Water explains two central mysteries of the earth

Water explains two central mysteries of Earth – Spectrum of Science

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reaction in depth: Water is behind two mysteries of the Earth’s core

Earth’s outer core is less massive than it should be. Nobody yet knows why. One possible answer lies in a substance you wouldn’t expect to find down there: water.

© iStock / johan63 (detail)

Water travels from the Earth’s surface to the edge of the Earth’s outer core and there reacts with silicon.

A previously unknown chemical reaction with water likely creates two mysterious features of the Earth’s core. In this way, the core, composed mainly of iron, is enriched with hydrogen and releases silicon into the Earth’s mantle. This would explain, on the one hand, why the outer core of the Earth is approximately ten percent lighter than expected and, on the other hand, what happens to a thin layer that, according to seismic data, is located on the surface from the earth. center.

A team led by Sang-Heon Shim of Arizona State University and Yongjae Lee of Yonsei University in Seoul examined the behavior of core and shell components at high pressures and temperatures in a diamond anvil cell. As the group reports in the journal Nature Geoscience, water reacts with silicon in the iron alloy of the Earth’s core. This creates silicate, which adheres to the rocks in the mantle, while the hydrogen remains dissolved in the iron.

Earth’s liquid outer core is made of an alloy of iron and nickel, although its exact composition is controversial. Its density is much lower than what would be expected from an alloy of this type. It is therefore assumed to contain large amounts of lighter elements, but it is not clear which ones and, above all, how they get there. Part of the answer may lie in a gigantic conveyor belt that extends from the Earth’s surface to the iron core. In the deep ocean trenches that mark the boundaries between Earth’s plates, water-rich rocks from the oceanic crust descend deep into the Earth’s mantle. They release much of this water into the Earth’s mantle. But some water-bearing minerals are so stable that they are transported along with the Earth’s ancient crust to the edge of the Earth’s core, 2,900 kilometers away.

Shim and Lee’s team now investigated what happens to the water there in high-pressure experiments. In a laser-heated diamond anvil cell, where pressures and temperatures at the core-shell boundary can be reached, water-containing minerals were allowed to react with an alloy of iron and nine percent silicon. Regardless of the exact source of water and the respective conditions, silicate was always formed on the one hand and hydrogen dissolved in iron on the other. In the experiment, the iron with the hydrogen dissolved also had a significantly lower density than the surrounding alloy, so the reaction could explain why Earth’s outer core is lighter than expected.

The results of this experiment also suggest that the significantly lighter and hydrogen-rich iron is poorly distributed throughout the Earth’s outer core. It is simply too light to be distributed by currents in the metal and instead remains on the surface of the liquid core. This finding fits surprisingly well with another puzzling observation from the 1990s. Consequently, seismic waves traveling through the Earth and diffracting in the different layers indicate the existence of a thin, stable layer of liquid around the Earth’s outer core.

Until now there has been no adequate explanation for this layer, called E’. However, the working group’s experiments now indicate that the liquid is the result of a gigantic process that has taken place over billions of years. Water, along with subducting rocks, travels from the surface to the edge of the Earth’s mantle. There it reacts with metal from the Earth’s core, producing a hydrogen-rich iron melt that floats on top of the iron-nickel alloy of the Earth’s core and is visible as a separate layer in the earthquake data.

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