No one was around to observe the formation of earth some 4.5 billion years ago, so the best we can hope for is observing the state of the planet today in order to learn about how our home came to be. An essential part of that process is finding out whether the inner core of Earth is a solid mass or some sort of shifting super-hot slurry. Researchers from Australian National University (ANU) think they’ve determined that the Earth’s core is most likely solid but a little squishy.
The key was detecting so-called “shear waves” emanating from the core. These are seismic waves that only propagate through solid rock, but the core’s waves are so minuscule no one has been able to detect them yet. It was not even clear there were any waves to detect. This is important because a liquid core would not be able to transmit shear waves. A solid core would. If you can prove shear waves propagate through the core, you’ve likely proven that it’s solid.
Associate Professor Hrvoje Tkalčić and Ph.D. Scholar Than-Son Phạm from ANU devised a method that involved waiting for an earthquake. A big one. The researchers call their approach to measuring Earth’s core the “wavefield method.” Following an earthquake, they used sensitive seismic probes to record every little rumble. However, they can’t start right after the quake. It takes about three hours for the major rumbling to settle down, allowing the seismograms to record more subtle signals bouncing around inside the Earth. This is very similar to techniques used to measure the thickness of Antarctic ice.
The study shows that seismic maps produced in this three-to-10-hour post-earthquake window can essentially fingerprint the planet. The map demonstrates the presence of shear waves emanating from the planet’s core, and further, it allowed Tkalčić and Phạm to infer the speed of the waves.
From this data, the researchers believe they’ve determined that the core is indeed solid. It’s not a monolithic, unmoving lump, though. The core has elastic properties similar to gold and platinum. We still don’t know the exact temperature of the core, its age, or how fast it solidifies. However, the new waveform method could make it feasible to learn those things in the future.
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