Earth has an even inner core, and it’s a solid ball of metal.

For generations, scientists have probed the structure and composition of the planet using seismic wave studies. This consists of measuring the shock waves caused by earthquakes as they penetrate and pass through the central region of the Earth. By noting differences in velocity (a process known as anisotropy), scientists can determine which regions are denser than others. These studies have led to the predominant geological model which incorporates four distinct layers: a crust and mantle (composed largely of silicate minerals) and an outer core and inner core composed of nickel-iron.

According to seismologists from the Australian National University (ANU), data obtained in a recent study has shed new light on the deepest parts of the Earth’s inner core. In an article published in Nature Communication, the team reports finding evidence of another distinct layer (a ball of solid metal) at the center of Earth’s inner core – a “deepest inner core”. These discoveries could shed new light on the evolution of our planet and lead to revised geological models of the Earth comprising five distinct layers instead of the traditional four.

The research was led by Dr Thanh-Son Pham and Dr Hrvoje Tkalcic, respectively postdoctoral fellow and professor in the Research School of Earth Sciences (RSES) at ANU. As they report, the team stacked seismic wave data from about 200 earthquakes over the past decade that were magnitude 6 or greater. The triggered waveforms were recorded by seismic stations around the world, which passed directly through the center of the Earth to the other side of the globe (the antipode) before returning to the source of the earthquake. .

Measurements of the anisotropy of the Earth’s inner core based on the travel times of these waves have revealed unprecedented insights into the Earth’s interior structure. This included the possible presence of a layered structure in the innermost part of the inner core. “The existence of an inner metallic ball in the inner core, the innermost inner core, was posited about 20 years ago,” Dr Pham said in an ANU press release. “We now provide another source of evidence to prove the hypothesis.”

The direction and travel times of seismic waves indicate differences in the arrangement of iron atoms at elevated temperatures and pressures or the preferred alignment of growing crystals. After examining the bouncing seismic waves, the team found that they repeatedly probe points near the center of the Earth from different angles. By analyzing the variation in travel times of different earthquakes, they deduced that the crystallized structure in the innermost region of the core is likely to have an outer layer.

According to the team, these findings could explain how waves speed up or slow down depending on their angle of entry as they enter the innermost core. Says Dr. Pham:

“By developing a technique to amplify signals recorded by densely populated seismograph arrays, we have observed, for the first time, seismic waves that bounce up to five times along the Earth’s diameter. Previous studies have documented only one antipodal bounce. The findings are exciting because they offer a new way to probe the Earth’s inner core and its most central region.

One of the earthquakes they studied originated in Alaska, which triggered seismic waves that “bounced” somewhere in the South Atlantic before returning to Alaska. According to the ANU team, these findings also suggest that there may have been a major global event in Earth’s past that led to a significant change in the crystal structure of Earth’s inner core. Therefore, Professor Tkalcic said, studying the Earth’s deep interior could tell us more about the evolutionary history of planet Earth:

“This inner core is like a time capsule of Earth’s evolutionary history – it’s a fossilized record that serves as a gateway to events in our planet’s past. Events that happened on Earth hundreds of millions to billions of years ago. There are still many unanswered questions about which Earth’s innermost core might hold the secrets to piecing together the mystery of how our planet was formed.

Further reading: ANU, Nature Communication