Rare helium leaking from Earth’s core holds clues to where we come from

According to a recently published study, a rare helium isotope, helium-3, is leaking out of the Earth's core, thereby adding more evidence to debates surrounding the Earth's origins.

Helium-3 has only one neutron in its nucleus as opposed to the two that are found in the isotope that is usually found on Earth. The isotope is so rare that it makes up for only 0.0001 percent of all the helium present on the Earth. The origin of helium-3 can be dated back to about 13.8 billion years ago when the Big Bang occurred, and its presence in the Earth's core throws more light on the formation of Earth itself.

How much helium-3 is in the Earth's core?

Helium-3 has been detected on the Earth's surface before. However, geologists were not sure whether the source of this rare, primordial gas was the Earth's core or its middle layers, called the mantle. The recently published study showed that about 4.4 pounds (2,000 grams) of helium-3 is released every year from the Earth's core.

Using the estimates, the researchers modeled that there are between 10 teragrams (10¹³) to a petagram (10¹⁵) of helium-3 in the core. The location of helium-3 is crucial since it provides clues about the planet's origins. As a planet grows, it accumulates material from its surroundings, and for the Earth's core to have accumulated helium-3, the planet itself would need to have been formed inside a thriving solar nebula and not its fringes, the study revealed. The solar nebula is a bast spinning and collapsed cloud that astronomers believe led to the creation of the solar system as we know it today.

How was the helium-3 amount estimated?

To determine the helium-3 levels, the researchers used a model that included two stages of Earth's history. One had helium-3 being accumulated by the Earth during its formation, and the other included the exodus of helium-3 after the moon's formation.

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The researchers attribute this to a Mars-sized object that collided with the Earth about four billion years ago. The result was the remelting of the Earth's crust that allowed the helium to escape, a process that continues to this date. This impact is also the reason for the formation of the moon.

The researchers are hopeful that further research on other gases that might be escaping the Earth's core from similar spots and at similar rates as helium-3 will add more evidence to their modeling.

The study was published in Geochemistry, Geophysics, Geosystems.

Study Abstract:

Volatiles from the solar nebula are known to be present in Earth's deep mantle. The core also may contain solar nebula-derived volatiles, but in unknown amounts. Here we use calculations of volatile ingassing and degassing to estimate the abundance of primordial ³He now in the core and track the rate of ³He exchange between the core and mantle through Earth history. We apply an ingassing model that includes a silicate magma ocean and an iron-rich proto-core coupled to a nebular atmosphere of solar composition to calculate the amounts of ³He acquired by the mantle and core during accretion and core formation. Using experimentally determined partitioning between core-forming metals and silicate magma, we find that dissolution from the nebular atmosphere deposits one or more petagrams of ³He into the proto-core. Following accretion, ³He exchange depends on the convective history of the coupled core-mantle system. We combine determinations of the present-day surface ³He flux with estimates of the present-day mantle ³He abundance, mantle and core heat fluxes, and our ingassed ³He abundances in a convective degassing model. According to this model, the mantle ³He abundance is evolving toward a statistical steady state, in which surface losses are compensated by enrichments from the core.

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A rare kind of helium leaking from the Earth’s core could help solve a major mystery

How much helium-3 is in the Earth's core?

How was the helium-3 amount estimated?