Is Earth’s Core Home to Hidden Primordial Helium?

The unexpected revelation that one of the lightest elements in the Universe—helium—can bond with iron under extreme pressure to form iron helide suggests a fundamental reevaluation of the geochemical processes occurring within the Earth’s profound depths.

This discovery indicates the potential presence of helium within the Earth’s core, where iron exists in its most pressurized state. A research team led by physicist Haruki Takezawa from the University of Tokyo suggests that our planet’s dense iron core may conceal a substantial reservoir of primordial helium.

On our planet, helium manifests in two stable isotopes. The predominant isotope, helium-4, features a nucleus comprising two protons and two neutrons, accounting for approximately 99.99986 percent of all helium on Earth. Conversely, helium-3, which contains two protons and one neutron, constitutes merely around 0.000137 percent of the total helium.

Helium-4 is primarily produced by the radioactive decay of uranium and thorium found within the Earth. In contrast, helium-3 is largely primordial, originating from the early moments after the Big Bang, with some produced as a by-product of the radioactive decay of tritium (hydrogen-3).

Notably, volcanic eruptions release small quantities of helium-3 in gases emitted from deep within Earth, leading scientists to theorize that primordial helium may be trapped within the mantle, remnants from the solar nebula from which Earth formed.

The research conducted by Takezawa and his team proposes a different source for this helium.

“I have dedicated many years to studying the geological and chemical processes occurring deep within the Earth. To replicate the intense conditions present, we utilize a laser-heated diamond anvil cell to apply such pressures to our samples,” explains physicist Kei Hirose from the University of Tokyo, where the experiments were performed.

“In this instance, we subjected iron and helium to pressures ranging from 5 to 55 gigapascals and temperatures between 1,000 and nearly 3,000 kelvins, equivalent to approximately 50,000 to 550,000 times atmospheric pressure. These elevated temperatures could even melt iridium, which is frequently utilized in spark plugs due to its exceptional thermal resistance.”

Previous studies have indicated that helium binds to iron in trace amounts, typically on the order of a few parts per million. In contrast, the team reported helium content in iron as high as 3.3 percent—nearly 5,000 times greater than documented in earlier research. This remarkable finding is attributed to the experimental design.

“Helium is prone to escape under normal conditions; everyone has witnessed a deflating balloon. Therefore, we needed a strategy to retain our measurements,” Hirose elaborates.

“While we conducted the material synthesis at elevated temperatures, the chemical measurements were performed at cryogenic temperatures to prevent helium from escaping, enabling us to detect the presence of helium in iron.”

This discovery implies that although helium is typically chemically inert at standard conditions, it can be induced to interact under extreme circumstances.

This could suggest that primordial helium was absorbed as Earth formed, binding with iron and becoming sequestered in the core during planetary differentiation. The same may hold true for the cores of the Moon and Mars.

If validated, this scenario might explain the presence of helium isotopes in volcanic gases as originating from the core rather than being trapped in the lower mantle. Furthermore, hydrogen—the lightest element—also exists in a primordial form. If primordial helium was abundant during Earth’s formation, primordial hydrogen may also have been present, potentially contributing to Earth’s early water supply.

Future research should delve into these intriguing possibilities.

The findings are detailed in the publication titled Physical Review Letters.