The heart of our planet may be releasing an element trapped for billions of years, eventually emerging through volcanoes.
To understand this, we need to go back to Earth's birth. At that time, our planet was largely covered by an ocean of molten rock. As it cooled, this material formed the mantle, the thick layer located between Earth's crust and its metallic core.
Earth in cross-section.
Image: Argonne National Laboratory / Flickr / CC 2.0
Even today, rocks sometimes rise from the depths and feed volcanoes. Some lavas, particularly in Iceland and on Baffin Island in the Canadian Arctic, have a surprising chemical composition. They contain an unusually low proportion of a particular form of hydrogen: deuterium.
Hydrogen does indeed exist in several forms. The most common is very light. Deuterium is slightly heavier because its nucleus contains an additional neutron. By comparing their proportions in rocks, researchers can find clues about the origin of their material.
There were two possible explanations for these unusual lavas. The first assumed that their composition had been fixed when the mantle formed, as the ocean of molten rock solidified. The second proposed that ancient hydrogen was escaping from the metallic core and joining the mantle over time.
Schematic model of the origin of the detected anomalies.
To distinguish between them, a team of researchers simulated the conditions that prevailed inside the young Earth. The calculations show that cooling the magma ocean alone is not enough to explain the low quantities of deuterium measured in some lavas.
Another clue points in the same direction: these rocks sometimes contain an unusual proportion of helium-3, a rare form of helium that may have been preserved deep within Earth since its formation. The combination of these two signatures makes the hypothesis of material originating from the core more convincing.
If this leakage exists, it may have been continuing for billions of years. Researchers must now understand how hydrogen could cross this boundary located thousands of kilometers beneath our feet. Earth's chemical history may thus be linked to deep exchanges.