New research quantifies the amount of a gas called helium three and proves that it comes from the Earth’s core.

Billions of years ago when the Earth was forming, it accumulated large quantities of an isotope of helium called helium three (He-3). The one place in the protosolar system where large amounts of He-3 would be is in the solar nebula, so the existence of this gas is a clue to the Earth’s formation.

Based on the amounts detected, the helium three in Earth’s core had to come from that solar nebula at formation while the planet was still fairly molten. Later on, the Earth cooled and stopped taking in helium until the impact of a Mars-sized object melted parts of the Earth again and also formed the Moon. This caused the Earth to begin to release He-3 from the core into the mantle, a process that continues to this day. From the mantle, helium three reaches the surface through a process called deep mantle convection.

According to the researchers, two kilograms of the gas comes out of the Earth every year. Two kilograms isn’t much – about enough to fill a balloon. The models in this study, published in the journal Geochemistry, Geophysics, Geosystems, indicate that there is up to a petagram – ten to the fifteenth – grams of the gas still locked up under the surface. Lead author Peter Olson notes: It’s a wonder of nature, and a clue for the history of the Earth, that there’s still a significant amount of this isotope in the interior of the Earth.

One complication with the model is that mantle plumes don’t bring much material from the locations where helium three is concentrated deep in the mantle. These regions, called Large Low Shear Velocity Provinces (or “blobs” from a story a couple of weeks ago), are relatively stable, remaining unchanged in the mantle for hundreds of millions of years. Instead, the plumes that bring the most helium three to the surface form away from these reservoirs, closer to the core where it is hotter.

Overall, there were three things needed for this model to work – helium three dissolving into the magma ocean and separating from the core metals, then helium three depletion of the mantle, and finally, the ability of the helium three to cross the core-mantle boundary. Honestly, that’s a lot of caveats for one model to hold, but it’s still plausible, especially due to Earth getting whacked by that Mars-sized body early on in its formation.

The researchers plan further studies looking at other gasses from nebulae in the core such as Xenon 129, which if it comes from the same places as the helium three, could provide more evidence for the solar nebula theory.

We look forward to seeing just where this research leads.

More Information

AGU press release

“Primordial Helium-3 Exchange Between Earth’s Core and Mantle,” Peter L. Olson and Zachary D. Sharp, 2022 March 28, Geochemistry, Geophysics, Geosystems