A team of geophysicists using seismic data from more than three decades of earthquakes have found evidence that the Earth’s inner core isn’t completely solid.
The new research sheds light on a hotly debated question in geology by showing that the surface of the inner core can change, and can do so on annual timescales.
The findings could improve our understanding of Earth’s magnetic field, the life-sustaining barrier between Earth’s surface and the Sun’s barrage of solar radiation.
A team led by John Vidale at the University of Southern California in the US originally set out to measure the inner core’s slowing rotation. “[We] didn’t set out to define the physical nature of the inner core,” says Vidale.
The Earth’s diameter is about 12,700km. To get to the inner core, you would have to travel about 5,000 kilometres (3,000 miles) through the rest of Earth’s layers. Beginning with the very thin crust on which we live, the next layer is the mostly solid mantle. Below the mantle lies the molten outer core, which is primarily responsible for the dynamo effect sustaining our magnetic field.
Finally, you’ll hit the inner core, a hot ball of iron and nickel slightly smaller than the Moon.
Of course, no one can travel to these layers to observe them directly and, until now, the inner core was mostly thought of as a solid sphere.
To study these inaccessible parts of the Earth, Vidale and colleagues measured the spread of seismic waves that occurred after 121 earthquakes originating in Antarctica between 1991 and 2024. The way these waves scatter as they travel through the Earth can be used to infer physical properties about the layers they pass through. (Wave scattering was measured by receiver stations in Alaska and Canada.)
“One dataset of seismic waves curiously stood out from the rest,” says Vidale. “Later on, I’d realize I was staring at evidence the inner core is not solid.”
Instead, the team’s analyses revealed that the inner core has a changing, viscous surface topography, likely caused by the outer core.
“The molten outer core is widely known to be turbulent, but its turbulence had not been observed to disrupt its neighbour – the inner core – on a human timescale,” says Vidale. “What we’re observing in this study for the first time is likely the outer core disturbing the inner core.”
The discovery opens the door to new findings about previously hidden dynamics between the inner and outer cores. This could lead to better models of our magnetic field and the spread of heat within the Earth.
Related to Vidale and colleagues’ original study question, the changing topography of the inner core could also improve our understanding of the inner core’s rotation and help answer why it is slowing.
The research is published in the journal Nature Geoscience.
