How to explain Mercury’s massive iron core?

Among the terrestrial planetssolar systemAnd Mercury is an exception. In its contrasting interior in silicate shell and metal core, the latter represents three quarters of its total mass! Much higher in the case of Venus and Earth – a third – as for Mars – a quarter. Even more surprising is its density, given the generally higher iron content, which exceeds all other planets. To explain this difference, two theories have followed one another. The first imagined a gradient of the fraction of iron available to form the planets according to the distance from the sun. The vision was then replaced by the idea of ​​collision (s) which blew up the rock mantle of Mercury. However, a new paradigm seems to dust off the oldest theory by tilting in its favor. But has the mystery of iron mercury really been solved?

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The magnetic field of the young sun. The solar dynamo creates a strong magnetic field (green lines) that attracts the iron-rich magnetic particles to the Sun in a planetary accretion disk. Source: NASA / JPL-Caltech.

The magnetic field of the young sun

Researchers at the University of Maryland and the University of Tohoku have developed a model that represents the chemical makeup of planets as a function of heliocentric position. As a starting point for their study, they postulated that the density, mass and iron content of the planet vary with its distance from the solar magnetic field. Specifically, about the young Sun, at the time when planets were still forming in a protoplanetary disk. “The Sun is made of plasma, highly conductive, capable of supporting and amplifying the magnetic field”, describes Joao Pedro Marquez, lecturer at the IAS and at the University of Paris-Saclay. However, macroscopic movements such as convection or rotation have been shown to amplify them more, which is called the dynamo effect. That is why young stars, spinning very quickly, have a strong magnetic field. But this electromagnetic dance is in reality a waltz, in which the partner of the solar star is only the disc that surrounds it …

Much like a spinning disc, the sun is absorbed into plasma, through which electric currents pass. “These two astrophysical bodies interact and create instability in the disc,” notes the astrophysicist. The resulting magnetic field will attract iron particles to the center of the solar system, producing a gradient visible on both planets and meteorites. Then the accumulated iron sinks into the core under the influence of gravity, at the same time expelling oxygen from the lighter. But is this process enough to get an iron core like Mercury? I am not entirely sure according to Joao Pedro Marquez: “Such a scenario does not exclude the addition of a celestial collision, because Mercury still contains a lot of iron! “.

How the Mercury Credits were formed: Left: NASA / JPL-Caltech Right: Reprinted with permission from Macmillan Publishers Ltd: Nature [4737348: 460-461,© 2011]Nasa.

From iron to earthly life

However, the results presented by the researchers raise questions according to Matthew Finsendon, who is also a lecturer at the IAS and the University of Paris-Saclay. Thus, the planetary orientation presented in the research mainly depends on the state of Mercury, which clearly stands out in terms of intensity. The scale used also curiously flattens the state of Venus which, unlike the defended scenario, is less dense than Earth – but more distant. A problem that seems to be solved by the unexpected absence of Venus in several characters … but it is not the only anomaly to count. “According to this model, the formation of Mercury will be completed in just two million years, which seems fast,” notes Matthew Finsendon. In addition, the migration of giant planets should be at the origin of important movements of matter, which would distort the appearance of the gradient. The reasons for questioning are piling up… However, there is one point in particular that looks promising: the state of exoplanets.

To validate their findings, the researchers propose to observe other star systems similar to ours, in order to see if the planetary iron gradient in our system is located elsewhere. If the density of the planets decreased with the distance from the star, they would be right… really? “There are still potential biases. Perhaps near the star, the outer layers of the planet risk disappearing after the collision, in which case the theory of the collision of Mercury remains plausible, ”suggests the astrophysicist. With its unusual nucleus, Mercury is one of the few planets that still has a magnetic field. The core of a planet is truly crucial to sustaining life… as we know it! The blue planet therefore has a magnetosphere through the liquid part of its iron core. Cosmic rays refract there before seriously damaging the health of living organisms. What if life existed without a magnetic field? With organisms living exclusively underground or resistant to radiation. Matthew Finsendon insists: “If there is life, it must adapt as much as possible to its development environment.


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