Four scenarios for the origin of geomagnetism

4.5 billion years ago, the Moon was born, after a collision between the primordial Earth and a star the size of Mars, called Theia. But some details are missing from the scenario: In a new study, researchers looked at Earth’s magnetism. Did he arrive after the collision or is he already there?

Our moon, the Moon, was born 4.5 billion years ago. The most plausible scenario to explain this assumes a collision between the primordial Earth and a Mars-sized star, nicknamed Theia. The impact would have generated a lot of debris that could then accumulate to form the moon. Whereas the core of Theia could have fused with the primordial earth core, explaining why The metal core of our satellite It is very small, with a diameter of about 660 kilometers. But one feature of the Earth remains unexplained in this hypothesis: what about Earth’s magnetism? Was it there before the collision? Was she, on the contrary, born through her? Study in the magazine penny Consider this question.

Convection inside the core can only sustain an already existing magnetic field

For this, the researchers first separated the different variants of collision with clothes. Because according to the models, the star escaped after the impact (scenarios are called hit and run) or partially merged with our planet, or even evaporated by the force of the shock! Then they studied convective fluid dynamics in Earth’s outer liquid core. “Our new idea is to indicate that our theoretical understanding of the Earth’s magnetic field today can actually tell us something about the formation of the Earth-Moon system itself, David Hughes explains in a communicationco-author of the study and a mathematician at the University of Leeds (England). At first glance, this seems somewhat surprising, and previous theories have failed to recognize this potentially important link. »

The Earth’s magnetization is maintained thanks to the convection of conductive fluids inside the outer core: this is the effect of earth dynamoAlso called geodynamo. A phenomenon that, however, can only sustain an existing magnetic field, explained Fausto Cattaneo, an astrophysicist at the University of Chicago and first author of the study. “A special property of the Earth’s dynamo is that it can maintain a strong magnetic field but not amplify a weak field.” Thus, the magnetization was created either by or before the shock, but which in this case means that the collision did not modify it enough to cancel it out. “Our hypothesis is that it reached this specific state initially, either before the impact or as a direct result of the impact,” David Hughes adds. 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 magnetic field As for ” Remaining “fossil” field, So present from the accumulation stage, describes the study, because there “Ancient magnetic evidence that the Earth was magnetised”Actually, it wasn’t at first. Then the researchers used another parameter: the type of convection. Being known about the current composition, this has allowed them, thanks to various models of evolution, to simulate how the Earth arrived in today’s conditions. For this, they relied on the equations of fluid mechanics and the magnetic Reynolds number, a single number used in magnetic hydrodynamics that characterizes the relationship between convection and diffusion in a magnetic fluid.

Thus, they concluded that “Earth’s dynamo, as long as it is driven by convective motions, must be deeply subcritical,” So it is unable to amplify the planet’s magnetization. Conclusion means that the core was first magnetized, and then convective movements made it possible to maintain this state. But it remains to be seen when and how this happened, and what this means for Earth and Moon System. “In any case, any realistic model of the formation of the Earth-Moon system must include the evolution of the magnetic field”, David Hughes adds.

Thus four main moments were identified: “The formation of the primordial Earth from disk accretion, convection of the liquid core before the collision, the effect itself and immediately after the collision”. Of these four epochs, the researchers summarized possible scenarios leading to Earth’s magnetization and conservation: they obtained four of them.

Each is marked by the time the Earth was magnetized : In the first scenario, the process starts during accumulation, but this imposes many limitations on the properties of the original accumulation disk. And the second in it Strong magnetization occurs after formation and from dynamo action driven by primary convection “It then forces a slow rotation of the Earth to avoid a subcritical situation, which is unlikely. But these two ideas add up “The very severe limitation that the effect cannot result in a significant disturbance of the fluid core”, Thus eliminating very intense collisions, which cause stars to evaporate.

What if, on the contrary, it was the effect that created the Earth’s magnetic field? In this case too “The same effect drives a dynamo that attracts the core, and the resulting spin instability does not demagnetize”This is it “The effect itself does not attract the Earth and the dynamo is driven by the instability of rotation that develops later.” It is impossible to decide at the moment. Finally, the researchers concluded the importance of future studies hydromagnetic From Earth, which will make it possible to choose between all these scenarios!