Earth's structure found to have a distinct 'fifth layer'

Seismologists from The Australian National University (ANU) believe they have documented evidence of a distinct layer inside Earth, known as the innermost inner core — a solid ‘metallic ball’ that sits within the centre of the inner core.

Not long ago it was thought Earth’s structure comprised four distinct layers: the crust, the mantle, the outer core and the inner core. The ANU findings, published in the journal Nature Communications, confirm there is a fifth layer.

“The existence of an internal metallic ball within the inner core, the innermost inner core, was hypothesised about 20 years ago,” said Dr Thanh-Son Phạm, from the ANU Research School of Earth Sciences. “We now provide another line of evidence to prove the hypothesis.”

Using data from about 200 magnitude-6 and above earthquakes from the last decade, the researchers analysed seismic waves that travel directly through the Earth’s centre and ‘spit out’ at the opposite side of the globe to where the earthquake was triggered, also known as the antipode. The waves then travel back to the source of the quake — similar to a ping pong ball bouncing back and forth.

“By developing a technique to boost the signals recorded by densely populated seismograph networks, we observed, for the first time, seismic waves that bounce back and forth up to five times along the Earth’s diameter,” Phạm said. “Previous studies have documented only a single antipodal bounce.

“The findings are exciting because they provide a new way to probe the Earth’s inner core and its centremost region.”

One of the earthquakes the scientists studied originated in Alaska. The seismic waves triggered by this quake ‘bounced off’ somewhere in the south Atlantic Ocean, before travelling back to Alaska.

The researchers studied the anisotropy of the iron–nickel alloy that comprises the inside of the Earth’s inner core — that is, how seismic waves speed up or slow down through the material of the inner core depending on the direction in which they travel. They found the bouncing seismic waves repeatedly probed spots near the Earth’s centre from different angles.

By analysing the variation of travel times of seismic waves for different earthquakes, the scientists inferred that the crystallised structure within the inner core’s innermost region is likely different to the outer layer. They said this might explain why the waves speed up or slow down depending on their angle of entry as they penetrate the innermost inner core.

ANU’s Professor Hrvoje Tkalčić said studying the deep interior of Earth’s inner core can tell us more about our planet’s past and evolution. Indeed, the findings suggest there could have been a major global event at some point during Earth’s evolutionary timeline that led to a significant change in the crystal structure or texture of the Earth’s inner core.

“This inner core is like a time capsule of Earth’s evolutionary history,” Tkalčić said. “It’s a fossilised record that serves as a gateway into the events of our planet’s past — events that happened on Earth hundreds of millions to billions of years ago.”

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