Earth’s inner core rotating slower than surface: study

PARIS: Earth´s inner core, a hot iron ball the size of Pluto, has stopped spinning faster than the planet´s surface and might now be rotating slower than it, research suggested on Wednesday.

Roughly 5,000-km below the surface we live on, this “planet within the planet” can spin independently because it floats in the liquid metal outer core. Exactly how the inner core rotates has been a matter of debate between scientists -- and the latest research is expected to prove controversial.

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 pass through the middle of the Earth.

Seeking to track the inner core´s movements, new research published in the journal Nature Geoscience analysed 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,” the study´s authors, Xiaodong Song and Yi Yang of China´s Peking University, told AFP.

“One cycle of the swing is about seven decades,” the authors said. The inner core started rotating slightly faster than the rest of the planet in the early 1970s, the study said.

But it had been slowing down before coming in sync with Earth´s rotation around 2009, it added. There has been a “negative trend” since, the study said, meaning the inner core is now rotating slower than the surface. The researchers predicted the next change would occur in the mid-2040s.