Bowels of the Earth: Meet the planet’s new layer, the innermost inner core

The idea that the Earth could hold a fifth layer was proposed in 2002; since then, researchers have been trying to get more data to learn about the elusive inner core
The four known layers of the Earth include the crust, the mantle, and the outer and inner core; the fifth layer lies within the inner core. Representative image. iStock.
The four known layers of the Earth include the crust, the mantle, and the outer and inner core; the fifth layer lies within the inner core. Representative image. iStock.

Scientists have excavated a new secret from the Earth’s inner world. The researchers, in a new study, have confirmed the existence of a fifth new layer.

The four known layers of the Earth include the crust, mantle, outer liquid and inner solid core. The fifth layer — the innermost inner core — lies at the Earth’s centre, within the inner core, the study published in Nature Communications on February 21, 2023, stated.

The fifth layer is made of iron and nickel, the same materials that comprise the rest of the inner core.

The difference between the two parts of the inner core could stem from how iron atoms are arranged to form a solid. Alternatively, the iron in the innermost inner core could have solidified and grown in a different direction than the rest of the inner core.

The inner core as a whole was liquid in the early years of the Earth’s existence, turning into a solid as the Earth cooled.

The idea that the Earth could hold a fifth layer was proposed in 2002. Since then, researchers have been trying to get more data to learn about the elusive inner core.

Scientists rely on seismic waves — shockwaves generated during an earthquake — to ‘see’ the Earth’s interiors. These waves behave differently as they pass through diverse materials. For example, they travel slower when they pass through hot materials.

“Our sampling of the innermost inner core is sparse,” Hrvoje Tkalčić from the Australian National University (ANU) and one of the study’s authors, told Down To Earth.

This stems from the fact that waves from large earthquakes following a few set paths have been studied repeatedly, leaving the rest of the inner core unexplored.

“If you want to study the Earth’s centre, you need waves that move from the earthquake’s hypocentre (the point of origin) on one side of the Earth to the opposite side,” Tkalčić explained. 

But the problem is that the opposite side is most likely located in the ocean, with no instruments to record the incoming seismic waves.

To overcome this limitation, Tkalčić and Thanh-Son Pham, a postdoctoral fellow at ANU, shortlisted waves from earthquakes that bounce from the point of origin to the opposite end multiple times.

“Only larger earthquakes will radiate enough energy for these waves to survive multiple journeys through the centre of the Earth,” Tkalčić noted.

Researchers then studied the time seismic waves took to traverse the internal Earth. 

They confirmed previous studies that showed that the innermost inner core and the rest of the inner core slow seismic waves in different directions.

Further, according to their analysis, the innermost inner core slows down the seismic waves at a point between the rotation axis (from pole to pole) and the equatorial plane (perpendicular to the poles). 

In contrast, the outer shell of the inner core slows down the waves in the equatorial plane.

The differences in speed suggest that the iron in the innermost inner core and the rest of the inner core could be arranged differently. 

“This is an interesting study that supports previous claims that the inner core has some internal structure,” Peter Driscoll, from Carnegie Institution for Science, told DTE. He was not involved in the study.

This phenomenon of differences in speed in a particular direction has been studied extensively in the upper mantle, he said. “Of course, it’s much harder to see in the inner core because there are so few waves that pass through it and they are much noisier than shallow upper mantle waves,” he explained.

Tkalčić and Thanh-Son Pham calculated the innermost inner core’s radius to be around 650 kilometres. The inner core, as a whole, is 1,221 km in radius, which is one per cent of Earth’s volume. The duo hope to learn more about the Earth’s centre. 

“We should understand the transition from the innermost inner core to the outer shell of the inner core better and this is possible to do with seismology,” Tkalčić explained. That transition, according to him, is likely, not sharp. He expects it to be gradual.

This could inform researchers of two things: The rate at which the transition happened and the age of the inner core during the transition. 

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