Something ancient and primordial lurks in Earth’s core. Helium 3 (3He) was created in the first minutes after the Big Bang, and some of it found its way through time and space to take up residence in Earth’s deepest regions. How do we know this?
Scientists can measure it as it slowly escapes.
About five billion years ago, the Sun was born in a cloud of gas called the solar nebula. The remainder of that gas is mostly long gone now, dispersed into space by the solar wind.
But some of the material from the solar nebula, including helium, became trapped inside the Earth. New research shows that the primordial helium is slowly leaking out of Earth. How it leaks and how quickly it leaks are clues to Earth’s formation and evolution and the formation and evolution of other terrestrial planets.
We understand how the Earth formed. It formed from the gas and dust in the protoplanetary disk. But many of the details of Earth’s formation are still unclear. The leaking helium is a clue that can help scientists uncover more details.
Helium exists in nine isotopes, but only two are stable: Helium 3 and Helium 4. 4He outnumbers 3He by a million to one in Earth’s atmosphere. 4He comes from the decay of heavy radioactive elements, mainly uranium and thorium. But not 3He. Most 3He dates back billions of years to the Big Bang. Most of the Universe’s 3He was created via nucleosynthesis in the first few minutes after the Big Bang.
Scientists know that Earth’s mantle contains primordial volatiles from the solar nebula. There’s evidence that volatiles are locked even deeper into the Earth, down in the core. But the amount of volatiles like 3He in the core is unknown.
A new study published in AGU’s Geochemistry, Geophysics, Geosystems journal ferreted out some of the details of Earth’s ancient helium. The study is “Primordial Helium-3 Exchange Between Earth’s Core and Mantle.” The lead author is Peter Olson, a geophysicist at the University of New Mexico.
“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.”
Peter Olson, lead author, UNM.
Earth’s primordial helium has a long and interesting history stretching from the Big Bang to our current times. In basic terms, there are three chapters in the 3He’s story: accumulation through in-gassing, loss due to impacts, and long-term loss due to out-gassing.
As Earth formed from the solar nebula, it continuously accumulated helium, and that accumulation is the first chapter. But sometime around 50 million years after Earth formed, there was a calamity. A protoplanet about one-third Earth’s size, named Theia, crashed into the Earth. Molten debris flew into space and rotated around the Earth. That material eventually coalesced into the Moon, while some of it fell back to Earth. But the initial impact between Theia and Earth was cataclysmic, and the intense heat it generated re-melted Earth’s crust. That allowed much of the helium to escape into space, and so did other, less-cataclysmic impacts. The loss of 3He due to impacts is the second chapter.
An artist’s concept of the collision between proto-Earth and Theia, which happened about 4 billion years ago. The heat from the impact melted Earth’s crust, allowing bulk 3He to escape into space. Credit: NASA
3He has leaked continuously from reservoirs in Earth’s interior over billions of years, and that is the third and longest chapter.
The 3He that remains inside Earth is locked in deep reservoirs. But where it is and how much of it there is have been unanswered questions. This study modelled the helium acquired during Earth’s formation and the helium lost due to out-gassing over Earth’s long history. Their goal was to determine how much 3He is escaping and determine where it’s coming from.
“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,” said lead author Olson.
3He is primordial and isn’t replenished inside Earth. When it escapes from inside the Earth, very little of it is recycled back into the mantle. So measuring
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