A rare gas that dates back to the Big Bang is leaking out of Earth’s core, offering fresh clues about the formation of our planet billions of years ago, reports a new study.
Helium-3, an isotope of helium, is escaping from Earth’s interior at a rate of just over four pounds a year, but even this small amount can help solve big mysteries about the environment from which our planet was born. This gas has been tucked away since Earth’s infancy and can shed light on whether, and how, our world emerged from within the solar nebula, which was a huge mass of dust and gas that existed 4.6 billion years ago and ultimately birthed the Sun.
To better understand the source and implications of Earth’s helium-3, Peter Olson and Zachary Sharp, a geophysicist and isotope geochemist at the University of New Mexico, modeled the isotope’s movements at key points in our planet's history. The research suggests that helium-3, and potentially other volatile materials, are wafting out of the core, adding weight to the notion that Earth formed within a thriving solar nebula, and not on its outskirts or a declining phase as proposed by some theories, according to a study published on Monday in Geochemistry, Geophysics, Geosystems.
Scientists have known for decades that helium-3 is leaking from the depths of our planet, mostly through the mid-ocean ridge system, which is an enormous range of underwater volcanoes. However, it has been unclear whether the isotope was coming from Earth’s mantle, which is the molten layer below the planet’s crust, or from the core itself, located nearly 2,000 miles underground.
“There are a number of previous studies that proposed He-3 in the core, and even a few that discussed how helium might escape the core,” Olson said in an email. “What we did (that is new) is attach some numerical estimates to all of this.”
In other words, Olson and Sharp modeled points in Earth’s history that were crucial for its stockpile of helium-3. One was the actual formation and accretion of our planet out of gas, dust, and eventually rock. Another was the creation of the Moon, which is thought to be a chunk of Earth that was blasted into space in the wake of a cataclysmic crash between our baby planet and another proto-world about 4.5 billion years ago, ejecting a huge amount of helium-3, among other materials, in the process.
Based on the modern leakage of the isotope, these ancient events, and Earth’s subsequent evolution, the team was able to estimate that there is up to a petagram (roughly 100 million tons) of helium-3 left in the core. That is an enormous amount that adds strong evidence to the idea that Earth was situated in the thick of a blossoming solar nebula when it formed, allowing it to be enriched with helium-3 and other elements.
“The basic question in this regard is: When did Earth form?” Olson said. “The solar nebula only lasted a few million years, according to all the astronomical evidence. So, if the Earth grew to appreciable size (a reasonable fraction of its present mass) within the solar nebula, then it had to form early and grow fast.”
“The most widely-invoked theories, on the other hand, say the Earth accreted over a much longer period of time,” about 20 to 100 million years, he added.
While some helium-3 may also originate in the mantle, the new study suggests that the core is a major source of the isotope, with implications for future studies seeking to reconstruct our planet’s birth.
Olson and Sharp caution that there are many uncertainties built into their estimate, and that the model’s prediction might overshoot the amount of helium-3 that is actually in the core. However, they added that as long as “the sum of these uncertainties does not fundamentally alter the relative amounts of helium retained by the mantle and core, then our primary result—that the core is a major source of primordial helium—will still hold,” according to the study.
Other elements also might be rising from the deepest reaches of our planet, packed with clues about its origins. Olson said one potential target could be “light hydrogen,” which is a hydrogen isotope that matches the deuterium composition of the solar nebula, in contrast to hydrogen most elsewhere on Earth with a much higher deuterium content.
“We think that nebular hydrogen leaking from the core is something that could be usefully investigated,” Olson concluded. “If geochemists could find evidence for ‘light hydrogen’ in volcanic rocks from hotspots such as Iceland or Hawaii, that would almost be a smoking gun, since those hotspots bring material up from great depths and get much of their heat from the core-mantle boundary region.”
Update: This article has been updated with comments from lead author Peter Olson.