Something strange is happening to Earth’s inner core

In an absolute sense, the inner core still rotates in the same direction as the mantle and surface. Imagine a bus and truck moving next to each other in the same direction. The truck slows down and the bus moves forward. From the perspective of the bus, the truck now appears to be moving backwards. But to a pedestrian, both vehicles appear to be moving forward.

Similarly, the new study suggests that if a person standing on Earth’s surface could see the inner core — similar to a bus driver looking at a truck — it would appear to be turning in the opposite direction it was several decades ago. before.

The 2023 study was a big hit in the headlines, but less appreciated by other researchers. Some, such as seismologist Lianxing Wen of Stony Brook University in New York, countered that the inner core was not rotating on its own and that the data could be explained by the changing shape of the inner core’s surface. Others were convinced that the rotation fluctuated over shorter periods of time. Another analysis of data from the 2023 study suggested a 20- to 30-year oscillation, in contrast to a study co-authored by Vidale from the previous year, which suggested the rotation oscillated over a 6-year period.

For the new study, Vidale and his colleagues looked at repeated earthquakes—those that struck in the same place but at different times—from 1991 to 2023 in the South Sandwich Islands near Antarctica. Seismic waves from those tremors rippled through the interior of the planet, with some passing through the inner core. When those waves arrived on the far side of the planet, instruments in Alaska recorded the ground shaking as sharp line graphs called waveforms.

Vidale and his colleagues looked for waveforms from months or years away that matched. If the inner core rotates independently of the other layers of the Earth, then waves from repeated earthquakes must pass through different parts of it. And because the anatomy of the inner core is thought to be non-uniform, those different wave paths should produce distinct waveforms. But if the 2023 study was correct and the inner core had changed its spin relative to the surface, there should be some identical waveforms from before and after the twist, marking when the inner core had reverted back to an old track .

From 200 waveform comparisons, the team found 25 matches. These data suggest that the inner core reversed its spin relative to the mantle sometime around 2008, after which it continued to spin less than half as fast in the new direction.

According to Vidale, the slower return could indicate that the inner core is being deformed by the gravitational pull of the mantle, which contains roughly 70 percent of Earth’s mass. Dense pockets of the mantle can cook the inner core as it mixes, distorting the oscillation, he says. “We know that the surface of the inner core is right at the melting point, so it’s natural to think that it’s soft in the outermost part.”

After observing how the waveforms match up over time, Vidale says he now agrees with the 2023 study’s conclusion: the rotation of the inner core probably oscillates on a roughly 70-year cycle.

As for Wen, “nothing has changed.” He insists that the swelling and shrinking of parts of the core’s inner surface can fully explain the data. These spots can grow or shrink by a kilometer or more over several months — changes significant enough to change the waveforms of repeated earthquakes, he says.

Geophysicist Hrvoje Tkalčić says: “There are many possibilities that the truth is somewhere in between”. Seismologists seem to be converging on the idea that the rotation of the inner core is distinct and oscillating, but “we need more data to find the final truth,” says Tkalčić, of the Australian National University in Canberra. Researchers have to make a lot of assumptions about inaccessible regions of Earth’s interior, he says, hence the different perspectives.

Some clarity may emerge in the coming years. If the rotation of the inner core oscillates at the frequency suspected by Vidale’s team, it could soon re-enter a vigorous part of the cycle, he says. About 20 years ago, the inner core appears to have spun briefly too quickly, and should soon do so again, Vidale says. “By looking at it for the next five or 10 years, we can probably get a better idea of ​​what happened then.”