The interior of the earth: is the earth's core rusting?

What goes on in the Earth's core, a sphere the size of Mars at the center of our planet, continues to puzzle researchers.

Today nobody doubts that the core of the earth is divided in two.

It consists of a solid inner core about 2,400 kilometers in diameter overlaid by the almost equally thick liquid shell of the outer core.

But what exactly happens in the center of our planet is in the dark.

Two American research groups have now shed new light on the extremely hot, unimaginably high-pressure bowels of the earth.

Researchers have long suspected that the inner core appears to move independently of the rest of our planet.

Among other things, this is indicated by precise measurements of the fluctuations in the length of the day.

Every six years, the days get longer or shorter by about a tenth of a millisecond.

About 30 years ago, researchers at Columbia University in New York attributed this change to the Earth's inner core rotating slightly faster than the rest of the planet.

They inferred this from changes in the travel time of earthquake waves traveling through the Earth's core.

Since then, however, the exact rate of movement of this Pluto-sized ball of iron and nickel has been debated.

John Vidale and Wei Wang of the University of Southern California at Los Angeles have now found that the inner core does not rotate evenly, but rather oscillates back and forth over the years like a rotating pendulum.

As the two researchers now write in the "Science Advances", this rotating oscillation of the earth's core, which occurs every six years, is about 0.3 angular degrees.

The researchers were only able to make this precise calculation because they did not use the recordings of earthquake waves for their investigations, but the registrations of the ground vibrations triggered by underground nuclear weapon tests.

The duration of earthquake waves is always associated with a certain degree of uncertainty, because the time at which an earthquake begins can only be roughly calculated using models.

In contrast, in the case of a nuclear weapon test, the time of detonation is known to the nearest millisecond.

Seismic waves detect chemical processes

Vidale and Wei evaluated measurements from American and Soviet nuclear tests from 1969 to 1974.

At that time, the American nuclear tests took place on the Aleutian island of Amchitka, the Soviet detonations on the Arctic island of Novaya Zemlya, located between the Barents and Kara Seas.

The earth vibrations from these large subterranean explosions, each measuring several megatons, were recorded by a now defunct, very dense seismometer network in the US state of Montana.

During the measurement period, the earthquake waves first passed through the inner core more slowly and later faster.

Today, however, such precise measurements would no longer be possible,

The second research group is dealing with another layer of the earth's core, namely the boundary layer between the outer core and the earth's mantle at a depth of around 2900 kilometers.

There is no longer any trace of the rhythmically oscillating movement of the inner core.

Instead, a series of chemical reactions take place at this so-called core-mantle boundary, because like its inner brother, the outer core consists of an alloy of iron and nickel, while the earth's mantle consists mainly of silicate rocks.

Jiuhua Chen and Shanece Esdaille of Florida International University in Miami have now also used earthquake waves to study some of these chemical reactions in more detail.

In fact, over the past few years, seismologists have found that seismic waves in the immediate vicinity of the core-mantle boundary can slow down significantly than what is predicted by typical models of the Earth's interior.

Above about one-fifth of the outer core of the earth there are thin layers that strongly slow down earthquake waves.

How these zones form and what happens in them is currently the subject of intense geoscientific debate.

The two researchers now provide a possible explanation in the journal "Eos".

According to this, the iron of the earth's core could gradually rust when it comes into contact with water-bearing minerals from the earth's mantle.

These minerals, in turn, are remnants of the Earth's oceanic crust that sunk into the Earth's mantle in subduction zones tens of millions of years ago.

However, instead of melting completely there, these water-bearing rocks sank deeper and deeper and finally reached the core-mantle boundary.

Only there did they dissolve, and the water they contained was able to oxidize the iron in the outermost layers of the earth's core.

The researchers write that laboratory tests have shown that seismic waves travel through layers of such rust much more slowly than they would in a typical iron-nickel alloy under the pressures and temperatures at the core-mantle interface.

Some wave types can be slowed down by more than 40 percent.

The explanation now proposed contradicts other hypotheses, for example that the lowest layers of the mantle at the boundary to the core have melted far more than the mantle rocks in higher layers.