Did the Earth's core start spinning upside down?
The exact mechanism of core rotation remains a matter of debate.
Far below the surface of the Earth, a giant may have started spinning the other way than us, according to a study whose conclusions should not put an end to the controversy which agitates the world. specialists on the subject.
Earth's core, a burning sphere the size of Pluto, has stopped spinning and may even have gone the other way, this study suggests Monday in Nature Geoscience.
This planet within the mostly iron planet some 5000 km below the surface is free to move as it floats within the liquid envelope of the outer core.
The exact mechanism of this rotation remains a subject of debate. Because the little we know about it is based on the fine analysis of seismic waves, caused by earthquakes, when they pass through the center of the planet.
Analyzing seismic wave data from the past 60 years, Xiaodong Song and Yi Yang of Peking University concluded that core rotation almost stopped around 2009 before starting again. in the opposite direction.
We believe that the central core is, relative to the Earth's surface, rotating in one direction and then in another, like a seesaw, have- they told AFP.
A full cycle (back and forth) of this swing is about 70 years, they say. The last rotation change before that of 2009 would have occurred in the early 1970s. The next would take place in the mid-2040s, completing the cycle, according to the Chinese researchers.
According to them , this rotation would be more or less timed to changes in the length of the day, minute variations in the exact time the Earth needs to rotate on its axis.
To date, there are few indications of an influence of this rotation on what happens on the earth's surface. But the two authors are convinced that there are physical links between all the layers that make up the Earth. We hope our research motivates researchers to design and test models that treat the Earth as an integrated dynamic system, they explain.
Independent experts have welcomed this research, but also a certain reserve.
This is a very careful study by excellent scientists who used a lot of data, John Vidale, a seismologist at the University of Southern California, told AFP. . But he says none of the existing models do a really good job of explaining all the available data.
John Vidale published a study last year suggesting that the inner core oscillates much faster, changing direction about every six years, according to seismic data from two nuclear explosions dating back to the late 1960s and early 1970s.
A tipping moment close to that indicated by the Chinese researchers' study, a coincidence, according to the American seismologist.
Another theory with a solid basis, according to Mr. Vidale: The inner core only moved significantly between 2001 and 2013, before stabilizing since.
For Hrvoje Tkalcic, a geophysicist at the Australian National University, the inner core cycle is around 20 to 30 years, rather than the 70 years proposed by the study.
These mathematical models are probably all incorrect because even if they explain the observed data, the latter may answer other models yet to be imagined, he says.
The geophysical community therefore, in his view, has every reason to be divided over this discovery, and the subject to remain controversial.
He compares the seismologists to doctors who study a patient's internal organs with imperfect or limited equipment. As if trying to understand the functioning of the liver only with the help of an ultrasound.
Without the equivalent of; a digitizer, our representation of the Earth's interior remains hazy, he says, expecting more surprises in this area. Like the theory that the inner core conceals within it an even smaller sphere of iron, on the model of Russian dolls.