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Sustaining Earth’s magnetic dynamo

Abstract

Earth’s magnetic field is generated by fluid motions in the outer core. This geodynamo has operated for over 3.4 billion years. However, the mechanism that has sustained the geodynamo for over 75% of Earth’s history remains debated. In this Review, we assess the mechanisms proposed to drive the geodynamo (precession, tides and convection) and their ability to match geomagnetic and palaeomagnetic observations. Flows driven by precession are too weak to drive the geodynamo. Flows driven by tides could have been strong enough in the early Earth, before 1.5 billion years ago, when tidal deformation and Earth’s spin rate were larger than they are today. Evidence that the thermal conductivity of Earth’s core could be as high as 250 W m−1 K−1 calls the ability of convection to maintain the dynamo for over 3.4 billion years into question. Yet, convection could supply enough power to sustain a long-lived geodynamo if the thermal conductivity is lower than 100 W m−1 K−1. Exsolution of light elements from the core increases this upper conductivity limit by 15% to 200%, based on the exsolution rates reported so far. Convection, possibly aided by the exsolution of light elements, remains the mechanism most likely to have sustained the geodynamo. The light-element exsolution rate, which remains poorly constrained, should be further investigated.

Key points

  • Numerical models of the geodynamo driven by thermo-chemical convection account for most of the observed properties of the present geodynamo.

  • The thermal conductivity in Earth’s core remains debated, with published values ranging between 20 and 250 W m−1 K−1. With a conductivity as high as 250 W m−1 K−1, motionless heat transport would prevail in the core implying that convection would not be able to sustain Earth’s magnetic dynamo for 3.4 billion years (Gyr).

  • Nevertheless, thermo-chemical convection caused by the slow cooling of Earth supplies enough power to the geodynamo when the thermal conductivity is lower than 100 W m−1 K−1. The exsolution of light elements increases this upper conductivity limit only marginally or by up to a factor of three, depending on the exsolution rate.

  • Flows driven by precession are too weak to drive the geodynamo.

  • Flows driven by tides could have been strong enough before 1.5 Gyr ago, when tidal deformation and Earth’s spin rate were larger than today, which calls for further investigation of tidally driven dynamos.

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Fig. 1: The Earth’s dynamo requires turbulent motion of liquid iron in Earth’s core.
Fig. 2: The Earth’s dynamo operates on a broad range of timescales.
Fig. 3: The morphology of the Earth’s magnetic field is best reproduced by convection-driven dynamos.
Fig. 4: Convection can power the geodynamo in the distant past when thermal conductivity is lower than 100 W m−1 K−1.
Fig. 5: Precession cannot drive a dynamo but tides generated strong flows that could drive a dynamo in the early Earth.

Code availability statement

The code for Figs 4 and 5 is available from https://doi.org/10.6084/m9.figshare.16722346.

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Acknowledgements

The authors thank J. Badro, Y. Gallet, S. Labrosse, G. Morard, A. Nilsson and P. Olson for useful discussions. M.L. was supported by the Programme National de Planétologie (PNP) of CNRS-INSU, co-funded by CNES. N.S., A.F. and H.-C.N. acknowledge support by the French Agence Nationale de la Recherche under grant ANR-19-CE31-0019 (revEarth). D.C. acknowledges support from the European Research Council (ERC) under grant agreement no. 847433 (Theia project).

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All authors contributed equally to the thinking and writing of the article. M.L. contributed particularly to the convection energy budget, N.S. and D.C. to precession energetics and dynamos, and A.F. to the review of geomagnetic data.

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Correspondence to Nathanaël Schaeffer.

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Nature Reviews Earth & Environment thanks H. Matsui, J. Wicht and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Glossary

Exsolution

Precipitation of a dissolved substance, due to a decrease in temperature and hence in solubility.

Turbulent flow

A high Reynolds number flow, which, because of instabilities, exhibits a wide range of lengthscales and timescales, with apparently random fluctuations requiring a statistical description.

Geomagnetic secular variation

Time variations of the magnetic field of the Earth with periods ranging from one year to hundreds of years.

Alfvén waves

In a conducting liquid or plasma, oscillations of the fluid and magnetic field that propagate together along magnetic field lines. Discovered by H. Alfvén in 1942, they earned him a Nobel Prize in 1970.

Mass anomaly flux

Thermal or chemical mass anomaly that passes through a surface area per unit of time (in kg s–1).

Mantle thermal catastrophe

In thermal-evolution models of the Earth, solutions in which the mantle becomes fully molten within the past 2 Gyr. These solutions are incompatible with petrological observations, and hence unacceptable.

Boundary layer

The fluid layer located in the vicinity of a bounding surface, where diffusive processes prevail.

Flow instabilities

In hydrodynamics, a simple fluid flow can become unstable when some quantitative condition is met, leading to more complexity, enhanced mixing and sometimes chaotic behaviour or turbulence.

Obliquity

The angle between the normal to the ecliptic plane and the axis of rotation of the Earth. Its present-day value is 23.5°.

Kinematic dynamo

A dynamo sustained by a prescribed velocity field, discarding any back-reaction of the magnetic field on the flow. A kinematic dynamo leads to unbounded growth of the magnetic field.

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Landeau, M., Fournier, A., Nataf, HC. et al. Sustaining Earth’s magnetic dynamo. Nat Rev Earth Environ 3, 255–269 (2022). https://doi.org/10.1038/s43017-022-00264-1

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