World's largest hydrogen deposit discovered

A new experiment has discovered the largest hydrogen reservoir on Earth. It contains 45 times as much H2 as the oceans. But the discovery has a major drawback.

The Earth's core represents the largest hydrogen reservoir on our planet: it contains more hydrogen than the Earth's mantle and all the oceans combined, as a high-pressure experiment has demonstrated. This experiment allows for a more precise determination of the hydrogen content in the Earth's core. According to the results, the core contains between 0.07 and 0.36 percent hydrogen by weight – that's nine to 45 times more hydrogen than in all the world's oceans, as the research team reports in "Nature Communications". 

The Earth's core is the most mysterious part of the Earth.

The Earth's core is the most extreme and enigmatic region of the planet. Many questions about its structure remain unanswered, such as when the solid inner core formed or the precise composition of its iron-nickel alloy. However, it is assumed that the core's metallic alloy must also contain other, lighter elements. This could explain why the solid core is relatively soft, or why silicon-rich iron flakes form at the edge of the outer core.

The question of which light elements are present in the Earth's core and in what quantities remains controversial. "The estimated proportion of hydrogen in the Earth's core varies greatly – from ten to 10,000 parts by weight per million," explain Dongyang Huang of ETH Zurich and his colleagues. These values ​​are largely based on high-pressure experiments that investigate how much hydrogen dissolves in iron alloys under the extreme conditions of the Earth's core.

Silicon as the key to measurement

Huang and his team pursued an alternative approach. In their high-pressure experiment, they simulated both the iron-rich Earth's core and the surrounding mantle. "To do this, we heated an iron sample encased in silicate glass using a laser," the scientists report at temperatures of up to 5,300 degrees Celsius and a pressure of around 85 gigapascals, a miniature "magma ocean" formed on the outside and a liquid Earth's core on the inside.

Under these extreme conditions, silicon, oxygen, and hydrogen dissolve from the silicate and migrate into the molten iron core. Upon cooling, nanoclusters form, containing a mixture of iron and the three lighter elements. "Hydrogen and silicon combine with oxygen, always in a 1:1 ratio," explain Huang and his colleagues.

Hydrogen for nine to 45 oceans

The crucial point: "The silicon content of the Earth's core has now been relatively well defined," the team explains. Furthermore, silicon can be measured more precisely than hydrogen in high-pressure experiments. Using their measurement data and model calculations, Huang and his colleagues were able to determine the hydrogen content in the Earth's core's metal alloy more accurately than before.

The results confirm that the Earth's core is the largest hydrogen reservoir on Earth. It contains more hydrogen than the oceans, crust, and mantle combined. "According to our measurements, the Earth's core contains between 0.07 and 0.36 percent hydrogen by weight," the researchers report. "That corresponds to nine to 45 times as much hydrogen as in the Earth's oceans." These values ​​are slightly below some earlier estimates and models.

For the Earth's core, this means: "To explain the density deficit of the Earth's core, it must contain a mixture of lighter elements, similar to the core of Mars," write Huang and his colleagues. According to them, hydrogen plays a significant role among the lighter elements in the Earth's core, but it is not the only one. 

Major drawback: Humans cannot use the hydrogen

Anyone who draws hope from the news that hydrogen abundance will solve humanity's energy problems will be disappointed. There is no known way to use hydrogen from Earth's core. 

Temperatures of 5,000 degrees Celsius and immense pressure, millions of times greater than at Earth's surface, make any access to materials at this depth impossible. Humans have not yet drilled deeper than 12.2 kilometers into the Earth's crust; the liquid outer core lies at a depth of approximately 2,900 to 5,100 kilometers. 

Where does the hydrogen come from?

The new measurements also provide clues as to how hydrogen got into the Earth's core. This has been a matter of debate: were hydrogen, oxygen, silicon, and other elements already trapped in the Earth's core during Earth's formation? Or did these lighter elements migrate from the Earth's mantle into the core later – a process that may still be ongoing today?

According to the researchers, their results suggest that the gas originated during the formation of our planet. "Earth must have received most of its water from the main accretion phase," they write. This indicates that dust and planetary building blocks from the primordial cloud already carried this water.

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