2023-07-23 15:00:16
We learned it from childhood: at the heart of life hides a seed. Plants, animals, babies, everything comes from there. What children – and adults – know less is that at the heart of our planet also hides a “seed”, as geologists call it. Under the crust, under the rocky mantle, under the outer core of molten metal, this inner core – its other name – of some 1,200 kilometers in radius occupies, it is true, only 1.7% of the Earth’s volume. But it concentrates a lot of mysteries. Its very existence was only demonstrated by the Danish seismologist Inge Lehmann (1888-1993) in 1936, six years following the discovery of Pluto.
“Even today, we don’t know its age, we don’t know its physical properties, we don’t know its composition, and the only information we have is seismic”, insists Marine Lasbleis, physicist for the certification company Bureau Veritas and author of a thesis on the subject. In an article published on July 6, in the review Nature, the Frenchwoman, postdoctoral fellow at the Planetology and Geosciences Laboratory in Nantes during this research, supported the team of Keith David Koper, at the University of Utah (United States), to lift a veil on the texture of this nucleus. And they discovered an unexpected heterogeneity there.
For this, they actually used seismic information. It must be said that the Earth has no shortage of them: a hundred earthquakes of magnitude greater than 5 occur there every week. You still need to be able to detect them. The researchers benefited from particularly sensitive sensors, installed almost everywhere on the surface of the globe by the Organization of the Comprehensive Nuclear Test Ban Treaty. Intended to locate possible wild tests, these seismographs are somewhat idle.
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Researchers periodically take them out of this Tartar desert to locate explosions of meteorites or earthquakes of human origin linked to the exploitation of shale gas, as well as to listen to colonies of pygmy sperm whales, improve climate predictions or follow the formation of icebergs. Here, the extreme sensitivity of these devices has therefore been used to study the composition of the famous seed, thanks to the monitoring of 2,455 earthquakes with a magnitude greater than 5.7.
Tiny signal
The principle is quite simple: when an earthquake shakes the earth’s crust, certain waves plunge towards the depths of the Earth. At each change of medium, part of the signal bounces back and returns to the surface, another continues on its way, just deviated by “refraction”, like light on the surface of water or glasses. This is what happens on arrival at the surface from the inner core, then inside it, at every little imperfection. The network of ultra-sensitive sensors then detects not only the main signal, but also a “coda”, indicating texture irregularities. A tiny signal, a few nanometers per second, is barely extracted from the ambient seismic noise. But for the sensor network of l’International Monitoring System, ces “echo babies”as Mr. Koper calls them, open a window into the heart of the world.
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