Recently detected vibrations show that Earth's inner core is solid

The researchers report observing seismic waves crossing the inner core of the Earth, which allowed them to understand what it looked like: solid, but softer than expected.

Scientists predicted that the Earth had a solid inner core and a liquid outer core 80 years ago, based on measurements that were incompatible with a single nucleus. The solid core should accompany a special type of seismic wave, called J waves, but it has been difficult to detect the waves, until now.

"The detection of the J waves confirms that the inner core of the Earth is solid, although it is elastically less rigid than the previous estimate," wrote the authors of the Australian National University in their article published in Science.

We have long deduced from observations that the Earth had layers: a crust; a thick warm coat; a liquid external core; and a solid inner core. These inferences are based on seismic measurements. Seismic waves are just vibrations that cross the Earth and could be caused by volcanoes, earthquakes or even human-made explosions. Geophysicists measure these waves to determine what the interior of the Earth should look like. But it is long between difficult to measure the waves produced in the solid inner core, since they would have a very small amplitude – they would be really small.

Rather than directly tracking the waves, Hrvoje Tkalčić and Thanh-Son Pham examined the overall seismographic data and correlated the results, which meant that they examined the differences between the readings. Among all the different ways that the waves could travel, outer core and inner core, they were able to detect specific signals representing the J waves propagating in the inner core.

This study was able to use these J waves to determine that the inner core is solid, but that the waves pass through it more slowly than the current reference models suggest. This means that the kernel can also be more flexible than expected.

Researchers say there is still work to be done, such as extracting more seismogram data and placing seismic measurement equipment in new locations. And while there are still questions about the nature of the Earth's inner core, it's an important result, writes Jessica Irving, assistant professor of geoscience at Princeton, in a Science commentary. "The observation of the J phases will provide an additional tool for evaluating the properties of the" soft heart "of the Earth."