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Towards a better understanding of the geodynamic driving force

The Earth's magnetic field is useful for life, enabling many animal and bacterial species to orient themselves in their environment. It is also capable of deflecting the solar wind, leading to the attenuation or amplification of atmospheric erosion.

Towards a better understanding of the geodynamic driving force

Publication date: 11/05/2022

Press, Research

Related teams :
Geological Fluid Dynamics

This magnetic field is generated in our planet’s outer core by movements of liquid metal. This process, known as geodynamo, has been active for at least 3.4 billion years. However, the mechanisms driving it are still debated.

Convection movements in the outer core, produced by the Earth’s cooling over geological time, could be one such mechanism. To understand this, all you have to do is look at a bowl of hot miso soup. As the soup cools, soy particles move around, revealing convection movements.

However, recent studies call into question the scenario of a convection-driven dynamo. They suggest that thermal conductivity in the core could be as high as 100 to 250 W/m/K. At such values, most of the core’s heat would be transported by conduction, leaving insufficient energy to power the convective geodynamo over the last 3.4 billion years.

A team of researchers from IPGP-Université Paris Cité and the Institut des Sciences de la Terre (CNRS Grenoble) have examined these mechanisms and demonstrated that convection could power geodynamo provided that thermal conductivity is less than 100 W/m/K.

Lunar tides and the Earth’s precession, a slow variation in the orientation of its axis of rotation, are other mechanisms generating flow in the core. Scientists show that this precession-induced flow is too weak to explain the geodynamo, but that the flow generated by the tides could have powered the dynamo of the primitive Earth, when the Moon was closer to the Earth.

Ref: Landeau M., Fournier A., Nataf HC., Cébron D. & Schaeffer N.. Sustaining Earth’s magnetic dynamo. Nat Rev Earth Environ 3, 255–269 (2022). DOI: 10.1038/s43017-022-00264-1

(a) The Earth's metallic core, composed of a seed and a liquid outer core, is surrounded by a rocky mantle. The movement of liquid metal in the outer core drives the geodynamo. Magnetic field lines (red to yellow) and flow intensity (blue to yellow) are extracted from a numerical simulation of the convection-driven dynamo. (b-e) Mechanisms proposed in the literature to drive geodynamo: (b) Convection forced by secular cooling and seed growth. (c) Convection forced by the exsolution of oxides such as MgO or SiO2. (d) Precession inducing liquid metal rotation along a rotational axis (blue) different from the mantle's rotational axis (red). (e) Lunar tides that induce deformation around the outer core in about one day. Landeau et al., 2022
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