DATE2025.11.21 #Press Releases
Most of Earth’s Hydrogen and Carbon Reside in the Core
-Estimating the Origin of Water and Carbon from Their Global Abundances-
Summary
A research team led by Yutaro Tsutsumi (then graduate student), Shunpei Yokoo (Assistant Professor), and Kei Hirose (Professor) of the Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, in collaboration with the isotope microscopy facility at Hokkaido University and the large synchrotron radiation facility SPring-8, has determined the partition coefficients of hydrogen and carbon between metallic iron and silicate melt. Metallic iron represents the core material, while silicate melt corresponds to the magma ocean that covered the early Earth during its formation.
The experiments revealed that, under the high-pressure and high-temperature conditions of Earth’s deep interior, both hydrogen and carbon readily form alloys with iron. Moreover, when hydrogen and carbon coexist, they strongly interact with each other, significantly reducing their respective partition coefficients compared to when each element is present alone.
Using these experimentally determined partition coefficients, the team performed modeling of Earth’s accretion and subsequent core formation. They successfully estimated the amounts of hydrogen and carbon currently contained in Earth’s core that can account for the observed quantities of water and carbon in the oceans, crust, and mantle, as well as other observational constraints such as the core’s mass fraction. (In the core, hydrogen is mainly alloyed with iron, while in the outer layers, it exists primarily as water.)
These results, for the first time, clarify the total amounts of water and carbon contained within the entire Earth—including the core—and enable researchers to infer the types of materials that built our planet. The findings indicate that, contrary to the conventional view that Earth’s water (hydrogen) and carbon were primarily derived from carbonaceous chondrites rich in volatiles, roughly half actually originated from non-carbonaceous chondrite-type materials.
Figure：(Left) Diamond anvil cell apparatus that reproduces conditions of Earth’s deep interior
(Right) Experimental sample used to observe chemical reactions between core and mantle materials under high pressure and temperature
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Journals
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Journal name Nature CommunicationsTitle of paper