It's also a bit squishy.
MIKE MCRAE
19 OCT 2018

For the first time, geologists have confirmed that our planet's inner core is indeed solid - although not quite as firm as previous models have suggested.

Thanks to a new method for detecting soft whispers of seismic waves, analysis of an elusive type of earthquake ripple has revealed key properties of our planet's deepest layer.

Researchers from the Australian National University (ANU) zeroed in on a low amplitude 'J-phase' seismic wave that passes through the planet's core, allowing them to finally put constraints on its solidity.

As the planet's crust grinds and groans on the surface, waves of energy are sent rippling their way through its gooey insides.

These come in various forms. Some, described as compressional waves, push back and forth through the planet's body like a series of jittering train carriages. Others, called shear waves, surge up and down like the ocean's surf along surfaces.

How one converts into the other according to various phase changes can tell you a lot about the properties of the material it's passing through.

One particular variation called a J-phase should pass through the planet's inner core, picking up details of the layer's elasticity. That's always been the theory, at least. The only problem is they're rather quiet, making them virtually impossible to detect, so geologists have seen their measurement as something of a 'Holy Grail' of seismology.

Two ANU Earth scientists have now worked out a clever way to listen to these incredibly faint waves in the hum of earthquake vibrations echoing through our planet.

The method relies on taking any two seismic receivers on the planet's surface and comparing notes several hours after the loudest rumbles have died away. With enough pairs of signals, a pattern can emerge.

"Using a global network of stations, we take every single receiver pair and every single large earthquake – that's many combinations – and we measure the similarity between the seismograms," says researcher Hrvoje Tkalčić.

"That's called cross correlation, or the measure of similarity. From those similarities we construct a global correlogram – a sort of fingerprint of the Earth."

A similar process was recently used to accurately measure the thickness of ice in Antarctica, providing a novel way to determine not just the properties of Earth's layers, but potentially of other worlds as well.

Getting a grip on the nature of our planet's guts is no easy task. We can barely dig more than 12 kilometres (about 7.5 miles) into the crust, which hardly scratches the surface, let alone reveals what's thousands of kilometres underfoot.

A century ago, it was thought our planet had a thick crunchy outer coating and a gooey centre made of molten metals.

That all changed in the 1930s, following seismic readings of a large earthquake in New Zealand, which threw up signs of compression waves that shouldn't have been there. A Danish seismologist by the name of Inge Lehmann suggested these patterns were most likely an echo bouncing off a solid centre.

This inner core has been firmly established in geological models of our planet's structure. It's about three quarters the size of our Moon, made of iron and nickel, and sizzles at a temperature roughly as hot as the Sun's surface.

There might even be a complexity to its structure, with differences in how its iron crystals align giving the inner core its own 'inner core'.

But even if all that is already established in geological models, it's nice to now get firm evidence that scientists have been on the right track - besides, we got a bit of a surprise, too.

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

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

All of this information is vital if we're to build a firm understanding of phenomena like planetary formation, or how magnetic fields work.

Our own protective bubble of magnetism reverses regularly, for example, and we still haven't nailed down exactly how this happens.

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

With a new way to listen to our planet's rumbling, we're almost certain to learn more about what its soft heart is telling us.

This research was published in Science Magazine.

https://www.sciencealert.com/seismic-j-phase-shear-wave-detection-reveals-elastic-solid-inner-core