Towards a better understanding of the engine of the geodynamo | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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  Towards a better understanding of the engine of the geodynamo

Earth's magnetic field is an essential aspect of Earth. It deviates solar winds, therefore affecting Earth's atmosphere. Many animals and bacteria depend on the geomagnetic field to navigate in their environment.

 

We know that Earth's magnetic field originates from flow motions of liquid metal in the outer core. This process, the so-called geodynamo, has been active on Earth for at least 3.4 billion years. However, the mechanism that drives the geodynamo is still debated.

 

The most accepted candidate is convection. The Earth formed very hot and is now cooling. This cooling drives convective flows in the outer core. It becomes clear that cooling can drive convection when we look at a warm miso soup. As the soup cools down, suspended soy particles move, visualising the convective cells.

 

 

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

However, recent investigations question the ability of convection to sustain the geodynamo. They suggest that the thermal conductivity of Earth's core could be as high as 250 W/m/K. With such a high value, motionless heat transport would prevail in the core. This implies that convection would not be able to sustain Earth's magnetic field for the last 3.4 billion years.

 

A team of researchers from the IPGP-Université Paris Cité and the Institut des Sciences de la Terre (CNRS Grenoble) has reviewed the mechanisms proposed to sustain the geodynamo and shows that convection could supply enough power to sustain a long-lived geodynamo if the thermal conductivity is lower than 100 W/m/K.

 

Precession, which is a slow variation of the orientation of Earth's rotation axis, and tides are other candidates to drive flow motions in Earth's core. The scientists show that the flow driven by precession is too weak to sustain the geodynamo but the flows driven by tides could have been strong enough in the early 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

 

Date de publication : 
11 May 2022