esquel meteorite.

Extended Data Figure 5 Flow diagram of the procedure used to identify the intensity and orientation of the field recorded by each region, as described in the Methods.

The blue box represents the input to the procedure (approximate field intensity). The orange boxes represent the parts of the procedure used to deduce the approximate direction of the recorded field. The red boxes represent the part of the procedure used to optimize the direction and intensity estimates. The green box represents the final outputs.

Extended Data Figure 6 200-km-radius body cooling rate.

Values derived from Fig. 4. Cooling rates between 0 K Myr−1 and 10 K Myr−1 are depicted by the colour scale; any value greater than 10 K Myr−1 is depicted by the dark red colour. The green line is the 800-K contour from Fig. 4, which corresponds to the temperature of the cooling rates inferred from the relationship to island size14. The core cools at a rate of between 0 K Myr−1 and 3.6 K Myr−1.

Extended Data Figure 7 Planetary magnetic parameters calculated from the dynamo generation model.

a, Local Rossby number, Rol, showing the combination of parameters resulting in predominantly dipolar (red region) to multipolar (blue region) fields. b, Magnetic Reynolds number, Rem, showing the combination of parameters that do (blue region) and do not (red region) result in dynamo activity. c, Dipolar Lorentz number, Lodip, showing the combination of parameters that do (blue region) and do not (red region) produce Lodip > 10−5. A four-hour rotation period is highlighted in each figure.

Extended Data Figure 8 Additional planetary cooling models.

a, b, Temperatures (a) and cooling rates (b) from the conductive cooling model with a 100-km radius. The cooling rate at 800 K (green line) reaches the inferred values of the Imilac or the Esquel meteorites after complete core solidification. c, d, Temperatures (c) and cooling rate (d) from the conductive cooling model with a 300-km radius. The depths (horizontal black lines) corresponding to the inferred cooling rates at 800 K (green line) for the Imilac and Esquel meteorites reach the tetrataenite formation temperature (593 K, red line) before the core solidification starts.

Extended Data Table 1 Average island sizes for each region of the Imilac and Esquel meteorite studied
Extended Data Table 2 Values of the parameters used in the planetary cooling and dynamo generation models

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Bryson, J., Nichols, C., Herrero-Albillos, J. et al. Long-lived magnetism from solidification-driven convection on the pallasite parent body. Nature 517, 472–475 (2015). https://doi.org/10.1038/nature14114

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