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All schoolchildren know that the Earth has a magnetic field – this is what allows the compass to align north-south and allows us to navigate the oceans. It also protects the atmosphere, and therefore life, from the powerful wind of the sun.
But what about other Earth-like planets in the galaxy? Do they also have magnetic fields to protect emerging life?
A new analysis examines a type of exoplanet – the super-Earth five times larger than our own planet – and concludes that they probably have a magnetic field, but generated in a totally new way: by the magma of the oceans planets.
The surprising discovery that melted rocks slowly growing on the surface or below the surface can generate a strong magnetic field also suggests that in the early years of the Earth, while it was essentially a piece of rock melted, it also had a magnetic field generated by the magma. This was in addition to its current field, which is generated in the outer core of liquid iron.
"This is a new regime for the generation of planetary magnetic fields," said Burkhard Militzer, a professor of Earth Sciences and Planets at the University of Berkeley. "Our Earth's magnetic field is generated in the outer liquid iron core, and in Jupiter it comes from the convection of liquid metallic hydrogen.On Uranus and Neptune, it is supposed to be generated in the layers of ice. now added molten rocks to this diverse list of field-generating materials ".
The link between the inside of a planet and its magnetic field also offers astronomers a way to learn about the composition and age of exoplanets too far away to be visited.
"It's very far into the future, but if someone observes an exoplanet and finds a magnetic field, it may indicate that there is an ocean of magma, even if it can not be." not see it directly, "said Militzer.
The findings also have implications for the life chances of other planets. As the magma oceans cool down from above, a life-friendly surface could appear as the melted mantle continues to disintegrate.
"A magnetic field is useful for protecting a planetary atmosphere from stellar winds," said François Soubiran, a former postdoctoral fellow at UC Berkeley, currently at the Ecole Normale Supérieure de Lyon, France. "Most of the super-Earths that we detect now are very close to their host stars and exposed to very strong stellar winds, so the possibility of the existence of a magnetic field is definitely a key part of the evolution of the planet and its habitability. "
Soubiran and Militzer published their findings on September 24 in the journal Nature Communications.
Internal Dynamo of the Earth
The current Earth magnetic field is generated in the molten iron outer core, where rising and falling masses of electrically conductive liquid iron, combined with the planet's spin, create a dynamo and a persistent magnetic field.
But Rocky Earth was melted after its initial formation, 4.5 billion years ago, and some layers may have remained melted and convected – like boiling water, but more slowly – during millions of years after his birth. Could the slowly convective ocean magma generate a magnetic field similar to that generated in the iron core today?
The same question arose after the discovery of super-Earths around other stars. The superlands are so massive that their interior, the mantle, should remain liquid and convective for a few billion years after its formation.
In both cases, slow boiling magma on a rotating planet can only generate a strong magnetic field if the liquid rock conducts electricity.
Nobody knew if it was true
Silicate experiments – a term referring to the thousands of silicon-based minerals that make up the rocky interior of the Earth – at high temperatures and pressures inside a super-Earth are difficult. Even determining whether a rock remains solid or liquid is not simple under the typical conditions of the interior of the planets: temperatures of 10,000 ° C and pressures 10 million times higher than those of the air around us.
"At normal temperatures and pressures, silicates are completely insulating, electrons are either tightly bonded to nuclei, or they are molecularly bonded and can not move freely and create macroscopic electric currents," said Soubiran. "Even though the high internal pressure could reduce the barriers to the movement of electrons, it was not necessarily obvious that silicates would conduct in the super-Earths."
But Soubiran and Militzer had access to computer models at the atomic scale of the minerals that allowed them to calculate the conductivity, in this case, quartz (silicon dioxide), magnesia (magnesium oxide) and carbon dioxide. 39, a silicon-magnesium oxide (post-perovskite). , which are all common in the rocks on the Earth, the Moon and probably all the planets in our solar system.
After long calculations for all three, they found that these silicates became moderately conductive when they passed from solid to liquid at high temperatures and pressures. When they integrated the conductivities into models of the interior of the Earth, they discovered that the rocks were sufficiently conductive to support a dynamo and thus a magnetic field.
"Our calculations showed that the disorganized structure of the liquid helped electrons become drivers," said Soubiran. Silicates liquid at 10 000 C and 10 million pressure atmosphere have for example only about one hundredth of the conductivity of liquid iron.
Soubiran noted that rotating planets with a period of two days or more would generate a magnetic field similar to the Earth: a dipole field with a clear north and south. A slower rotation, however, could create a more disorganized field that would be harder to detect from afar.
Bruce Buffett, a Berkeley expert on the dynamics of the Earth's interior who did not participate in the research, said planets could only generate magnetic fields if they had the right balance of Electrical conductivity and fluid velocity to create the return necessary to maintain a magnetic resistance. field.
"Many geophysicists expected that, at least in terrestrial conditions, the conductivity of liquid silicates would fall more into the category of good, if you had very large fluid movements to compensate for low conductivity, you could have a magnetic field, "said Buffett, a professor of earth sciences and planets.
"This is the first detailed calculation for higher temperature and pressure conditions, it shows that the conductivity seems to be a bit higher, and the fluid movements that you would need to make it work are maybe a little less extreme. "
Research Report: "Electrical Conductivity and Magnetic Dynamos in the Magmatic Oceans of the Super-lands", Francois Soubiran and Burkhard Militzer, September 24, 2018, Nature Communications
Related Links
University of California, Berkeley
Lands Beyond Beyond – Additional Solar Planets – News & Science
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