Researchers report that they’ve observed seismic waves traversing the Earth’s inner core, allowing them to figure out what it’s like: Solid, but softer than previously thought.
Scientists predicted that the Earth has a solid inner core and liquid outer core 80 years ago, based on measurements inconsistent with just one single core. The solid core should accompany a special kind of seismic wave, called J waves, but it’s been difficult to actually detect the waves — until now.
“Detection of J waves confirms that Earth’s inner core is solid, although elastically less stiff than previous estimate,” the authors, from the Australian National University, wrote in their paper published in Science.
We’ve long inferred based on observations that the Earth has layers: A crust; a thick, hot mantle; a liquid outer core; and a solid inner core.
These inferences are based on seismic measurements. Seismic waves are just vibrations that travel through the Earth, which could be caused by volcanoes and earthquakes or even human-caused explosions. Geophysicists measure these waves in order to deduce what Earth’s insides must look like.
But it’s long been difficult to measure the waves produced in the solid inner core, given that they would have a very small amplitude — they’d be really weak.
Rather than spotting the waves directly, Hrvoje Tkalčić and Thanh-Son Pham looked at global seismogram data and correlated the results, meaning they looked at the differences in the readings. Among all of the different ways that the waves could travel, outer core and inner core, they were able to pick out specific signals representing the J waves travelling through the inner core.
This study was able to use those J waves to determine that the inner core is solid, but that waves travelled slower through it than current reference models suggest. This means the core may also be softer than previously thought.
The researchers explain that there’s further work to be done, such as taking more data from more seismograms and placing seismic measuring equipment in new places.
And while there are still questions about the nature of Earth’s inner core, it’s an important result, Jessica Irving, assistant professor in geosciences at Princeton, wrote in a Science commentary. “The observation of J phases will provide an extra tool to assess the properties of Earth’s ‘soft heart’.”