The Earth’s inner core – a sphere of nearly pure iron more than 3,100 miles (5,000 kilometers) deep that is hotter than the Sun – has slowed down and may be spinning in the opposite direction to the planet’s surface, according to a study published today in Nature Geoscience. This slowdown may have global effects, such as shortening the days by a few fractions of a second and influencing the climate and sea level.
The authors of the paper, Yi Yang and Xiaodong Song, from Peking University’s Institute of Theoretical and Applied Geophysics in China, have tried to solve an enigma that has been puzzling scientists since it was confirmed a few decades ago that the Earth contains a fifth layer in its inner core.
The inner core is a solid sphere about 750 miles (1,200 kilometers) in diameter that spins freely in a sea of molten iron and other metals, which is known as the outer core. The rotation of this gigantic sphere is like a dynamo that generates the Earth’s magnetic field, which protects it from space radiation and allows life to exist on its surface. The Earth’s mantle extends around this nucleus, measuring almost 1,900 miles (3,000 kilometers) in thickness, and finally, there is the outer crust, with a mere 25 miles (40 kilometers) of average thickness.
Getting to the center of the Earth is an impossible challenge. In the late 1970s, Soviet scientists began digging a well on the Kola Peninsula in northern Russia. After years of work, they reached a depth of 7.5 miles (12 kilometers), the maximum that has been dug into the bowels of the planet. It appears impossible to go much further without the walls of the hole collapsing under the pressure.
To understand what happens in deeper areas, scientists typically analyze earthquakes. The variation of seismic waves as they pass through the planet reveals the internal composition of the core and its rate of rotation.
In 1996, Xiadong Song, then working in the United States, was one of the authors of a groundbreaking study that analyzed seismic signals and showed that the Earth’s inner core rotates faster than the crust. In 2005, this scientist confirmed these observations and detailed that the core rotates one more time than the rest of the planet every 900 years, approximately. This lack of synchrony is partly due to the fact that the tides and the progressive distance of the Moon have been slowing down the crust, which means that the days do not last exactly 24 hours – 1,400 million years ago, a day was less than 19 hours. Parallel to this phenomenon, the days have been shortening by a few fractions of a second for a few years without anyone knowing why.
For the new study, Song analyzed nearly 200 earthquakes in the South Sandwich Islands, a remote Atlantic archipelago near the South Pole, that took place between the 1960s and the present. These quakes happened in pairs and produced identical waves. But when they were captured in measuring stations in Alaska, near the North Pole, their waves arrived slightly out of step, as if, when passing through the nucleus, the nucleus rotated faster than the crust.
Analysis of these tremors with computer models show that in 2009 the Earth’s core slowed down and since then it has rotated a little slower than the crust. This observation has an unusual implication. “Seen from space, the core rotates at almost the same rate as the rest of the planet. But from the point of view of the surface, where the seismic stations are, the core is now rotating in the opposite direction; to the west,” Song explains to EL PAÍS.
Scientists also detected a similar difference in the mid-1970s. “We infer that there is an oscillation cycle that lasts about seven decades,” Song explains. “This suggests that there is a resonance that connects all the layers of the Earth that occurs with that rhythm.”
This phenomenon can have global effects. “In recent years, the days are getting shorter, and it is possible that this is partly due to the Earth’s core,” he says. According to Song, thanks to the core anomaly, a day is one thousandth of a second shorter now than it was in 1970. “The rotation of the inner core within the outer core also alters the inner gravitational field and causes deformations on the surface, which in turn can influence sea level. These changes could also affect the global temperature of the planet,” he adds.
In early 2022, John Vidale and Wei Wang of the University of Southern California analyzed tremors caused by pairs of nuclear bombs detonated by the Soviet Union between 1971 and 1974. The results suggested that, in that period, the inner core of the Earth had stopped and began to rotate in the opposite direction with respect to the crust, a result similar to that found by the current study. “The new study is very good, but even so it is difficult to confirm if what it proposes is true,” Vidale warns today. The problem is that the number of earthquakes that can be analyzed for these studies is limited. In addition, these jobs require extensive computing time with powerful computers to fully simulate the Earth. “It is possible that in five or 10 years, with more data and better simulations, we will be able to know if, as it seems, the Earth’s core follows these cycles,” adds the American scientist.
Puy Ayarza, director of the Department of Geology at Spain’s University of Salamanca, believes that the new work “is innovative and provocative.” What the Chinese geologists have observed, he says, “fits well with proven facts such as the fact that the Earth’s magnetic field has been changing very rapidly in recent decades and that its dipole character [with north and south poles] is weakening.” “This dipole character is given by the differential rotation of the inner nucleus within the outer one, so it could be true that it is rotating more slowly. It seems that the movement of the inner core may not be as uniform as we thought. The work is a step forward and provides a lot of data. We will see if their conclusions are true,” he adds.
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