Earth's Inner Core May Have Started Rotating in the Opposite Direction

The rotation of the Earth's inner core may be reversing, scientists have found in a study that sheds new light on geological processes occurring deep within our planet.

The results of the research, published in the journal Nature Geoscience, indicate that changes in the rotation of the inner core could take place on a scale of decades. The study's authors told Newsweek the findings have implications for our understanding of how the core influences the other layers of the Earth.

Our planet consists of several layers. The thin, outer layer, known as the crust, mostly consists of solid rock and is generally around 20 to 30 miles thick in continental areas, although in oceanic regions the average thickness is roughly four miles.

Below the crust lies the mantle, which extends downward for roughly 1,800 miles, making up 84 percent of the Earth's total volume. This layer consists of rock material that is denser than that in the crust and mostly solid, although melting can occur in some localized regions because of high pressures.

Below the mantle is the Earth's core, which has an inner and outer section. The outer core extends for around 1,400 miles and mostly consists of liquid iron and nickel. At the center of our planet lies the dense inner core, which is around 750 miles thick and thought to be solid. It primarily consists of iron and small amounts of nickel, among other elements.

In the mid-1990s, one of the authors of the latest study—Xiaodong Song, who is affiliated with the SinoProbe Lab at the School of Earth and Space Sciences at Peking University, China—in collaboration with Paul Richards at Columbia University provided the first observational evidence of the inner core's independent rotation.

They found that the inner core was rotating in the same easterly direction as the Earth itself, albeit slightly faster than the other solid layers—the mantle and surface. This spin is driven primarily by magnetic and electrical effects within the surrounding liquid outer core, as well as gravitational interactions with the mantle.

Since this discovery, however, there have been several unresolved matters regarding our understanding of the inner core rotation when it comes to factors such as the speed of rotation and whether or not it varies.

Such issues motivated Xiaodong and his collaborator on the Nature Geoscience paper—Yi Yang, who is also affiliated with the SinoProbe Lab—to gather more data over a longer duration to test different models, resulting in the latest publication. For their study, they analyzed seismic waves produced by natural, repeating earthquakes that have passed through the Earth's core since the 1960s.

"Seismic waves from a pair of repeating earthquakes usually have identical waveforms arriving at the same station," the authors told Newsweek. "However, when the waves from the repeating earthquakes interact with the Earth's inner core, they may show different waveforms and arrival times because they sample different inner-core structures."

Using these seismic waves, the researchers were able to infer the pattern of the inner core rotation over the past several decades, revealing new details about this process and its connection to the other layers of the Earth.

Their findings indicate that the inner core was rotating faster than the Earth's mantle and surface—in an eastward direction relative to the surface—from the early 1970s to around 2009. The rotation then appeared to pause from 2009 to 2011 or so. Since this period, the rotation seems to be gradually reversing, their data suggests.

"The evidence for the reversing since 2009 is quite strong—statistically over 95 percent confidence level," the authors said.

They also said these changes are likely part of an oscillation that takes place over roughly seven decades, with a previous turning point occurring in the early 1970s.

"These results help us better understand how the inside of the Earth operates and how the different layers of the system interact as a whole," the authors said. "Such multi-decadal oscillations also exist in the other Earth layers, such as the outer core, mantle and surface, indicating a possible resonating Earth system."

Two major forces act on the inner core. The first is the electromagnetic force. The Earth's magnetic field is generated by fluid motion in the outer core. The magnetic field acting on the metallic inner core is thought to drive that core to rotate.

The other force acting on the inner core is gravity. The mantle and inner core differ significantly in their physical characteristics, so the gravity between their structures tends to drag the inner core to a position of gravitational equilibrium.

"If the two forces are not balanced out, the inner core will accelerate or decelerate," the researchers said. "The 70-year oscillation of the inner core is likely driven by the electromagnetic and gravitational forces."

This oscillation correlates with similar periodic changes in other geophysical observations, such as magnetic field variations or the length of the day, the scientists said.

The inner core rotation is linked to the magnetic field because of the electromagnetic force between the inner core and the magnetic field generated in the outer core. And the rotation is also linked to the length of the day because of the gravity between the inner core and the mantle. The gravitational interaction between these two regions may also affect the deformation of the Earth's mantle and surface, linking it with surface processes.

"Thus, the Earth may be in resonance from the surface to the deepest part of the Earth with a multidecadal periodicity," the authors said. "[The study] implies that the Earth is an integrated system, and there are dynamic links between Earth layers."

Despite the findings, the authors said there are some limitations to the study, including the fact that the duration of the available seismic data is limited.

"We have not observed a full cycle of the proposed seven-decade oscillation, with the data covering less than six decades," the scientists said. "The modern digital seismic stations started to be deployed globally since the 1990s. More ancient seismic data are in paper records, but they are quite sparse and hard to access."

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