A 650-km thick solid metallic ball forms the earth’s innermost inner core

Until recently, it was thought that there were four layers: crust, mantle, outer core, inner core.

An earthquake  in Alaska caused seismic waves to penetrate the earth’s innermost inner core and bounce back from somewhere in the South Atlantic.  | Photo Credit: Drew Whitehouse, Thanh-Son Pham, and Hrvoje Tkalcic.

Until recently, it was thought the earth’s structure comprised four distinct layers: the crust, the mantle, the outer core, and the inner core. Analysing data from seismic waves earthquakes cause, seismologists from the Australian National University (ANU) confirmed that there is a fifth layer. This innermost inner core is a 650-kilometre thick solid “metallic ball” that sits within the centre of the inner core. These findings have been published in Nature Communications.

“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 Thanh-Son Pham of the ANU Research School of Earth Sciences. The ANU’s Hrvoje Tkalcic said: “This inner core is like a time capsule of the earth’s evolutionary history; it’s a fossilised record that serves as a gateway into... events.... that happened on the earth hundreds of millions to billions of years ago.”

The researchers analysed seismic waves that travel directly through the earth’s centre and emerge at the other side of the globe exactly opposite to the source point of the quake, known as the antipode. The waves “bounce off” from the antipode and then travel back to the epicentre just like the bouncing back and forth of 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,” Pham said.

The researchers studied the anisotropy of the iron-nickel alloy that comprises the inside of the inner core by analysing how seismic waves sped up or slowed down through the material depending on their direction of travel. This could be caused by a different arrangement of iron atoms at high temperatures and pressures or preferred alignment of growing crystals. 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 waves for different earthquakes, the scientists inferred that the crystallised structure within the inner core’s innermost region was different from the outer layer.

According to the ANU team, the findings suggest there could have been a major global event at some point during the earth’s evolutionary timeline that led to a “significant” change in the crystal structure/texture of the inner core. “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,” Tkalcic said.