The composition of Earth's core is more complex than previously believed. Recent research indicates that it may serve as a significant reservoir for hydrogen, potentially containing the equivalent of up to 45 oceans of water, trapped in metallic layers beneath our feet.
Led by Motohiko Murakami from ETH Zurich, the study employed laboratory experiments to replicate the extreme conditions present during the formation of Earth. Findings suggest that hydrogen likely integrated into the core early in the planet's history, moving alongside silicon and oxygen as the Earth's interior differentiated into layers.
How Can We Detect Hydrogen in an Inaccessible Core?
Direct access to the core is impossible, but seismic waves provide indirect insights. The team utilized a state-of-the-art laser-heated diamond anvil cell to simulate core conditions, applying immense pressure and heat. A water-bearing crystal capsule was used to melt a piece of metallic iron, allowing hydrogen, oxygen, and silicon to blend into the liquid metal. The researchers then quickly froze the sample to analyze the atomic arrangement.
"Using advanced tomography, we successfully visualized atomic behavior within metallic iron," stated Dongyang Huang, the study's lead author.
The crucial discovery is that hydrogen is not present as free gas or water molecules in the core but is chemically integrated into the metal, forming iron hydrides linked to silicon- and oxygen-rich nanostructures. This finding provides a mechanism for hydrogen to be transported downward during core formation rather than remaining near the surface.
By analyzing the hydrogen-to-silicon ratio and combining it with prior estimates of silicon in the core, researchers believe hydrogen constitutes about 0.07% to 0.36% of the core's mass. While this percentage may seem small, the core's vast size translates this into roughly 9 to 45 oceans of water.
A New Perspective on Earth's Water Sources
The origins of Earth's water have long been debated. This study supports the theory that a substantial amount of hydrogen was present during the planet's formation, rather than being delivered solely by comets and asteroids after the core's development. While cometary contributions are acknowledged, this research suggests they may not be the primary source of Earth's water.
Hydrogen stored within the core could significantly influence various Earth systems over geological timescales. The ETH team notes potential connections to the generation of Earth's magnetic field, mantle dynamics, and the slow cycling of hydrogen between the deep Earth and surface over billions of years.
Moreover, understanding hydrogen's behavior under high pressure in metals aids scientists in modeling rocky exoplanets, as the presence of light elements in a planet's interior can impact core formation and evolution.
Despite the captivating notion of "dozens of oceans," the estimate is grounded in a series of scientific evidence, including laboratory measurements and imaging of minute structures. The next phase involves validating these findings to ensure their applicability to the complex systems of our planet.
Ultimately, this research emphasizes that the visible water on Earth's surface may only represent a fraction of the entire hydrogen narrative, with a significant portion concealed deep within the core. "These findings deepen our understanding of the deep Earth," Murakami concluded. "They offer insights into the distribution of water and other volatile substances in the early solar system and how Earth acquired its hydrogen."