For decades, scientists have studied the behavior of Earth’s inner core, a solid iron ball at the heart of our planet that spans about 1,500 miles wide—nearly 70 percent of the moon’s size. Some say that deep beneath our feet, this innermost layer is spinning—and at a different speed than the rotation we experience on the surface.
In a new study, researchers hypothesize that over the past decades, the pace of the inner core’s rotation has in turn gotten gradually slower, fallen into sync with the surface rotation and then slowed even further. Now, the inner core lags slightly behind, they say.
The idea isn’t cause for alarm—the change is a normal part of a 70-year cycle, the scientists proposed Monday in Nature Geoscience. The inner core switches between spinning just a little faster and a little slower than the surface, matching the surface’s speed roughly every 35 years, they write.
But not all scientists agree on the details of how the inner core is spinning, and some aren’t convinced that it’s spinning at all.
“No matter which model you like, there’s some data that disagrees with it,” John Vidale, a seismologist at the University of Southern California who did not contribute to the new research, tells the New York Times’ Robin George Andrews.
Buried about 3,200 miles below the planet’s surface, the inner core reaches an estimated 9,000 to 13,000 degrees Fahrenheit. It can’t be accessed for samples or direct measurements of its spin. Instead, researchers have used seismic waves from earthquakes to learn about it. An earthquake sends these waves deep into the Earth and through the inner core, and scientists pick them up with sensors on the other side of the planet.
In 1936, scientists used this technique to discover the inner core’s existence. Sixty years later, in 1996, two researchers realized that the time it took for seismic waves to travel through the Earth’s center changed over time, which signaled shifts in the inner core, according to Science News’ Nikk Ogasa. Based on these findings, the pair theorized that the inner core was spinning slightly faster than the rest of the Earth.
One of those scientists, Xiaodong Song, a geophysicist at Peking University in China, co-authored the new study as well. In it, the researchers examined digital seismic records from the 1980s through 2021, as well as paper records of seismic activity from the 1960s and 1970s. The data suggest that around 2009, the inner core had slowed its spinning to roughly the same speed as Earth’s surface. Since then, it has been moving slightly slower, the researchers say.
“Most of us assumed that the inner core rotated at a steady rate that was slightly different from the Earth,” Paul Richards, a seismologist at Columbia University who did not contribute to the new paper but co-authored the 1996 paper with Song, tells the Washington Post’s Carolyn Y. Johnson. “This paper shows that the evidence for [faster] rotation is strong before about 2009 and basically dies off in subsequent years.”
The same synchronization happened in the early 1970s, they found, suggesting the inner core’s spin coincides with that of Earth’s surface roughly every 35 years.
This periodic change in rotation might be due to a tug-of-war effect between the Earth’s liquid outer core and solid mantle, writes the Times. As molten metals move in the outer core, they generate electromagnetic forces that influence the inner core to spin. But the gravity of the mantle pulls the opposite way, slowing the inner core’s rotation. One full cycle of this process takes about 70 years, write the researchers in the new study.
Other scientists have different interpretations. Based on data from 1970s nuclear explosions, Vidale co-authored a paper in June that posits the inner core’s spin follows a six-year cycle instead of a 70-year one, per Inverse’s Jon Kelvey.
Lianxing Wen, a seismologist at Stony Brook University who did not contribute to the study, does not think the inner core spins differently from the surface at all, he tells the Post. Instead, changes in the surface of the inner core over time might drive the seismic patterns the team found.
“This study misinterprets the seismic signals that are caused by episodic changes of the Earth’s inner core surface,” Wen says in an email to the Post.
Scientists study the inner core because its properties may affect how fast the Earth spins—and thus how long a day is—as well as the planet’s magnetic field, per Science News.
But they still have more to learn. “I keep thinking we’re on the verge of figuring this out,” Vidale tells Nature News’ Alexandra Witze. “But I’m not sure.”