Earth’s Russian nesting doll structure has a fifth layer we never knew about – the “innermost inner core”

Geography textbooks will have to be re-written as scientists from the Australian National University (ANU) have confirmed that the Earth’s structure is made up of five, not four, layers.

We’ve all seen the cross-sectional diagrams of the Earth which show its four layers: the crust, the mantle, the outer core, and the inner core.

It seems these diagrams are incomplete.

In his 1864 science fiction novel, The Journey to the Centre of the Earth, about his own imaginings of what lies within our planet, Jules Verne wrote: “Science, my lad, has been built upon many errors; but they are errors which it was good to fall into, for they led to the truth.”

While Verne’s adventure story’s proposed wondrous world full of life in the caverns of Earth’s interior is not what we’ve found, his essential point remains cogent.

Seismologists at ANU used data from about 200 earthquakes of magnitude 6 or higher to dive deep into Earth’s innards and rewrite what we thought we knew. Measuring the different speeds at which the waves travelled through the planet, the scientists were able to map out the Earth’s inner structure and found that there is an additional distinct layer deep within.

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This solid “metallic ball” sits within the inner core and is known as the “innermost inner core.”

As the innermost inner core likely contains information from millions, even billions, of years ago, the discovery may help us understand how our planet formed.

“The existence of an internal metallic ball within the inner core, the innermost inner core, was hypothesised about 20 years ago,” says ANU’s Dr Thanh-Son Pham. “We now provide another line of evidence to prove the hypothesis.”

“This inner core is like a time capsule of Earth’s evolutionary history,” adds Professor Hrvoje Tkalčić, also from ANU. “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.”

When an earthquake occurs, its waves penetrate through the Earth, reaching the other side of the Earth before bouncing back – like a ping pong ball.

“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. Previous studies have documented only a single antipodal bounce,” Phạm explains.

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

By analysing the variation of travel times of seismic waves, the scientists were able to infer the iron-nickel alloy structure within the inner core’s innermost region is different to the outer layer.

“There are still many unanswered questions about the Earth’s innermost inner core, which could hold the secrets to piecing together the mystery of our planet’s formation,” says Tkalčić.

The findings are published in Nature Communications.