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Research on ‘Earth's inner core rotation’ has these flaws

While a new study by two Chinese researchers, based on the analysis of seismic waves, argues that the Earth's inner core has stopped

By Ground report
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Research on ‘Earth's inner core rotation’ has these flaws

While a new study by two Chinese researchers, based on the analysis of seismic waves, argues that the Earth's inner core has stopped and may have changed its rotational motion relative to the surface, other scientists disagree: they say that the relative rotation of the inner core is very small or non-existent and that a much longer observation period would be needed to confirm a change in rotation or a reversal. They believe that getting past the debate could take a decade.

Two researchers from Peking University in China have published a controversial study in the journal Nature Geoscience, arguing that the Earth's inner core appears to have slowed to a halt, and may even be inverting. They also indicated that there would be physical links between all of Earth's layers, from the inner core to the surface, and that the inner core would rotate differently from Earth's crust and mantle.

The results of the research come from the analysis of dozens of earthquakes, and, according to its authors, this variation is correlated with small changes in geophysical observations on the Earth's surface, such as the magnetic field or the increase or decrease in the duration of the days.

"A complete cycle (in one direction and in the other) of this movement lasts around seven decades," according to the researchers. The last rotation change occurred in the early 1970s. And the next one will take place in the mid-2040s, completing the cycle, according to Chinese scientists. 

What happens if the core of the Earth stops?

“There are two main forces acting on the inner core,” Yang and Song said. “One is the electromagnetic force. Earth's magnetic field is generated by the movement of fluids in the outer core. The magnetic field acting on the inner core is expected to cause it to rotate by electromagnetic coupling. The other is the force of gravity. Both the mantle and the inner core are very heterogeneous, so gravity between their structures tends to drag the core to the position of gravitational equilibrium, which is known as gravitational coupling."

"If the two forces do not balance, the inner core will either speed up or slow down," they added. “Both the magnetic field and the Earth's rotation have a strong periodicity of 60-70 years. We believe that the proposed 70-year oscillation of the inner core is driven by electromagnetic and gravitational forces."

Song has spent decades trying to unlock the mysteries of the inner core by studying the seismic waves that traverse this distant region. He was part of the team that first reported evidence for inner core rotation in 1996 by measuring slight time (or "temporal") changes in these waves, which are generated by earthquakes.

Do we have to worry?

No way. There is no need to panic. It's something that's been going on for a long, long time, so don't expect any doomsday from this rotation change. It could influence the length of Earth's days and its magnetic field, although some researchers are sceptical. More research work is needed to pin down what mechanisms might be responsible.

A giant in the heart of planet

What little we know about the inner core comes from measuring the tiny differences in seismic waves created by earthquakes, or sometimes nuclear explosions, as they pass through the centre of the Earth. It is a huge ball of hot iron the size of Pluto, located approximately 5,000 kilometres deep.

Until today, scientists have not agreed on exactly how the inner core spins, although it can apparently spin independently because it floats on the outer core, made of liquid metal.

Cycles of seven decades

According to Yi Yang and Xiaodong Song, the authors of the new study, the rotation of the inner core almost stopped around 2009, and then it rotated in the opposite direction. They maintain that the inner core rotates, in relation to the Earth's surface, from one side to the other, as if it were a swing, according to the scientists' statements to AFP, which is reproduced by Phys.org.

According to his vision, the oscillation cycle is approximately seven decades long: this would mean that it changes direction approximately every 35 years. Chinese researchers indicated that it previously changed direction in the early 1970s, and predicted that the next change in direction would take place in the mid-2040s.

Specialists believe that a small imbalance in the electromagnetic and gravitational forces would be enough to slow down and then reverse the rotation of the inner core. Cycles of approximately seven decades in length would coincide with other observable periodic changes on the Earth's surface, such as in day length and the magnetic field, both of which have a periodicity of six to seven decades.

Counter arguments

Although his colleagues confirm the basic data regarding the slowdown of the inner core, they disagree on other points. According to University of Southern California scientist John Vidale, who was not involved in the research (Multidecadal variation of the Earth’s inner-core rotation), seismic waves from nuclear tests show that the inner core can reverse its rotation approximately every three years, he told Science News.

Along the same lines, Hrvoje Tkalcic, a geophysicist at the Australian National University, has published research that suggests that the cycle of the inner core is 20 or 30 years, instead of the 70 proposed by Chinese scientists, highlighting the AFP.

Meanwhile, other researchers have indicated that the inner core is not moving at all. They argue that changes in the shape of the inner core surface could explain differences in the travel times of seismic waves.

Natural vibrations

The controversy seems endless: according to an article published by the Swiss newspaper Neue Zürcher Zeitung, researcher Rudolf Widmer-Schnidrig, from the Schiltach Community Geoscientific Observatory in the Black Forest, the Chinese authors have omitted something essential: the Earth has natural vibrations, similar to a giant bell. Investigations of these oscillations in no way indicate that the Earth's inner core rotates any differently than the Earth's crust and mantle.

For Widmer-Schnidrig, the natural vibrations of the Earth are no less sensitive to signals from the relative rotation of the inner core than seismic waves. Consequently, he believes that studies of natural vibrations would need to be included in the discussion before definitive conclusions could be drawn.

For his part, Seiji Tsuboi, from the Japanese Agency for Earth and Sea Science and Technology, also quoted by the Swiss newspaper, maintains that the relative rotation of the inner core is very small or non-existent. In either case, a much longer observation period and more data would be needed to confirm a change in rotation.

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