The Earth’s core is a huge hidden reservoir of hydrogen

For decades, scientists have been debating the amount of hydrogen (H) present in the Earth's core and how it got there. Some theories suggest that water (H₂O) was brought to Earth by comets and asteroids after the core had formed. Others argue that the Earth was more likely to have already been rich in water during its formation and that a significant portion of this water sank into the planet¡¯s deepest interior as elemental hydrogen (H) during the core formation process.

However, it is not possible to directly measure the amount of hydrogen present in the Earth¡¯s core. Indirect measurements using seismic waves are also difficult because the pressure and temperature conditions in the Earth¡¯s core differ significantly from those in the Earth¡¯s mantle.

A research team led by Motohiko Murakami, a professor in the Department of Earth and Planetary Sciences at ETH Zurich, has conducted an experiment to determine the form of hydrogen present in the Earth¡¯s core. The findings helped him to calculate the amount of hydrogen in the core. His study has just been published in the journal external page Nature Communications

Hydrogen trapped with silicon and oxygen

In the experiment, the researchers simulated the conditions that prevailed during the formation of the Earth. They discovered that hydrogen does not exist in the core as a standalone element; instead, it forms nanostructures with silicon and oxygen that are incorporated within molten iron. Consequently, hydrogen is not found in the core as a gas or as a water molecule (H₂O) but instead exists as an iron hydride directly within the molten metal. Conditions similar to those inside the Earth.

For their experiment, the ETH researchers used a laser-heated diamond anvil cell. This device allows scientists to generate pressures more than a million times greater than atmospheric pressure and temperatures hotter than those on the surface of the Sun.

To mimic the Earth¡¯s core, the researchers used a water-containing crystal capsule in which a tiny piece of metallic iron was embedded. They then heated this capsule with a laser until the iron became liquid, causing the elements silicon, oxygen and hydrogen to migrate from the capsule into the molten iron.The researchers then immediately quenched the capsule, making the individual atoms visible in three dimensions.

¡°The biggest challenge was to detect hydrogen under  extreme conditions in the nanoscale. Using state-of-the-art tomography, we were finally able to visualise how these atoms behave within metallic iron,¡± says Dongyang Huang, a former postdoctoral researcher in Murakami's research group and first author of the study.

Hydrogen content greater than expected

To determine the total hydrogen content of the Earth¡¯s core, the researchers used two key values: first, the ratio of hydrogen to silicon, which they determined in their experiment; and second, the silicon content of the Earth¡¯s core as established by previous studies.

The findings suggest that hydrogen makes up between 0.07 percent and 0.36 percent of the core¡¯s mass. If this hydrogen were to combine with oxygen to form water, it would correspond to approximately 9 to 50 times the volume of water found in all of today¡¯s oceans. This indicates that the Earth¡¯s core likely contains significantly more hydrogen than older models previously estimated.

Hydrogen arrived early

The findings also change our understanding of how the Earth was formed. If a significant amount of hydrogen entered the core during the Earth¡¯s development, it suggests that most of this hydrogen must have been present very early on in Earth¡¯s history.¡°This challenges the notion that hydrogen originated from comets that struck the young Earth after its formation,¡± Murakami emphasises. The results further indicated that the core may contain the largest reservoir of hydrogen on the planet ¨C surpassing the volume found in the oceans, atmosphere and mantle combined.

¡°The findings enhance our understanding of the deep Earth. They provide clues as to how water and other volatile substances were distributed in the early solar system and how the Earth acquired its hydrogen,¡± Murikami explains.

Hydrogen in the core influences the magnetic field

Hidden hydrogen in the Earth¡¯s core may affect many processes, from the formation of the magnetic field to the long-term exchange of hydrogen between the core and the mantle. Over billions of years, some of this deeply stored hydrogen could gradually rise to the surface, potentially impacting volcanism and the dynamics of the Earth¡¯s mantle.

The results also help to model exoplanets, as the distribution of hydrogen and other elements is crucial in determining whether a planet has a metallic core or lacks one entirely. Additionally, this study lays new groundwork for geochemical models of the Earth¡¯s mantle and the global water cycle.

¡°The water we see on the Earth¡¯s surface today may be just the visible tip of a gigantic iceberg deep inside the planet,¡± the ETH professor emphasises.