Our knowledge of the inner workings of our planet depends on earthquakes. Seismic waves produced during earthquakes bounce off boundaries within the planet or bend like light waves reflected or refracted by a change in environment. But the deeper you go, the more difficult it becomes to study, both because only waves that pass almost directly through the center of the planet are affected, and because of the noise created by interactions at farther boundaries.
As a result, even though it was assumed for more than 20 years that there was an inner core inside the Earth, this issue remained unclear. from the Australian National University, Dr. Thanh Son Phạm and Professor Hrvoje Tkalčić claim they have finally confirmed this in a new paper. To arrive at this conclusion, they used delays in the arrival times of seismic waves from 200 earthquakes of magnitude 6 and above that bounced back and forth on Earth, and in some cases passed five times.
Phạm and Tkalčić describe this innermost core as a solid “metal ball” 650 kilometers wide, making it smaller than Ceres but larger than other main belt asteroids. Seismic P waves travel 4 percent slower along a path that is 50 degrees to the Earth’s axis of rotation. The authors interpret this as a sign that it has a crystalline structure, resulting from the way iron atoms arrange themselves at very high temperatures and pressures. Movement through the rest of the solid core occurs slowest in the direction of the equatorial plane.
“This inner core is like a time capsule of Earth’s evolutionary history,” Tkalčić said. It is a fossilized record that serves as a gateway to events in our planet’s past. “We’re seeing events that took place on Earth hundreds of millions to billions of years ago,” he says. The authors suggest that a major spherical event caused the innermost core to differentiate from the rest of the inner core, but they do not yet know what that is.
It is believed that the inner core as a whole grows as parts of the liquid outer core solidify. However, we do not know whether the boundary between the innermost core and the shell surrounding it has changed.
The authors suggest that future research examine the boundary between the innermost core and the surrounding crust.
Last year, Tkalčić and another colleague challenged notions about Mars’ interior structure, presenting evidence that its mantle still has mobile magma rather than cooling into a solid crust. The research was published in the journal Nature Communications.
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