Earth’s Inner Core May Be Slowing Its Spin, Another Study Suggests

The only way to know firsthand would be to take a real Journey to the Center of the Earth—but for now, a new study suggests our planet’s inner core might be spinning more slowly than it used to.

The Earth’s inner core, a solid ball of iron and nickel about 70 percent of the size of the moon, rotates along with the rest of the planet, but scientists disagree about whether it might spin at a slightly slower, faster or equal speed, compared to the outer layers. The study, published in June in the journal Nature, uses seismic waves to add evidence for the idea that the inner core switches between spinning faster and slower than Earth’s crust.

Such changes in the inner core’s spin could have a small impact on the Earth’s magnetic field—and they could, very marginally, alter the length of days up on the surface.

The inner core is “a planet within a planet, so how it moves is obviously very important,” Xiaodong Song, a seismologist at Peking University in China, told the New York Times’ Robin George Andrews last year.

But scientists have been struggling to pin down the specifics of the inner core’s motion. The controversy of whether or not the heart of the planet spins differently than the rest of the Earth reignited in January 2023, after Song and others proposed in Nature Geoscience that the core’s rotation oscillates in a 70-year cycle, alternating between leading and lagging the spin of the planet every 35 years. The most recent switch, from faster to slower than the outer part of the Earth, occurred about 15 years ago, according to those researchers.

“This paper shows that the evidence for [faster] rotation is strong before about 2009 and basically dies off in subsequent years.” Paul Richards, a seismologist at Columbia University who was not involved with either paper, told the Washington Post’s Carolyn Y. Johnson in 2023.

Not all scientists who study the interior of the planet agreed, however. Dongdong Tian, a seismologist at the China University of Geosciences, and Lianxing Wen, a geoscientist at Stony Brook University, wrote in a comment to the journal Geophysical Research Letters that fluctuations on the surface of the inner core could explain the observed changes, even if its spin was constant.

Now, the new study provides support in favor of last year’s paper, suggesting the inner core has been decreasing its speed since about 2008—and it fuels further debates on the nature of Earth’s center.

Researchers have no way of directly observing the planet’s innermost layers, so they instead rely on tracking the energy released by earthquakes. These seismic events generate waves of energy that pass through the Earth’s crust, mantle and core.

To view these waves, geologists use a tool called a seismometer to record the shaking of the Earth’s surface that’s generated by an earthquake. The instrument produces wiggles on a line graph, which they call waveforms. The researchers understood that if they tracked waveforms from seismic waves that passed along the same path through the center of the Earth in different years, they might be able to see how the inner structure of the planet is evolving over time.

For the study, they observed patterns in the waveforms from 121 earthquakes that originated in the South Sandwich Islands near Antarctica from 1991 to 2023. These were recorded on a seismometer in Alaska after traveling through the Earth’s interior. If the inner core is rotating at a different speed than the layers above it, then the repeated seismic waves should have passed through different areas of the inner core during the 32-year period, leading to differences in the detected waveforms, even while they take the same overall path through the planet, per Science News’ Nikk Ogasa.

But if the inner core was shifting its speed in a cycle, some of these waveforms would repeat. And sure enough, the team was able to match more than 25 patterns before and after 2008, showing a symmetry they say is explained by a recent slowing of the inner core’s speed.

“When I first saw the seismograms that hinted at this change, I was stumped,” study co-author John Vidale, an earth scientist at the University of Southern California, says in a statement. “But when we found two dozen more observations signaling the same pattern, the result was inescapable. The inner core had slowed down for the first time in many decades.”

Still, not everyone is convinced. Wen tells Science News that even with the new study, “nothing has changed”—he maintains that the expanding and contracting of the inner core’s surface is significant enough to explain the patterns observed in the waveforms.

Debates like these tend to polarize the scientific community. But Hrvoje Tkalčić, a geophysicist at the Australian National University who was not involved with the study, tells Science News “it is very likely the truth is somewhere in between.” To try to understand the unreachable depths of Earth’s interior, scientists tend to make a lot of assumptions, he says. “We need more data to find the ultimate truth.”

Rudy Molinek | READ MORE

Rudy Molinek is Smithsonian magazine's 2024 AAAS Mass Media Fellow.