Earth’s inner core may be superionic

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(ORDO NEWS) — Perhaps the deepest bowels of our planet are both in liquid and solid phases: heavy elements form a crystal lattice, and light ones flow freely between them.

Many of the mysteries associated with the Earth‘s inner core can be related to the fact that it is in a special superionic phase. Heavy iron and nickel remain solid, while light elements move more or less freely along their crystal lattice, changing the nature of seismic waves passing through them.

We remember from school that our planet consists of a thin crust, under which there is a mantle surrounding the core. The core is composed of iron and nickel and is divided into a liquid outer and a solid inner.

This knowledge was obtained indirectly: the deepest boreholes do not reach the mantle, much less the core. Geologists study its structure using laboratory experiments, modeling and, of course, seismic waves.

Passing through matter with different properties, they change their speed and other characteristics, allowing them to “shine through” the planet like an x-ray.

Similar work has shown that the Earth’s inner core is not as dense as one would expect from iron and nickel at enormous temperature (5400 ° C) and pressure (about 350 gigapascals). Apparently, it contains a considerable admixture of lighter elements.

These may include silicon, sulfur, carbon, oxygen and hydrogen, but it is not clear which ones, in what quantities and form, are present there. In any case, their presence affects not only the density, but also the melting point, thermal conductivity of the core, and the nature of the passage of seismic vibrations through it.

As seismograph measurements show, movement through the inner core slows down the propagation of transverse S-waves, which oscillate in a plane perpendicular to the direction of their propagation. Neither pure iron nor iron-nickel in various combinations should behave this way.

Therefore, in 2021, John Brodholt and his colleagues attributed this feature to the fact that the inner core is in a “superionic” state, like water in the form of ice XVIII . In it, oxygen ions form a crystal lattice, as in a solid phase, and hydrogen ions movably flow between them, as in a liquid.

The inner core can be arranged similarly: heavy iron and nickel ions are in the solid phase, while particles of light elements move relatively freely. This picture was confirmed by the new work of Yuya He (Yu He) and his colleagues from the Chinese Academy of Sciences.

The scientists simulated the behavior of a structure of 64 atoms, including nickel, iron and a set of light elements, at temperatures and pressures that are considered characteristic of the inner core.

And although they used a different combination of light elements than Brodholt et al., they managed to again obtain a superionic state of matter in which the free movement of light ions causes S-waves to slow down.

This may explain another feature of the inner core – the fact that its influence on seismic vibrations changes over time. According to one of the assumptions, this may be due to the fact that the speed of its rotation differs from the general speed of rotation of the planet.

But if the inner core is indeed in a superionic state, then the same results can be obtained simply by a constant flow of the liquid phase in it.

There is another mystery of seismic vibrations passing through the inner core: in the direction from pole to pole they move faster than in the equatorial plane. According to Yu He and his colleagues, this is probably due to the uneven distribution of light “liquid” elements.

If they are concentrated mainly closer to the very center of the planet and form a sphere flattened from the poles there, then they can lead to anisotropic propagation of seismic waves. But how and why this liquid is collected in such a “lens” is unknown.


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