How was the Earth able to maintain its magnetic field for billions of years?

The magnetic field, which has protected the atmosphere as well as terrestrial life since its appearance, has its source in the heart of our Planet. More precisely, it is generated by the movements turbulent iron liquid which makes up the outer core. This geodynamo has been active for more than 3.4 billion years, and despite frequent and repeated inversions, she never flinched. However, the mechanisms capable of sustaining these fluid movements over the very long term are still not fully understood. Several processes are mentioned, without their participation being clearly quantified.

The new kernel paradox

The convection natural is certainly the most accepted hypothesis at present. Numerical simulations in support, it appears that this mechanism, generated by the slow cooling of the Earth’s core, is indeed capable of producing on its own a magnetic field which resembles that which is measured on the surface of the Earth. However, a question shakes the scientific community. This continuous convection over several billion years is intimately dependent on the thermal conductivity of the kernel, the value of which is currently highly debated. Thermal conductivity represents the ability of rocks to conduct heat heat and evacuate it outside the Earth.

However, published data range over a very wide range, between 20 and 250 Wm^-1.K^-1. If the lower values ​​question the maintenance over time of an efficient convective system, the higher ones also pose a problem, by suggesting that the majority of the heat flux would then escape too quickly to produce a convection capable of maintaining in a strong magnetic field over time. This problem, known as the “new core paradox”, perhaps highlights the fact that convection is not the only mechanism at work.

Scientists have therefore focused on the analysis of the various possible solutions and their respective contribution to the geodynamo effect.

Exsolution, tides and precession: a significant participation?

Among the mechanisms considered, there is the exsolution of light elements. This process describes leakage of certain chemical elements light out of the core, which leads to a gradual densification of the metallic liquid. The tides earth generated by the gravitational attraction of the Moon as well as precession of the Earth, are also alternative mechanisms to be considered. Precession represents the slow variation of the rotational orientation of the Earth. It appears that these last two orbital factors would indeed be able to produce turbulent flows within the outer core, and therefore participate in the geodynamo effect. It remains to be seen whether this participation is significant or not.

In a study published in the journal Nature Reviews Earth and Environmentresearchers from the Institute of Physics of the Globe of Paris and ISTerre, in Grenoble, take stock of the contribution of these different mechanisms to the long-term generation of the Earth’s magnetic field.

Convection, yes, but with the help of exsolution

It thus appears that the Earth’s precession is a minor parameter, the flow it produces within the outer core being far too weak to participate significantly in the geodynamo effect. The tides, on the other hand, may well play an important role. The results suggest that they may have been a major mechanism during Earth’s earliest times, more than 1.5 billion years ago, when tidal forces terrestrial were much more important than today.

Regarding the convection mechanism, it would indeed be able to support the geodynamo on its own over the long term, but on condition that the thermal conductivity is less than 100 Wm^-1.K^-1, which is not yet proven. The scientists show, however, that the problem is solved if we take into account the exsolution of the light elements which increase this limiting conductivity value by 15 to 200%. As a result, convection, aided by the exsolution of light elements from the core, would be the mechanism mainly involved in the maintenance and stability over time of the earth’s geodynamo. The effect of the tides, which could be significant, is still poorly constrained today, as is the rate of exsolution.