Four Scenarios for the Origin of Earth’s Magnetism

4.5 billion years ago, the Moon was born, following an impact between the proto-Earth and a star the size of Mars, called Theia. But some details are missing from the scenario: in a new study, researchers have looked into the magnetism of the Earth. Did he arrive after the collision, or was he already there?

Our satellite, the Moon, was born 4.5 billion years ago. The most accepted scenario to explain it assumes a collision between the proto-Earth and a star the size of Mars, nicknamed Theia. The impact would have generated a lot of debris which could then accrete to form the Moon. While the core of Theia would have merged with that of the proto-Earth, explaining why the metallic core of our satellite is so small, about 660 kilometers in diameter. But a characteristic of the Earth remains unexplained in this hypothesis: what about the magnetism of the Earth? Did it exist before the collision? Was it, on the contrary, generated by it? A study in the journal Pnas considered this question.

Convection within the core can only maintain an already existing magnetic field

For this, the researchers first separated the different variants of the collision with Theia. Because according to the models, the star fled after the impact (the scenarios called hit and run) or partially completely merged with our Planet, or even was vaporized by the force of the shock! They then looked at the dynamics of convective fluids in Earth’s outer liquid core. “Our new idea is to point out that our theoretical understanding of the Earth’s magnetic field today can actually tell us something about the very formation of the Earth-Moon system, explains David Hughes in a press release, co-author of the study and mathematician at the University of Leeds (England). At first glance, this seems somewhat surprising, and previous theories had failed to recognize this potentially important link. »

The magnetization of the Earth is maintained thanks to the convection of conductive fluids within the external core: it is the effect of the terrestrial dynamo, also called geodynamo. A phenomenon which, however, can only maintain an existing magnetic field, as explained by Fausto Cattaneo, astrophysicist at the University of Chicago and first author of the study. “A special property of the earth dynamo is that it can maintain a strong magnetic field but not amplify a weak one. » Thus, the magnetization was created either by the impact, or before, but implying in this case that the collision did not modify it enough to cancel it. “Our hypothesis is that it arrived at this particular state at the start, either before the impact or as an immediate consequence of the impact,” adds David Hughes. This allowed the team to narrow down the number of possible variables, so they ended up with just four scenarios!

Four scenarios for the Earth-Moon system

It all depends on when the Earth became magnetized. First, it is impossible for the magnetic field to be a ” residual “fossil” field”, therefore present from the accretion phase, describes the study, because there are “paleomagnetic evidence that the Earth was magnetized”, in fact, it was not initially. The researchers then used another parameter: the type of convection. The current configuration being known, this allowed them, thanks to different models of evolution, to simulate how the Earth arrived at the conditions of today. For this, they relied on the equations of fluid mechanics and the magnetic Reynolds number, an adimensional number used in magnetohydrodynamics which characterizes the relationship between convection and diffusion in a magnetic fluid.

They thus concluded that “the Earth’s dynamo, as long as it was driven by convective motions, must have been strongly subcritical”, therefore unable to amplify the magnetization of the planet. A conclusion which means that the core was first magnetized, then the convective movements made it possible to maintain this condition. But it remains to be seen when and how this happened, and what this implies for the Earth-Moon system. “In any case, any realistic model of the formation of the Earth-Moon system must include the evolution of the magnetic field”, adds David Hughes.

Four key moments have thus been identified: “formation of the proto-Earth from disk accretion, convection of the liquid core before the impact, the impact itself and immediately after the impact”. From these four epochs, the researchers recapitulated the possible scenarios leading to the magnetization of the Earth and its maintenance: they obtained four of them.

They are each distinguished by the time when the Earth was magnetized: in the first scenario, it is during accretion that the process begins, but this imposes many constraints on the properties of the accretion disk original. The second, in which the strong magnetization occurs after formation and from dynamo action driven by core convection » then imposes a slow rotation of the Earth to avoid a subcritical condition, which is unlikely. But these two ideas then add “the very strong constraint that the impact cannot lead to a significant disturbance of the liquid core”, therefore eliminate too extreme collisions, leading to a vaporization of the stars.

What if, on the contrary, it was the impact that had created the Earth’s magnetic field? In this case, either “the impact itself drives a dynamo which magnetizes the core, and the resulting rotational instabilities do not demagnetize it”that is “the impact itself does not magnetize the Earth and the dynamo is driven by rotational instabilities which subsequently develop”. Impossible to decide for the moment. Finally, the researchers conclude on the importance of future hydromagnetic studies of the Earth, which will make it possible to choose between all these scenarios!

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