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Earth’s inner core is less solid than previously thought

The surface of the Earth’s inner core may be changing, as shown by a new study from scientists that detected structural changes near the planet’s center.

The findings were published Feb. 10 in Nature Geoscience by a team of researchers led by the University of Southern California and from Cornell Engineering.

The changes of the inner core have long been a topic of debate for scientists. However, most research has been focused on assessing rotation. John Vidale, professor of earth sciences at USC and principal investigator of the study, said the researchers didn’t set out to define the physical nature of the inner core.

“What we ended up discovering is evidence that the near surface of Earth’s inner core undergoes structural change,” Vidale said. The finding sheds light on the role topographical activity plays in rotational changes in the inner core that have minutely altered the length of a day and may relate to the ongoing slowing of the inner core.

“These findings also put essential constraints on the inner core viscosity, core thermal evolution, and mechanisms of how Earth’s geomagnetic field generates,” said Guanning Pang, postdoctoral researcher in the Department of Earth and Atmospheric Sciences at Cornell. “These will help constrain the Earth’s long-term evolution.”

Redefining the inner core

Located 3,000 miles below the Earth’s surface, the inner core is anchored by gravity within the molten liquid outer core. Until now the inner core was widely thought of as a solid sphere.

The original aim of the scientists was to further chart the slowing of the inner core. “But as I was analyzing multiple decades’ worth of seismograms, one dataset of seismic waves curiously stood out from the rest,” Vidale said. “Later on, I’d realize I was staring at evidence the inner core is not solid.”

The study utilized seismic waveform data – including 121 repeating earthquakes from 42 locations near Antarctica’s South Sandwich Islands between 1991 and 2024 – to give a glimpse of what takes place in the inner core. As the researchers analyzed the waveforms from receiver-array stations located near Fairbanks, Alaska, and Yellowknife, Canada, one dataset of seismic waves from the latter station included uncharacteristic properties the team had never seen before.

“At first the dataset confounded me,” Vidale said. It wasn’t until his research team improved the resolution technique did it become clear the seismic waveforms represented additional physical activity of the inner core.

Deformed inner core

One hypothesis for the activity is the interaction between the gravitational coupling between the inner core and mantle and the geomagnetic field.

“The inner core should remain in an equilibrium position relative to the mantle under the gravitational and electromagnetic coupling effects,” Pang said. “However, when disbursement occurs, the inner core will offset from equilibrium and oscillate. Then, the gravitational force tends to drag the inner core back into the equilibrium, and if the shallow inner core viscosity is low, it can deform.”

This article was adapted from a version written by Will Kwong with permission from the University of Southern California.

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