For decades, scientists have puzzled over traces of primordial helium—a rare isotope known as ³He—escaping from volcanic rocks in places like Hawaii and Iceland. Unlike the more common ⁴He, which is continuously produced by radioactive decay, ³He is not naturally formed on Earth, leading researchers to suspect it originates from deep within the planet.

Now, a groundbreaking study published in Physical Review Letters offers a surprising explanation: Earth’s core may contain vast amounts of helium, trapped for billions of years since the formation of the solar system. A research team from the University of Tokyo, led by Professor Kei Hirose, has demonstrated that helium can bond with iron under extreme pressures, overturning long-held assumptions that helium is too chemically inert to form stable compounds.

This discovery not only reshapes our understanding of Earth’s deep interior, but also suggests that ancient gases from the early solar nebula may still be trapped beneath our feet—offering new insights into the formation of planets, the origins of Earth’s atmosphere, and the evolution of its core.

A High-Pressure Experiment That Changed Everything

To investigate how helium behaves inside Earth, the researchers turned to an experimental technique that simulates the intense conditions of the planet’s deep interior. By using a laser-heated diamond anvil cell, they subjected iron and helium to crushing pressures and scorching temperatures similar to those found in the lower mantle and outer core.

“I have spent many years studying the geological and chemical processes that take place deep inside the Earth. Given the intense temperatures and pressures at play, experiments to explore some aspect of this environment must replicate those extreme conditions. So we often turn to a laser-heated diamond anvil cell to impart such pressures on samples to see the result,” said Hirose.

The team applied pressures ranging from 5 to 55 gigapascals—equivalent to 50,000 to 550,000 times atmospheric pressure—and heated the samples to temperatures between 1,000 and nearly 3,000 Kelvin (1,340–4,900°F).

“In this case, we crushed iron and helium together under about 5–55 gigapascals of pressure and at temperatures of 1,000 Kelvin to nearly 3,000 Kelvin. Those pressures correspond to roughly 50,000–550,000 times atmospheric pressure and the higher temperatures used could melt iridium, the material often used in car engine spark plugs due to its high thermal resistance,” Hirose explained.

Previous studies had detected only tiny traces of helium mixed with iron, at levels of about seven parts per million. But this experiment revealed a staggering concentration—up to 3.3% helium, an amount 5,000 times higher than previously observed. Even more remarkable, the helium remained stable inside the iron’s crystal lattice even after pressure was reduced, suggesting it could stay locked in Earth’s core for billions of years.

A New Clue to Earth’s Mysterious Helium Leak

The presence of ³He in volcanic rocks has long suggested that some primordial material from the early solar nebula is still present inside the planet. However, the question remained: where exactly is this helium stored, and how does it escape?

This study provides a compelling answer: Earth’s core itself may act as a vast reservoir of trapped helium, slowly leaking through the mantle and reaching the surface through volcanic plumes. If true, this means Earth’s core may still be releasing trapped gases from the solar system’s formation, providing a direct link between our planet’s interior and its earliest origins.

Beyond helium, this discovery raises a bigger question: could other noble gases, like neon or hydrogen, also be stored in Earth’s core? If so, this could help explain the origins of Earth’s atmosphere and even provide insights into how water may have formed on our planet.