Scientists confirm existence of a fifth layer in Earth's core

The Earth is composed of four layers - the crust, the mantle, the outer core and the inner core - each with unique characteristics and properties. A new study has now confirmed the existence of a fifth layer, called the innermost inner core.

By measuring the different speeds at which seismic waves created by large quakes penetrate and pass through the Earth's inner core, seismologists from the Australian National University (ANU) believe they have documented evidence of the fifth layer - a solid metallic ball that sits within the inner core.

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

The researchers analyzed seismic waves that propagate through the Earth's center and emerge at the opposite side of the Earth from where the earthquake occurred - called the antipodal point. These waves then travel back to the origin of the earthquake - a process that the team describes as similar to a ping pong ball bouncing back and forth.

In this study, the team analysed data from about 200 magnitude-6 and above earthquakes from the last decade. Among these quakes was one that originated in Alaska. Seismic waves triggered by this earthquake bounced off somewhere in the south Atlantic Ocean, before returning back to Alaska.

The inside of Earth's inner core is composed mainly of iron and nickel. The team studied the anisotropy of the iron-nickel alloy and 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 infer the crystallised structure within the inner core's innermost region is likely different to the outer layer, according to a post.

According to the researchers, it could potentially explain why seismic waves accelerate or decelerate based on their angle of entry as they penetrate the innermost part of the Earth's inner core.

The findings are published in the journal Nature Communications.