Scientists are now confident that they can prove that the central core of the Earth is solid.

• Science says that there is now evidence that the inner core of the Earth is solid.
• But it's sweeter than expected.
• UNA scientists have come up with a way to detect J waves, a type of wave that can only travel through solid objects.

Science is now sure that the center of the Earth is solid and not hollow, as described by fiction writers.

Associate Professor Hrvoje Tkalčić and doctoral student Than-Son Phạm of the Australian National University (ANU) are convinced that they have direct evidence of this.

They have developed a way to detect shear waves, or "J-waves" – a type of wave that can only cross solid objects – in the inner core.

"We found that the inner core is indeed strong, but we also found that it was softer than previously thought," says Tkalčić.

"It turns out that if our results are correct, the inner core shares similar elastic properties with gold and platinum.

"The inner core is like a time capsule, if we understand, we will understand how the planet was formed and how it evolved."

Internal shear waves are so small and so weak that they can not be observed directly. Their detection has been considered the holy grail of global seismology since scientists predicted for the first time that the inner core was solid in the 1930s and 1940s.

The scientists explain:

The researchers had to come up with a creative approach.

Their correlation field method examines the similarities between signals from two receivers after a major earthquake, rather than direct wave arrivals. The same team used a similar technique to measure the thickness of ice in Antarctica.

"We throw the first three hours of the seismogram and we observe it between three and ten hours after a major earthquake. We want to get rid of the big signals, "says Dr. Tkalčic.

"Using a global network of stations, we take each pair of receivers and each major earthquake – that is, many combinations – and we measure the similarity between the seismograms. This is what is called cross-correlation, or the measure of similarity. From these similarities, we build an overall correlogram – a kind of Earthprint. "

The study shows that these results can then be used to demonstrate the existence of J waves and infer the speed of the shear wave in the inner core.

Although this specific information on shear waves is important, Dr. Tkalčić explains that the results of this research on the inner core are even more exciting.

"For example, we do not yet know what is the exact temperature of the inner nucleus, what is the age of the inner nucleus, or how fast it solidifies, but with these new advances in global seismology, we are slowly getting there. "Dr. Tkalčić says.

"The understanding of the Earth's inner core has direct consequences on the generation and maintenance of the geomagnetic field – without this field, there would be no life on the surface of the Earth."

Jules Verne's 1864 novel, Journey to the Center of the Earth, describes a vast underground world.

The ANU research is published in Science Magazine.

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