It looks like a sci-fi movie made real, Earth’s inner core might not only have stopped spinning in a so-called super-rotating state but it might just be about to spin in reverse, realizing in fact a cycle extending over several decades. This is an old question on which there is still no consensus among in-house geophysicists.
It’s been the buzz of the week since Monday January 23, 2023 following a publication in the very serious newspaper Nature Geoscience. It has led to the publication of numerous articles stating that the heart of the TerreTerre, also called “nucleus”, may have not only stopped rotating but also reversed its direction of rotation. Anyone old enough to have attended the release of The Core in 2003 and who are not a little versed in the internal geophysics of the Earth are perhaps asking the question with dread of knowing whether reality is not in the process of joining fiction, that is to say the end of our Blue Planet which will soon be roasted by the wind and the solar flaressolar flares due to the disappearance of its magnetic shield, a shield generated precisely by movementsmovements in the earth coreearth core.
The trailer of Fusionthe French version of The Core. © Paramount Pictures
So already, the North and South poles are weakly protected from solar particles, which means that we see northern Lightsnorthern Lightsand polar explorers are clearly dying there of cold and no combustioncombustion. So even if the earth’s shield disappeared it would not be the end of the biosphere and in fact it has several times been very weak at the time of the arrival of magnetic reversals. Then the article of Nature raises questions about a subject that is still controversial today and that concerns what is called the super-rotation of the inner core of the Earth, that is to say its seed, which is something quite different from the nucleus or the heart which is made up of two main structures: the outer heart which is fluid and fusionfusion thousands of degrees and the internal heart, the seed, enveloped by this fluid composed like it essentially of a alloyalloy of ferfer and of nickelnickel.
Ultrasounds of the Earth’s interior thanks to seismic waves
The seed is solidsolid (there are arguments, however, to suggest that in reality it is more complicated) with a radius of about 1,200 kilometers comparable to that of PlutonPluton but much hotter because with a surface temperature estimated at already 5,500 K, i.e. almost the temperature of the surface of the SoleilSoleil. We know of its existence since the work of the seismologistseismologist Danish Inge Lehmann who detected it using so-called P seismic waves, compression waves in an elastic medium arriving first in seismographs studying earthquakes.
Seismic P waves are physically very close to sound wavessound waves in fluids and can be described with the same equationsequations. We can therefore use the waves of earthquakes to make a kind of ultrasound of the interior of the Earth and draw conclusions about its structure and its internal composition, the vitessevitesse of propagation of these waves and even the direction of this propagation depending on the composition and the structure as well physiquephysique than chemical mattermatter traversed, crystallized or not.
It was by analyzing changes in the propagation times of P-type seismic waves on known trajectories that Xiaodong Song and Yi Yang, of Peking University, concluded that the rotation of the seed would have almost stopped towards 2009 before perhaps starting to go back in the opposite direction. It would in fact be a pendulum swinging between the two directions of rotation over a period of several decades each time.
Xiaodong Song is not unknown and it is not his first feat of arms about what is called the super-rotation of the seed because in 1996, already with a colleague seismologist with him at the era at the famous Lamont-Doherty Earth Observatory of the University ColumbiaColumbia in Palisades (New York), Paul G. Richards, he had published an article in Nature on the question by asserting that they indeed saw a differential rotation of the seed compared to the coatcoat of the earth.
A seed super-rotation that has ceased?
But what exactly is called differential rotation? Already, quite simply from the fact that the surface of the seed, and therefore the seed itself since it is immersed in the upper nucleus liquidliquid, is rotating relative to the Earth’s mantle. It is possible since it is not rigidly linked to it materially. We speak of super-rotation when it rotates faster than the Earth and of under-rotation when it rotates less quickly, which relative to the mantle causes it to rotate in the opposite direction.
Yang and Song believe that their work implies that the seed recently entered a non-rotational transition phase and heralds the transition from super-rotation to under-rotation. But their colleagues are not convinced that this is the case or that a rotational movement exists, let alone the magnitude of the possible movements. The controversy has actually been going on for decades on this subject.
To give some idea of this debate, we need only go back to the year 1995 when Gary Glatzmeier at Los Alamos and Paul Roberts at Ucla caused a stir by publishing the results of the first three-dimensional model. digitaldigital of the geodynamo inside the Earth reproducing the phenomenon of inversion of the magnetic fieldmagnetic field of our Blue Planet, before the similar results obtained with the VKS experiment.
Glatzmeier and Roberts also showed that the magnetic fields generated by the movements of convectionconvection of the metallic fluid of the outer core showed themselves capable of exerting a torque on the seed causing its super-rotation. Indeed, as noted, the following year Song and Richards presented seismic evidence of a super-rotation of 0.4 to 1.8 degrees per year.
However, the same year, other researchers showed that the field of gravitationgravitation of the earth. Indeed, the mantle is not homogeneous, with therefore variable densities of matter and not only with depth. However, the celestial mechanics involved in predicting the trajectories of the satellites on orbitsorbits bass knows well that for this reason the movements of the satellites are not those expected if one considered an Earth as a perfect sphere and of constant density, which leads to complex movements and instabilities of certain trajectories orbitalsorbitals.
The gravitygravity of the mantle must therefore also disturb the movements of matter in the core and, in fact, the calculations show that if the seed is solid and cannot change shape, no super or under-rotation is possible.
In 2008, work by researchers from the Institut de Physique du Globe in Paris and Johns-Hopkins University in Baltimore (United States) based on a Numerical simulationNumerical simulation even allowed to interpret the seismic data indicating a rotation of the seed differently as Futura explained it in an article. According to the researchers, the rotation of the seed would actually be a weak oscillation, without any long-term average displacement.
In short, the works of Xiaodong Song and Yi Yang remain unquestionably interesting, but we still have to be careful about them when they tell AFP: ” We think that the central core is, relative to the surface of the Earth, rotating in one direction and then in another, like a seesaw. A full cycle (back and forth) of this swing is about seventy years. ».
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