The Cosmic Watchdog

WASHINGTON, D.C. — Earth’s inner core, a solid metal ball gyrating within the molten outer core, may be both slowing down and changing shape.

Recent analyses of earthquake waves have suggested that around 15 years ago, the inner core’s rotation may have slowed so much that it appeared to pause or reverse direction relative to the surface. But a new analysis suggests something more must be changing at Earth’s center.

The most probable explanation is that the inner core is not only rotating differently — its surface is probably also morphing, geophysicist John Vidale of the University of Southern California in Los Angeles reported December 9 at a meeting of the American Geophysical Union. The finding could help resolve a long-standing debate over what is changing at the inner core.

No instrument can physically probe Earth’s core. So, researchers study it using seismic waves from earthquakes. Scientists typically use quakes that occur in the South Sandwich Islands near Antarctica, which repose on the opposite side of the planet from instrument arrays in Alaska. The earthquake waves travel through the planet like sonar waves through water, with some passing through the inner core on their way to Alaska. Instruments there then record the waves as squiggly signatures called waveforms, which contain information about what the waves encountered on their journey through Earth.

For robust detections of changes in the inner core, researchers compare similar-sized quakes that occurred in the same place but at different times. Such twin temblors, known as doublets, should generate the same waveforms if their journeys through Earth were identical. But researchers have observed that some doublets in the South Sandwich Islands generate different waveforms in Alaska, indicating that something in the inner core had changed between the times the two quakes in those doublets occurred.

In 2023, geophysicists reported that the waveform differences stemmed from the inner core’s rotation slowing down so much that it appeared to have stopped moving — or even reversed — relative to the surface sometime around 2009 (SN: 1/23/23). Then, earlier this year, Vidale’s team seemingly confirmed the reversal. They were able to match some waveforms before and after the turnaround, identifying times when the inner core had reassumed a previous orientation toward the surface (SN: 6/25/24).

For the new study, Vidale and colleagues analyzed about 200 pairs of earthquakes that occurred from 1991 to 2024. They examined matching waveform pairs from before and after the reversal, recorded at two separate receiver arrays located near Fairbanks, Alaska, and Yellowknife, Canada.

Intriguingly, 10 doublets showed subtle differences in waveforms at Yellowknife that were not present in the Fairbanks waveforms. The team knew that the waves reaching these arrays, which are roughly 1,600 kilometers apart, take slightly different paths through Earth: Waves reaching Fairbanks penetrate deep into the inner core while those reaching Yellowknife graze its exterior.

“The simplest explanation is deformation to the shallow inner core,” Vidale says.

It’s possible that the entire geoid-shaped inner core is deforming, like a football being reshaped so its ends point in two new directions. Alternatively, he says, patches of the inner core’s surface may be swelling or contracting. That would be like small bumps and dents forming on the metaphorical football. It’s also possible that both processes are occurring. Such changes could be driven by the gravitational pull of the mantle — Earth’s largest internal layer — or by material flowing in the outer core, Vidale says.

This is not the first time researchers have reported that the inner core’s surface changes over time. In 2006, geophysicist Lianxing Wen of Stony Brook University in New York reported that patches of the region’s surface may rise or subside by hundreds of meters per decade, possibly due to material exiting the inner core as it cools. But unlike Vidale, Wen and colleagues like geophysicist Xin Zhang of the University of Science and Technology of China in Hefei maintain that the inner core does not rotate differently than Earth. “Surface changes can fully explain all the results,” Zhang says.

Geophysicist Xiaodong Song of Peking University in Beijing, who was one of the first to report that the inner core rotates differently from the rest of the planet, generally agrees with Vidale’s findings. While the waveform differences are probably caused mostly by rotational changes, other processes such as surface changes may also be occurring, he says. “It’s not either or.”

As for how all of this affects life on Earth, “we don’t know that this is going to affect anything on the surface,” Vidale says. “But we can’t say for sure until we figure out what’s happening.”