In The Core Much Of The Earth’s Water (Earth Science)

Using devices called diamond anvil cells, a team of researchers simulated the formation of the Earth’s core, demonstrating for the first time that, under extreme conditions of temperature and pressure, hydrogen can bind to the iron it contains. A result that supports the hypothesis that much of the water that arrived on Earth billions of years ago could be contained in the outer core in the form of hydrogen

According to a new study published in the pages of the journal Nature Communications , most of our planet’s water could be found inside it, dissolved in the iron and silicates of the outer core and mantle, respectively .

Given the extreme depths, temperatures and pressures involved, studying the heart of our planet directly is impossible. Much of the information we have today about its structure, composition and density has been obtained indirectly thanks to seismology – a branch of geophysics that studies the ways of propagation of seismic waves within the earth – and through laboratory experiments.

Thanks to this data, we know that the core is mainly made of iron and that its density, particularly that of the liquid outer core, is lower than expected. This has led the researchers to believe that, alongside the iron, there must be an abundance of some other light chemical element in the core. One possibility is that this element could be hydrogen: according to some hypotheses, the one contained in the water that reached Earth billions of years ago, during the numerous astronomical impacts.

To shed light on this possibility, a team of researchers from the University of Tokyo examined the behavior of water in the presence of iron and silicate compounds, using high temperature and high pressure experiments conducted inside anvil cells of diamond , thus simulating the metal-silicate reactions (core-mantle) that occurred during the formation of the Earth. “At the temperatures and pressures we are used to on the surface, hydrogen does not bind to iron” explains Shoh Tagawa, post-doc at the Department of Earth and Planetary Sciences of the University of Tokyo and first author of the study. “We wondered if this link can take place in more extreme conditions. Such extreme temperatures and pressures are not easy to reproduce and the best way to achieve them in the laboratory is to use a diamond anvil cell, a device that can produce pressures of 30-60 gigapascals at temperatures of 3,100-4,600 kelvins – a good simulation of the formation of the Earth’s core ».

Diamond anvil cell.  On the left, the chamber into which the samples are placed under conditions of pressure and temperature similar to those of the outer core of the Earth.  On the right, the diamonds of the anvil.  Credits: 2021 Hirose et al.

The team of scientists, led by  Kei Hirose of Kyoto University , used in particular iron and silicate compounds similar to those found in the Earth’s core and mantle respectively, which they compressed in the special chamber and simultaneously heated with a laser. To see what was happening in the sample, they used high-resolution images produced by applying a technique called secondary ion mass spectroscopy .

What happened during the experiment is that when the water met the iron and the molten silicates inside the anvil cell, the hydrogen of the water molecules reacted mainly with the iron, thus acting as a siderophilic element . while oxygen has entered the composition of the silicates.

The results obtained, explain the researchers, show for the first time that hydrogen can bind strongly with iron even in the extreme conditions of the Earth’s core, supporting the hypothesis that most of the water that arrived on Earth during its formation could be stored in this layer in the form of hydrogen. The presence of hydrogen in this layer could also explain its reduced density compared to pure iron.

“This discovery allows us to explore something that concerns us very much,” concludes Hirose. “The fact that hydrogen is a high-pressure siderophile tells us that much of the water that came to Earth during its formation through astronomical impacts may now be present in the core as hydrogen. We estimate that there could be as much hydrogen enclosed there as 70 oceans . If this had remained on the surface as water, our planet would have had no land and life as we know it would never have evolved ».

Featured image: The internal structure of the Earth. Credits: Wikipedia.

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