Over 70% of the Earth’s surface is covered with water, most of it contained in oceans. If we gathered all the water into a sphere, it would have a diameter of around one-tenth of the Earth. Now, multiply that by nine, and that might be the minimum amount of hydrogen in Earth’s core, with higher estimates suggesting 45 such oceans, making it the planet’s largest hydrogen reservoir, as per recent estimates that scientists made.
Hydrogen makes up around 0.07% to 0.36% of Earth’s total core weight, as scientists reported in Nature Communications on Tuesday. While most theories suggest that the Earth got its supply of water through comet impacts on the planet’s surface, this theory suggests otherwise. The Earth might have acquired water, which is the largest source of hydrogen on the planet, as the planet formed.
“Earth’s core would store most of the water in the first million years of Earth’s history,” said lead study author Dongyang Huang, an assistant professor in the School of Earth and Space Sciences at Peking University. Next in water abundance is the mantle and crust. “The surface — where life resides — contains the least,” he said.
More than 4.6 billion years ago, gas and dust around the Sun collided and clumped to shape the Earth’s core, mantle, and crust. Under immense pressure and temperature in the extreme depths of the Earth, a hot, molten metal core began to churn. The core was comprised mostly of iron and nickel, and became the engine of Earth’s protective magnetic field. Hydrogen could have only entered the fluid metal core if it was present during the Earth’s main growth phases and participated in core formation.
To understand the formation and evolution of life on Earth, scientists need to study the origin and distribution of hydrogen. Chemical interactions in iron have been used before to estimate how much hydrogen is buried in the Earth’s molten core. However, the core is too deep for direct observation, and recreating the high-pressure conditions in a lab is extremely difficult. Routine analytical methods fail to deliver because hydrogen is the smallest and lightest element.
Low density of the core hinted at the abundance of hydrogen. However, it was tricky to estimate the amount compared with other known core elements like silicon and oxygen that are easier to measure. X-ray diffraction techniques were used to look at the lattice structure of iron crystals, which expands more in the presence of hydrogen. The interpretations had a wide range, suggesting that there was not enough hydrogen in the core to form one ‘ocean’, or there was enough to fill over 120 such ‘oceans’.
Huang took a different route. First, his team sharpened core-like iron samples into needles with a 20-nanometer diameter, and then placed them under finely controlled high voltage. Next, the atoms in this sample were ionized and counted individually. This method was then replicated under the high-temperature, high-pressure environment of the core in a diamond anvil cell. An iron sample, along with hydrous silicate glass, was used to represent early magma oceans. Lasers were used to melt the sample in the core-like conditions. Using atom probe tomography, Huang and his team captured 3D images and measured chemical composition at the atomic scale.
This approach depends on how atoms are arranged in the Earth’s core and how hydrogen, silicon, and oxygen are dispersed there. The interaction of hydrogen with silicon and oxygen in nanostructures, as the metal cooled, indicated that the hydrogen-to-silicon ratio was approximately 1:1. To approximate the amount of core hydrogen, scientists combined observations of these ratios in the samples with previous estimates of silicon in the core.
The interactions among hydrogen, silicon, and oxygen in iron nanostructures offer insights into how heat is released from the core and how this process helped establish the Earth’s magnetic field, which shields the planet from solar wind and cosmic radiation and supports life. However, the scientists want to tread cautiously, as an an indirect approach like this has uncertainties and does not account for other interactions that may affect the core hydrogen estimate.
The estimated amount may still be lower than the core’s actual hydrogen content, since the amount of hydrogen that escaped the iron samples during decompression was not included. Previous estimates by Kei Hirose, a professor at the University of Tokyo’s School of Science, estimated that around 0.2% to 0.6% of the weight of the core consists of hydrogen.
If the measurements and hypothesis hold, science community will acknowledge that hydrogen was delivered throughout Earth’s growth. Gas from nebulae, along with water from comets and asteroids, may have also provided hydrogen to our planet.
Hydrogen is an essential element for life on Earth and is an integral part of organic compounds. From carbohydrates to proteins, carbon and hydrogen form the backbone of biomolecules. The origin story of hydrogen will be a pivotal piece of the puzzle of life on Earth.
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