A new modeling study shows an extremely rare type of helium created shortly after the Big Bang leaked from Earth's metallic core.
Most of this gas in the universe, called helium-3. It is a primordial gas and was created right after the Big Bang about 13.8 billion years ago.
Some of this helium-3 combines with other gas and dust particles in the solar nebula, the vast, rotating, collapsing cloud that is thought to have led to the creation of our Solar System.
"The discovery that the Earth's core may contain a large reservoir of helium-3 is further evidence to support the idea that Earth formed within the expanding solar nebula, not at its periphery or during its waning phase," the researchers said.
Helium-3, according to the researchers, is a natural wonder and could be a clue to the history of Earth's formation. "There are still large amounts of these isotopes in the interior of the Earth," said study lead author Peter Olson, a geophysicist at the University of New Mexico.
Helium-3 is an isotope, or variant of helium that has one neutron instead of the usual two neutrons in its nucleus. It is a rare gas, making up only 0.0001% of the helium on Earth.
This gas comes from various processes, one of which is the radioactive decay of tritium, a rare radioactive isotope of hydrogen. But since helium is one of the earliest elements to exist in the universe, it's very likely that helium-3 came from the Big Bang.
"Scientists already know that about 2 kilograms of helium-3 leaks from Earth's interior every year, mostly along the mid-ocean ridge system where tectonic plates meet. This is enough to fill a table-sized balloon," Olson said.
But scientists aren't sure exactly how much helium-3 comes from Earth's core, and how much helium-3 is in Earth's reservoirs.
To investigate, the research team modeled helium abundance during two important phases of Earth's history: the early formation of the planet, while it was still accumulating helium, and after the formation of the Moon, when our planet lost a lot of this gas.
Scientists think that the Moon was formed when a colossal object the size of Mars collided with Earth about 4 billion years ago. This event would melt Earth's crust and allow most of the helium on our planet to escape.
However, Earth didn't lose all of the helium-3 at that time, as it still retained some of the rare gas that kept seeping out of Earth's bowels.
The researchers combined modern helium-3 leakage rates with models of helium isotope behavior. These calculations reveal that between 10 teragrams and 1 pentagram of helium-3 are in the Earth's core. Surely this is a very large number, indicating that the Earth formed in a solar nebula with a high concentration of gases.
However, because these results are based on modelling, they are not rigorous. The team had to make a number of assumptions, such as that Earth picked up helium-3 as it formed in the solar nebula, that helium entered the core-forming metals and that some of the helium left the core into the mantle.
These assumptions, in addition to other uncertainties, including how long the solar nebula lasts relative to the rate at which Earth formed, mean there may be less helium-3 in Earth's core than they think.