Earth’s core contains up to 45 times more hydrogen than the oceans do — and it got there early in the planet’s formation, study finds
The Earth’s core contains up to 45 times more hydrogen than the oceans, making it the largest reservoir of hydrogen on the planet, a new study suggests.
The researchers found that this large amount of hydrogen entered the core when it formed about 4.5 billion years ago and did not arrive via comets that hit it. Earth once the core is established. This discovery could settle the debate over when and how hydrogen was delivered to our planet.
This debate continues because hydrogen deep within the Earth is extremely difficult to quantify. Hydrogen is the smallest and lightest element in the universe, so most techniques lack the resolution to properly detect it in high-pressure, high-temperature environments such as the Earth’s core.
But estimating how much hydrogen is locked inside the core is key to understanding how the hydrogen got there, Huang said.
Previous research used a technique called X-ray diffraction to estimate the amount of hydrogen in the Earth’s core. This method quantifies the minerals and other substances present in a material by analyzing how that material scatters
The downside of X-ray diffraction in this case is that it relies on a few crucial assumptions, Huang said. First, it assumes that researchers have a precise understanding of iron crystal structures and how they react under certain conditions. Second, it assumes that silicon and oxygen, both present in the nucleus, do not distort the crystal structure when they dissolve into iron – which turns out to be the case.
For the new study, Huang and his colleagues used an alternative method known as atom probe tomography. This technique can “provide nanoscale 3D compositional mapping of all elements in the periodic table” and is “ideal for high-pressure samples,” Huang said.
The researchers simulated the conditions that likely existed during the formation of the Earth’s core. To begin, they covered a tiny sample of metallic iron with hydrated silicate glass to model the magma-covered core. Next, they placed this object in a diamond anvil cell, a device in which two diamond crystals squeeze together to generate extreme pressure similar to that found in the Earth’s core. To create high temperature conditions, scientists used lasers that heated the object to about 8,730 degrees Fahrenheit (4,830 degrees Celsius).
The researchers used atom probe tomography in this context. They discovered that hydrogen, oxygen and silicon dissolve simultaneously in iron crystal structures under extreme conditions, altering the crystals in a previously unknown way.
Importantly, equal amounts of hydrogen and silicon entered the “core” from the “magma” in the experiment, which helped the researchers estimate that hydrogen makes up 0.07% to 0.36% of the Earth’s core by weight.
The results, published Tuesday February 10 in the journal Natural communicationssuggest that the Earth’s core contains 9 to 45 times more hydrogen than the planet’s oceans. If comets had delivered hydrogen to the Earth after the formation of the core, the hydrogen would be found mainly in the shallower layers of the Earth. But the discovery that the core is Earth’s largest reservoir of hydrogen indicates that the hydrogen was delivered before the core was fully formed, Huang said.
“This is the first time that the mechanism by which hydrogen enters the nucleus has been identified,” he said.
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