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A huge celestial body may have hit the earth, and the moon may have been born from this impact. (photo source: NASA / JPL-Caltech) 0.000137% of all helium on earth is of particular concern to earth scientists because they may be related to the origin of the earth.

Residents are planning a big celebration in the small town of Dexter, Kansas, in May 1903. Not long ago, abundant underground gas was discovered here, and people took it for granted that it contained natural gas, and expected that natural gas resources would give them a good life.

According to the plan, the highlight of the celebration will be the ignition test of the gas well. However, when the mayor finished his speech and threw the lighted hay into the outlet, there was no fire. The mayor repeated it several times, and the result was the same. The residents of the town had to accept the "disappointing" result. The unignited gases were then sent to the University of Kansas for identification, where chemists at the University of Kansas found an unknown ingredient.

This unknown component is helium. Town residents would not have thought that helium, which they "hated" at the time, has now become a more valuable resource than natural gas. But for earth scientists, 0.000137% of helium on earth is of particular concern-an isotope of helium, helium-3.

Balloons filled with helium can float and are not flammable, so they are widely welcomed. But some researchers point out that this is a huge waste of resources (photo: pixabay) every year, about 2 kilograms of helium-3 leaks from the depths of the earth, along mid-ocean ridges and mantle hot spots (hot spot) to the earth's surface. The problem is that we still don't know whether the helium-3 comes from the mantle or the core, or how much helium-3 is still stored in the depths of the earth, and these problems may be related to the origin of the earth.

Shortly after the big bang about 13.8 billion years ago, hydrogen and helium, the lightest in the periodic table, began to appear, with helium made up of two stable isotopes: mostly helium-4 and a small amount of helium-3. Billions of years later, when the universe cools to a certain extent, these "primitive" helium collapses with huge clouds of gas and dust, forming a fiery core in the center-- and that's how our sun was born. Material that does not enter the core of a star may become planets, asteroids and other debris-including our Earth.

The center of the Hubble Lagoon Nebula (photo: NASA, ESA) these helium from the beginning of the universe are also sealed in the Earth's interior. Although the helium-4 produced by the decay of uranium and thorium dilutes the original helium-4, most of the helium-3 on Earth still comes from the solar nebula, thus becoming the first-hand record of the birth and evolution of the Earth.

Geologists already know that there is a lot of helium-3 in the depths of the mantle today. However, in the history of the Earth, helium-3 did not stay in the interior of the Earth obediently, and it left in two main ways: one is a huge impact event, such as the Theia protoplanet impact that may have "hit the moon" (it is speculated that the Theia protoplanet is more than half the size of Mars It may even be close to Mars-obviously the kind of asteroid at the end of the Cretaceous is no longer qualified) The other is slower, such as mid-ocean ridges, mantle hot spots and other tectonic movements that bring the deep mantle to the surface.

The problem is that no matter which way you leave, once on the surface, most of the helium will go directly into the earth's atmosphere and easily leave the earth's system, and very little helium can be recycled back to the mantle. So, if the helium-3 in the deep mantle has been "giving" and never getting "in return" over the past billions of years, where does it come from?

Some researchers have suggested that large amounts of helium-3 are stored in the core and are continuously supplied to the deep mantle. Peter Olson and Zakary Zachary Sharp, earth scientists at the University of New Mexico, are proponents of this view, arguing that the core is like a safe: "it is less vulnerable to large-scale impact events than the rest of the earth's system and is not involved in plate tectonics on the earth's surface." Under this assumption, quantity becomes a key question: how much helium-3 was obtained during the formation of the core and how much helium-3 was transported to the mantle?

The core is a "safe" some time ago, Olson and Sharp built a model to try to explain the process in more detail. The study is published in Geochemistry, Geophysics, Earth Systems (Geochemistry, Geophysics, Geosystems). In their model, they mainly focus on the changes of helium-3 in two important stages: the process of obtaining helium-3 in the early accretion process of the earth, and the process of losing helium-3 later.

The process of obtaining helium-3 in the early accretion of the earth (left) and the subsequent loss of helium-3 (right) (image source: original paper) according to the current theory of the formation of terrestrial planets, the surface was still a hot sea of magma at the time of Earth's formation, and if the original Earth grew to enough mass, it could attract an atmosphere rich in hydrogen and helium. In such an environment, the surface of the magmatic sea can dissolve helium in the high-pressure atmosphere. Calculations show that some of the helium may be stored in the mantle, and a small amount of helium may eventually enter the procore.

Olson and Sharp's model speculates that an impact about 4.5 billion years ago released helium-3murin from the mantle-the Theia protoplanet impact, and that the debris thrown by the impact may have formed the embryonic form of the moon. However, the protoplanet is not enough to "leak" the core: the impact only melts the earth's crust and escapes most of the helium from the mantle. At this time, the core is like a safe, preserving the earth's last reserves of helium-3.

However, although the impact did not directly "leak" the core, it may still lead to the leakage of helium-3 in the core: if a large amount of helium in the early mantle is consumed, there will be a significant imbalance in helium concentration across the core-mantle boundary; in the following Earth's history, helium-3 in the core will have to leak into the mantle and then come to the surface with the mantle convection.

A schematic diagram of the earth's classical circle structure, the real structure of the earth may be much more complex (image source: Kelvinsong / wikipedia) what Olson and Sharp want to do is to use the current helium-3 leakage rate and the possible helium-3 concentration in the deep mantle to reverse how much helium-3 is transported from the core to the mantle, so as to know how much helium-3 entered the core in the earth's infancy. Calculations show that in the early days of the formation of the earth, the pronucleus contained 10 billion to 900 billion kilograms of helium-3. Such a large number suggests that the Earth was formed inside a booming solar nebula, when there is a high density of gas in the solar nebula.

Limited to the model, we have to admit that, like most model studies, this conclusion is still lack of valid evidence. The results of the model operation are based on various assumptions-for example, the Earth obtained helium-3 during accretion, helium entered the procore, and so on. "these assumptions, as well as other uncertainties, including how long the solar nebula forms relative to the Earth, mean that helium-3 in the core may be lower than the calculated results." This is what the paper says.

But Olson and Sharp still hope to find more clues to support their point of view. For example, hydrogen produced at the same time as helium during the Big Bang might be a good entry point. "if gases from other solar nebulae, such as hydrogen, can be found to leak from similar locations at a rate similar to helium-3, it could be 'conclusive evidence'."

It didn't leak completely, but it did.

Reference link:

Https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GC009985

Https://phys.org/news/2022-03-ancient-helium-leaking-core-clues.html

Https://www.acs.org/content/acs/en/education/whatischemistry/landmarks/heliumnaturalgas.html

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by: Erqi, revision: Wang Yu

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