Seismologists might have identified the deepest layer of Earth’s core

In high school science class, textbooks often feature a recognizable image of the Earth and all its layers—currently, that’s the crust, outer and inner mantle, and outer and inner core. But a new study published February 21 in Nature Communications might leave all of those graphics a little outdated. Seismologists at the Australian National University analyzed the reverberating waves from powerful earthquakes and found what they believe to be evidence of a distinct innermost inner core.

Each inner division of the Earth plays its own unique role in our lives. We exist on top of the thin, outermost layer called the crust. Although there have been past efforts to dig deep enough to break into the mantle, no one has succeeded yet. The mantle, both outer and inner, are made up of liquid rock, and the convection currents present there are responsible for the jostling and bumping of the crust’s tectonic plates. Finally, there’s the core. The liquid outer layer of the core is responsible for producing Earth’s magnetic field, which is further stabilized by the solid inner section. 

[Related: The Earth’s inner core could be slowing its spin—but don’t panic]

We can’t easily study the inner structure of the Earth, so geologists research the mantle by examining samples of rock from volcanic eruptions that may have come from that far underground. On top of that, they study the seismic waves produced by earthquakes. When an earthquake starts at an epicenter deep underground, the movement creates waves that shake the surface. Those waves can be measured by seismometers all around the globe, and by measuring just how fast the seismic waves are moving, seismologists can figure out a surprising amount about just what the center of the Earth looks like.

That is, when the numbers make sense. For a while, seismologists had noticed that when they measured earthquake waves passing through the very center of the inner core, their models would be less accurate. All waves, seismic or otherwise, travel at different speeds through different materials, but a phenomenon called anisotropy means that waves can also travel at different speeds in different directions. In 2002, researchers proposed the existence of the innermost inner core as a way to explain the anisotropic effects they had found when examining some powerful earthquakes.

Now, more research seems to be supporting that theory. As the number of seismic recording stations has increased in recent years, it’s become easier to triangulate exactly how fast and in what direction a wave is moving. The seismologists at the ANU looked at earthquakes above a magnitude of 6.0 over the last decade to determine the exact path of the seismic waves. Because of the increase in equipment, scientists were able to track the waves as they bounced around the Earth up to five times. And indeed, their findings supported that as the waves passed through the center of the Earth, their path was altered as if there was an innermost inner core. The researchers think the divide comes from a different crystal arrangement of the iron and nickel atoms that make up the core.

Some seismologists aren’t completely convinced by the findings because it’s still not clear that this is a hard boundary rather than a gradual transition. But discovering a new layer of the earth doesn’t happen often, and if the innermost inner core continues to be backed up by evidence, the authors argue it might just give geologists more insight into the geologic structure of the earliest days of the planet.