A recent scientific investigation suggests the Earth’s core harbors up to 45 times the volume of hydrogen found in our oceans, establishing it as the planet’s most substantial hydrogen reservoir.
The team determined that this massive hydrogen quantity was incorporated into the Earth’s core during its initial accretion approximately 4.5 billion years ago, rather than being delivered later by cometary impacts occurring subsequent to core solidification. This finding is poised to resolve ongoing debates regarding the timing and process of terrestrial hydrogen introduction.
“The prevailing view is that Earth’s hydrogen, including that sequestered within the core, arrived during the planet’s formation,” commented leading author Dongyang Huang from Peking University in China. “The disagreement among researchers centers precisely on the specific phase within Earth’s formation when the hydrogen delivery occurred.”
This scientific divergence persists because quantifying hydrogen deep within the Earth presents immense logistical challenges. Being the universe’s lightest and smallest element, most detection methodologies lack the requisite sensitivity to accurately observe it under the extreme pressure and temperature regimes characteristic of the Earth’s core.
However, Huang noted that accurately estimating the hydrogen content locked within the core is crucial for understanding the mechanism by which hydrogen initially entered this region.
Prior studies attempting to gauge the hydrogen volume in the core relied on X-ray diffraction. This technique quantifies minerals and other constituents by analyzing how materials scatter X-rays. Given that the core is predominantly iron, researchers added hydrogen to iron samples in a lab setting and calculated the potential core hydrogen loading based on the measured expansion of the iron’s crystal structure.
Huang pointed out inherent weaknesses in using X-ray diffraction for this purpose. Firstly, it mandates precise prior knowledge of iron’s crystal structure and its behavior under specific conditions. Secondly, it operates under the assumption that silicon and oxygen present in the core do not distort the crystal lattice upon dissolving into the iron—a distortion that this new research has shown actually occurs.
In the current study, Huang and his colleagues employed an alternative technique called Atom Probe Tomography. This method allows for “three-dimensional, nanoscale compositional mapping of virtually all elements in the periodic table” and is “perfectly suited for high-pressure samples,” according to Huang.
The researchers then simulated the environmental conditions likely present immediately following core formation. They began by coating a minute iron sample with hydrated silicate glass to mimic a magma-coated core. This composite was then placed within a diamond anvil cell—a device utilizing two compressed diamond crystals to generate pressures akin to those found in the Earth’s core. To achieve the necessary high temperatures, lasers were employed to heat the sample to approximately 4830 degrees Celsius.
Under these simulated core conditions, the team utilized Atom Probe Tomography. They discovered that hydrogen, oxygen, and silicon simultaneously integrated into the iron’s crystal structures, inducing lattice modifications in ways previously unrecognized.
Crucially, the experiment revealed an equal influx of hydrogen and silicon from the simulated “magma” into the “core” analogue. This observation allowed the researchers to estimate that hydrogen constitutes between 0.07% and 0.36% of the Earth’s core mass.
The findings, published in Nature Communications, indicate that the Earth’s core contains 9 to 45 times more hydrogen than the planet’s oceans. If comets were the source, delivering hydrogen after core formation concluded, hydrogen should predominantly reside in shallower Earth layers. The core’s status as the premier hydrogen reservoir strongly implies that delivery occurred before the core fully coalesced, Huang stated.
“This marks the first time a mechanism for hydrogen sequestration into the core has been established,” he concluded.
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