Within Earth’s inner core may lurk an ‘innermost’, fifth layer
Things look relatively familiar on the surface of the Earth as far as scientists’ understanding of our planet goes — but deep down is a cradle of layer upon layer of revelations about its structure.
Less than a month after a study published in the journal Nature Geoscience suggested Earth’s inner core may have slowed down its spin, seismologists at The Australian National University(ANU) believe they’ve documented evidence of a distinct fifth layer lurking underneath it — the “innermost inner core”.
For the longest time since Danish scientist Inge Lehmann used seismic waves to discover Earth’s inner core in 1936, our rocky spheroidal planet was thought to have four layers: the crust, the mantle, the outer core and the inner core.
Findings published Tuesday in Nature Communications confirm the existence of a distinct fifth layer — a solid ball of iron measuring 650 km across.
"The existence of an internal metallic ball within the inner core, the innermost inner core, was hypothesised about 20 years ago. We now provide another line of evidence to prove the hypothesis," Thanh-Son Pham, lead author of the paper and ANU postdoctoral researcher of geophysics, said.
Earth’s inner core — which is about 70% the size of the Moon with a diameter of 2,440 km — was previously thought to be the hottest and densest of all four concentric layers. Its interaction with the 6,970 km-wide molten outer core drives convection currents — heat released from solidification— to create and maintain Earth’s magnetic field.
Temperatures that far deep ought to be extreme, making this steaming cauldron of iron and nickel currently inaccessible.
Analysing seismic waves caused by earthquakes have helped scientists push this frontier deeper for over a century and a growing network of seismographs — instruments that record these vibrations — made the discovery of the innermost inner core possible.When a powerful earthquake rattles the Earth, it sends ripples that travel directly through the planet’s centre and “spit out” at the antipode — the point on the opposite side of the globe from the epicentre.These waves then bounce off, heading straight back to the source, to create much fainter “reverberations”.In the past, scientists have not recorded more than two passages of a seismic event, but that changed when the ANU team studied over 200 magnitude 6 and above earthquakes — including one that originated in Alaska."By developing a technique to boost the signals recorded by densely populated seismograph networks, we observed, for the first time, seismic waves that bounce back and forth up to five times along the Earth's diameter,” Pham explained.
A time capsule
Research co-author and professor of geophysics at the university, Hrvoje Tkalčić, said the findings create room for further research — which could lay bare the mystery of how a major global event caused a significant change in its composition."This inner core is like a time capsule of Earth's evolutionary history — it's a fossilised record that serves as a gateway into the events of our planet's past… [and hides information related to the] events that happened on Earth hundreds of millions to billions of years ago."