There is something ancient and primordial in the core of Earth. Helium 3 ( 3 He) 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?
As it slowly escapes, scientists can measure it.
The Sun was born in a cloud of gas about five billion years ago. The solar wind has dispersed most of the gas into space.
The material from the solar nebula became trapped inside the Earth. New research shows that the primordial helium is leaking. How it leaks and how quickly it leaks are clues to the formation and evolution of Earth and other planets.
We know how the Earth was formed. The gas and dust in the disk formed it. Many of the details of Earth's formation are not known. Scientists can use the leaking helium as a clue.
Only two of the nine isotopes are stable: Helium 3 and Helium 4. He outnumbers 3 by a million to one in Earth's atmosphere. He comes from the decay of heavy radioactive elements. Not 3He. The Big Bang happened billions of years ago. He was created in the first few minutes after the Big bang.
Scientists know that the Earth's mantle contains primordial volatiles. There is evidence that volatiles are locked down in the core. The amount of volatiles 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 about the exchange of helium between Earth's core and mantle.
“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 all the way to our current times. There are three chapters in the 3 He's story: accumulation through in-gassing, loss due to impacts, and long-term loss due to out-gassing.
The first chapter is about the accumulating of helium as Earth formed from the solar nebula. Around 50 million years after Earth formed there was a calamity. Theia was a planet about one-third the size of the Earth. The debris flew around the Earth. Some of the material fell back to Earth. The impact between Theia and Earth was so big that it melted the Earth's crust. That made it possible for a lot of the helium to escape into space. The second chapter deals with the loss of 3.
He has been leaking from the interior of Earth for billions of years.
There are three He that remain inside Earth. There are unanswered questions about where it is and how much it is. The study looked at the helium lost due to out-gassing and the helium acquired during Earth's formation. They wanted to determine how much he is escaping and where it is coming from.
It is a wonder of nature, and a clue for the history of the Earth, that there is still a significant amount of this isotope in the interior of the Earth.
He isn't replenishible inside Earth. Most of it is recycled back into the mantle when it escapes from the Earth. How much 3 He Earth started with is related to the surface flux. 3 He's release is connected to plate tectonics and magmatic activity.
He isn't in doubt about the loss of 3. The mechanism for that loss is not known. Where does it come from?
There are three reasons scientists look at the core as a possible source. The core is impervious to impacts. The core of the planet is isolated from impacts, which speed the loss of 3 He, but melt the planet's surface. The core is mostly isolated from the process of plate cycling. The core has remained mostly liquid, allowing it to hold onto more of its helium.
Many details are still unclear, but the presence and behavior of 3 He in Earth's mantle is well-known. The authors think that the core is a different matter.
The Theia impact was the most significant impact and it played a large role in diminishing the mantle's volatiles. A study shows that an impactor greater than 5% of Earth's mass would strip the planet of its atmosphere. The Theia impact would have removed most of Earth's atmosphere. It wouldn't have happened at once. It was a long-drawn-out process that was powered by the solar wind.
There is a lot of uncertainty around how the mantle lost its helium. Scientists think that the mantle became very weak. The chemical potential across the core-mantle boundary was affected by the depletion. He moved from the core back into the mantle.
The authors explain that he leaves the core and is carried to the surface of the mantle. The 3 He mixes with 4 He are in the mantle. They are released into the ocean by a formation called the Mid-Ocean Ridge Basalt. Not all of the 3 He from the core escapes according to the authors. It stays in the mantle until the formation of Ocean Island Basalts.
 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 outlines of rock units, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=79658206)
The details of 3 He's escape and accumulation are beyond the scope of a single study. The two most distinct phases in the history of 3He are the in-gassing of the same helium and the accretion phase. Their modelling works in time. That might sound confusing, but there is a reason for it.
There are several reasons why separate model treatments are required for the helium exchange processes illustrated in Figures 1a and 1b. They apply at different points in Earth's history, operate on vastly different timescales, and involve different physical and chemical conditions. Modelling the exact nature of ancient impacts and the resulting 3 He loss is extremely difficult and requires its own focused effort.
The authors modeled how much the Earth lost to de-gassing and how much it gained. The results will give book-ends for the history of Earth, even though the researchers haven't modelled late accretion impact events specifically.
The variables that can change the results are acknowledged by the authors. The in-gassing of 3 He is influenced by a number of factors, including the size of the young core and mantle, their densities, and the lifetime of the solar nebula. The values for these parameters were set by the researchers.
Earth's core gained 3 Pg during accretion, which is one billion metric tons. Some of that was lost after it leaked into the mantle.
The study goes on to de-gassing. The de-gassing is governed by the mantle. The thermal properties of the core, mantle, and core-mantle boundary are important. 3 He is released into the ocean via a combination of MORBs and OIBs. The helium can escape into space when it enters the atmosphere.
Billions of years have passed since these processes began. Each year, the Earth leaks out of it's shell. About enough to fill a balloon the size of your desk, according to the lead author. What does this tell us about the formation of Earth?
“There are many more mysteries than certainties.”
Peter Olson, lead author, UNM.
Questions remain about how much of the solar nebular gas was present when Earth formed. An enormous quantity is a petagram, or one billion metric tons, of 3 He in the Earth's core. The existence of it points to the fact that Earth was formed in the presence of the solar nebula. It would have been possible for the 3 He gas to build up deep in the planet.
The authors acknowledge the variables involved in their models in their paper. The effects of giant impacts, the atmosphere's erosion rate, and the lifetime of the solar nebula are included. The main conclusion is that Earth's core is a significant source of primordial helium.
Future studies could help strengthen the conclusion. He isn't the only gas like hydrogen that scientists will look for. It could be the smoking gun if they find it leaking in the same locations and at the same rates.
Even with these models in hand, there are many more mysteries.
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