Helium in the Earth's core

Researchers from Japan and Taiwan reveal for the first time that helium, usually considered chemically inert, can bond with iron under high pressures. They used a laser-heated diamond anvil cell to find this, and the discovery suggests there could be huge amounts of helium in the Earth’s core. This could challenge long-standing ideas about the planet’s internal structure and history, and may even reveal details of the nebula our solar system coalesced from.

If you’ve ever seen a volcanic eruption and wondered what might be coming out of it, you’d be right if you thought it’s mostly rocks and minerals. Would you be surprised to know there are often traces of what is known as primordial helium as well? That is, helium which differs from normal helium, or ⁴He, so called because it contains two protons and two neutrons and is continuously produced by radioactive decay. Primordial helium, or ³He, on the other hand is not formed on Earth and contains two protons and one neutron.

Given the occasionally high ³He/⁴He ratios found in volcanic rocks, especially in Hawaii, researchers have long believed there are primordial materials containing ³He deep within the mantle. However, graduate student Haruki Takezawa and members of Professor Kei Hirose’s group from the University of Tokyo’s Department of Earth and Planetary Science have now challenged this view with a new take on a familiar experiment — crushing things.

“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. “In this case, we crushed iron and helium together under about 5-55 gigapascals of pressure and at temperatures of 1,000 kelvins to nearly 3,000 kelvins. 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.”

Previous studies have shown only small traces of combined iron and helium, in the region of seven parts per million helium within iron. But in this case, they were surprised to find the crushed iron compounds contained as much as 3.3% helium, about 5,000 times higher than previously seen. Hirose suspects this is at least in part due to something novel about this particular set of experiments.

“Helium tends to escape at ambient conditions very easily; everyone has seen an inflatable balloon wither and sink. So, we needed a way to avoid this when taking our measurements,” he said. “Though we carried out the material syntheses under high temperatures, the chemical-sensing measurements were done at extremely cold, or cryogenic, temperatures. This way prevented helium from escaping and allowed us to detect helium in iron.”

This finding has implications for understanding Earth’s origins. The presence of helium in the core suggests the young Earth likely captured some gas from the solar nebula of hydrogen and helium that surrounded the early solar system. This could also mean that some of Earth’s water may have come from hydrogen in this ancient gas, offering a new perspective on the planet’s early development.

 

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Journal article:

Haruki Takezawa, Han Hsu, Kei Hirose, Fumiya Sakai, Suyu Fu, Hitoshi Gomi, Shiro Miwa, and Naoya Sakamoto, “Formation of Iron-Helium Compounds under High Pressure”, Physical Review Letters, DOI: https://doi.org/10.1103/PhysRevLett.134.084101

Funding: This work was supported by JSPS grant 21H04506 to K.H.; H.H. is supported by National Science and Technology Council of Taiwan under grants NSTC 113-2116-M-008-010, 112-2112-M-008-038, and 111-2112-M-008-032.

Useful links:
Department of Earth and Planetary Science - https://www.eps.s.u-tokyo.ac.jp/en/
Graduate School of Science - https://www.s.u-tokyo.ac.jp/en/index.html

Research contact:
Professor Kei Hirose
Department of Earth and Planetary Science, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
kei@eps.s.u-tokyo.ac.jp

Press contact:
Mr. Rohan Mehra
Public Relations Group, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan
press-releases.adm@gs.mail.u-tokyo.ac.jp

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