Jakarta – Like an onion composed of layers upon layers, it seems that the internal structure of the Earth is endless. Recent studies have shown the existence of a component known as the ‘inner core’ consisting of the innermost core and the middle core (which is still called the inner core).
Confused, right? So the picture is, the Earth has a core inside the core, inside the core, and so on. Human knowledge of the inner workings of our planet depends on earthquakes.
Seismic waves generated during earthquakes bounce off planetary boundaries or bend, just as light waves reflect or refract from a changing medium.
However, the deeper it is, the more difficult it is to study, both because only waves passing almost directly through the center of the affected planet, and because of the noise created by interactions at more distant boundaries.
As a result, the innermost core has been hypothesized for more than 20 years, its existence remains uncertain. Dr Thanh-Son Phạm and Professor Hrvoje Tkalčić from the Australian National University claim in a recent scientific paper that they have finally confirmed it.
Schematic of the layers of the Earth. The inner core is about half the diameter of the inner core, but smaller than the outer core and mantle. Photo: Drew Whitehouse/National Computational Infrastructure’s Vizlab/Australian National University
To come up with an answer, they used the arrival time delay of seismic waves from 200 earthquakes of magnitude 6 or greater that bounce back and forth through Earth. Phạm and Tkalčić describe the inner core as a dense “ball of metal” 650 kilometers long, making it smaller than Ceres, the dwarf planet in the Solar System, but larger than other main belt asteroids.
Seismic P waves pass through it 4% slower when along a path angled 50 degrees to the Earth’s axis of rotation. The study authors interpreted this as a sign it has a crystal structure caused by the way the iron atoms arrange themselves at very high temperatures and pressures. Movement through the rest of the dense core is slowest toward the equatorial plane.
“This inner core is like a time capsule of Earth’s evolutionary history, it is a fossil record that serves as a gateway to past events of our planet. Events that occurred on Earth hundreds of millions to billions of years ago,” said Tkalčić as quoted by IFL Science.
Researchers think a major global event caused the inner core to be different from other inner cores, though they don’t yet know what it is.
The inner core as a whole is believed to grow as part of the solidified outer core fluid. However, researchers
don’t know if the boundary between the innermost core and the shells around it shifted.
This is possible, due to the increasing number of seismic sensors scattered around the planet. By casting wide nets and combing the results from multiple locations, the authors were able to track seismic waves in unprecedented detail, including some traveling north-south almost directly through Earth.
They also developed techniques to amplify signals from distant seismic waves to distinguish them from local noise. The authors propose that future research may investigate the boundary between the innermost core and the shell around it.
Last year, Tkalčić and other colleagues challenged notions about Mars’ internal structure, after providing evidence that the mantle still contains moving magma, rather than cooling into a solid husk.
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