Seismic waves passing through the Earth’s solid inner core reveal it is textured, rather than homogenous. Little is known about the nature of these variations at this stage – simply discovering they exist is quite an achievement when you’re dealing with something shielded from our eyes by thousands of kilometers of rock.
Most of the Earth’s core remains liquid. However, in 1936 it was discovered there is a solid (or very nearly so) inner component where the immense pressures overcome the very high temperatures. This solid sphere makes up less than 1 percent of the planet’s volume, and not much more than that of its mass, but its importance far exceeds its size. Without it, the geomagnetic field that protects us from space radiation would be a fraction of its strength, making complex life unlikely.
Consequently, geologists are keen to learn the inner core’s age, composition and structure, but have frustratingly little to go on. In a new study, however, a team led by then University of Utah student Guanning Pang have used tiny echoes of powerful earthquakes to establish that the inner core is anything but homogenous.
Distortions and reflections of seismic waves created by large earthquakes have been used for a century to understand the Earth’s interior – it’s how we discovered the inner core exists at all. The level of detail has gone up dramatically since the establishment of the International Monitoring System (IMS), which was built to check if anyone was testing nuclear devices. The IMS has been a boon for scientists – it’s so sensitive it was used to identify whale species – and Pang and co-authors exploited its capacity to the full.
Observing the effect on passage through the core of waves from every earthquake larger than magnitude 5.7 measured by the IMS, the team established there is an unevenness to the core. This can be seen on a scale of “grains”, some slightly smaller than 10 kilometers (6 miles). It’s likely this inhomogeneity exists at scales smaller still, but even with the power of the IMS the team were not able to get finer resolution.
It looks very old-school, but this seismometer at the University of Utah is part of a network that provides us with our one way to explore the inner workings of the Earth.
Image credit: DAVE TITENSOR/UNIVERSITY OF UTAH
“For the first time we confirmed that this kind of inhomogeneity is everywhere inside the inner core,” Pang said in a statement.
Whether this texturing has anything to do with Pang and Dr Keith Koper’s previous discovery, the way the inner core’s rotation got out of step with the planet as a whole, possibly triggering changes to the length of the day, remains uncertain.
“It’s like a planet within a planet that has its own rotation and it’s decoupled by this big ocean of molten iron,” said Koper.
The team were able to detect the reflections of 2,455 earthquakes, a testimony to the IMS’s sensitivity. “This signal that comes back from the inner core is really tiny. The size is about on the order of a nanometer,” said Koper.
“Our biggest discovery is the inhomogeneity tends to be stronger when you get deeper. Toward the center of Earth it tends to be stronger,” Pang said. The authors attribute this to initial rapid growth within the inner core.
Patches remained liquid despite the intense pressure. Once these did solidify, their composition and structure were different from the material around them, forming grains. The paper proposes the initial inner core formed through a process of supercooling, like water that lacks impurities to nucleate around, and then freezes very suddenly once the process starts.
Expansion continues, thanks to the slow run-down of radioactivity in the core, but now that it happens more slowly there is less differentiation between neighboring areas.
The granularity increases sharply 500-800 kilometers (805-1,287 miles) beneath the boundary between the inner and outer core, suggesting rapid growth of the inner core up to that point. The authors best estimate is that what they call the innermost core starts 570 kilometers (917 miles) below the boundary with the outer core.
The study is published in Nature.
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