Earth’s inner core, a hot ball of iron the size of Pluto, has stopped spinning faster than the planet’s surface and may now be rotating slower than it, new research shows.
Roughly 5,000 kilometers below the Earth’s surface, the inner core can spin independently because it floats in the liquid metal outer core.
What little is known about the inner core comes from measuring the tiny differences in seismic waves – created by earthquakes or sometimes nuclear explosions – as they travel through the middle of the Earth.
The research analyzed seismic waves from repeating earthquakes over the last six decades.
“We believe the inner core rotates, relative to the Earth’s surface, back and forth, like a swing,” said the study authors, Xiaodong Song and Yi Yang of China’s Peking University.
“One cycle of the swing is about seven decades,” they said.
According to the study, the inner core started rotating slightly faster than the rest of the planet in the early 1970’s. But it had been slowing down before coming in sync with Earth’s rotation around 2009. There had been a “negative trend” since then, meaning the inner core is now rotating slower than the surface.