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.”
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 3He’s surface flux relates directly to how much 3He Earth started with. 3He’s release is directly connected to magmatic and hydrothermal activity tied to plate tectonics. “…much of the observed 3He flux results from upward transport of helium by the general convective circulation of the mantle,” the authors write.
The loss of 3He isn’t in doubt. Neither is the mechanism for that loss. The question is, where does it come from?
Scientists look at the core as a possible source for three reasons. Firstly, the core is immune to impacts. Impacts like the Theia impact melt the planet’s surface and hasten the loss of 3He, but the core is isolated from impacts. Secondly, the core isn’t part of tectonic plate cycling; it’s also mostly isolated from that process. Thirdly, the core has remained mostly liquid, allowing it to hold onto more of its helium.
The presence and behaviour of 3He in Earth’s mantle is reasonably well-understood, although many details are still unclear. But the core is a different matter. “…it seems increasingly likely that 3He could have been deposited in the core,” the authors write. “Key remaining questions concern amounts: How much 3He did the core acquire as it formed, and since that time, how much has it surrendered to the mantle?”
This figure from the study illustrates core-mantle helium exchange processes. (a) 3He acquisition during Earth’s accretion by in-gassing from the nebular atmosphere and transport through the magma ocean to the proto-core, and (b) 3He transport from the core to the mantle and from the mantle to the ocean after accretion. Image Credit: Olson and Sharp 2022.
Impact events, of which the Theia impact is the most significant, played a large role in depleting the mantle’s volatiles. A 2021 study showed that an impactor exceeding about 5% of Earth’s mass would almost completely strip the planet’s nebular atmosphere. The Theia impact would’ve removed nearly all of Earth’s atmospheric 3He. It wouldn’t have happened all at once, either. It was a long-drawn-out process fuelled by the solar wind. “And if that atmosphere removal mechanism consisted of a Parker wind driven by impact heat deposited in the interior, it would likely have continued to expel helium from the Earth system for millions of years following the impact,” the study explains.
This all happened billions of years ago, and there’s significant uncertainty around how the mantle lost its helium. But scientists are confident that the mantle became strongly depleted of 3He. The depletion created an imbalance or disequilibrium in chemical potential across the core-mantle boundary (CMB.) Over geologic timescales, 3He moved from the core back into the mantle.
“Much of the 3He leaving the core is then carried toward the surface within the general convective circulation of the mantle,” the authors explain. In the mantle, the 3He mixes with 4He. Then they’re released into the ocean by Mid-Ocean Ridge Basalt (MORB) formation. The authors say that not all of the 3He from the core escapes. Some of it stays in the mantle until eventually released through another geologic process, the formation of Ocean Island Basalts (OIBs.)
Earth’s primordial 3He resides in the core. Over time, it makes its way into the mantle, mixing with 4He. Then it’s released into the ocean through Mid-Ocean Ridge Basalt (MORB) formation, thanks to plate tectonics. Image Credit: By 37ophiuchi BrucePL – Based on diagram File:Mittelozeanischer Ruecken – Schema.png. “I translated it from German to English and revised the outlines of rock units.” CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=79658206
The details of all the aspects of 3He’s accumulation and escape are beyond the scope of a single study. Instead, the researchers modelled the two most distinct phases in 3He’s history: nebular in-gassing during Earth’s accretion phase and the de-gassing of the same helium. Their modelling starts now and works backward in time. That might sound confusing, but there’s a reason for it.
“The helium exchange processes illustrated in Figures 1a and 1b require separate model treatments for several reasons. First, they apply at different points in Earth’s history, operate on vastly different timescales, and involve different physical and chemical conditions,” the authors explain. The second reason is that modelling the exact nature of ancient impacts (late accretion events) and the resulting 3He loss, though extremely important, is extremely difficult and requires its own focused effort. “In spite of their importance to Earth’s helium budget, modelling these late-accretion events is beyond the scope of this study.”
Instead, the authors modelled how much 3He the Earth accreted during its formation and how much it lost to de-gassing. Those results will provide book-ends for the history of Earth’s 3He, even though the researchers haven’t modelled late accretion impact events specifically. “Comparing the post-accretion mantle abundance with the in-gassed abundance provides an estimate of 3He lost from the mantle during late accretion,” the paper states.
These figures from the research show some of the detail of Earth’s 3He accretion. Left: Red and green lines show Earth’s temperature and pressure respectively. The pink line shows 3He accretion into the proto-core in petagrams, and the proto-mantle in exagrams, for Earth’s first 12 million years. The vertical dashed line shows the end of the solar nebula phase, and the yellow shaded area shows the duration of Earth’s global magma ocean. Right: The blue line shows decreasing 3He accretion to Earth’s mantle over time, and the red line shows the same for the core. The blue shaded area represents the present-day 3He mantle abundance. Image Credit: Olson and Sharp 2022.
These are models, and the authors acknowledge the variables that can change the results. A host of factors influence the in-gassing of 3He, including the sizes of the young core and mantle, their densities, and things like solar luminance and the lifetime of the solar nebula. The researchers set the values for these parameters and others according to previously published research.
Their results show that Earth’s core gained 1 Pg of 3He during accretion, which is one billion metric tons. Some of that leaked into the mantle over time and then was lost.
Then the study turns to de-gassing. De-gassing is governed mainly by mantle convection. The thermal properties of the core, the mantle, and the core-mantle boundary (CMB) play significant roles. As outlined earlier, 3He is released into the ocean via MORBs and OIBs. The helium then enters the atmosphere, where it can escape into space.
This figure from the study shows the de-gassing of Earth’s 3He over time. Note that the horizontal axis is time, but reversed. The far-right is Earth’s beginning, and the left is modern times. The left vertical axis is 3He in teragrams, and the right vertical axis is the Crust/Mantle 3He ratio. The dashed line at 4.4 Ga denotes the nominal onset of de-gassing by mantle convection. 3He loss has remained relatively stable since about 3 billion years ago, and the mantle 3He content is slowly rising. The C/M ratio decreases with age, indicating that more than 90% of the 3He in the present-day mantle was initially deposited in the core and later leaked across the CMB. Image Credit: Olson and Sharp 2022.
These processes took billions of years to unfold. About two kilograms of 3He leak out of the Earth each year. That’s “about enough to fill a balloon the size of your desk,” lead author Olson said. What does this tell us about Earth’s formation?
“There are many more mysteries than certainties.”
We know Earth formed out of the protoplanetary disk, but questions remain about how much of the solar nebular gas was still present when it formed. A petagram, or one billion metric tons, of 3He in the Earth’s core, is an enormous quantity. Its existence points to the fact that Earth formed in the presence of the solar nebula. High concentrations of the 3He gas would have allowed it to build up deep in the planet.
In their paper’s conclusion, the authors acknowledge the variables involved in their models. These include the solar nebula’s lifetime relative to the accretion rate, the atmosphere’s erosion rate, the effects of giant impacts, and many others. But none of the variables can change their main conclusion: Earth’s core is a significant source of primordial helium.
Artist’s impression of the Solar Nebula. Image credit: NASA
Future studies might help strengthen this conclusion. 3He isn’t the only nebular gas, and scientists will undoubtedly look for others, like hydrogen. If they find it leaking in the same locations and at roughly the same rates, that could be the smoking gun.
But for now, even with these models in hand, “There are many more mysteries than certainties,” according to Olson.