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A new study by scientists at the University of New Mexico found that ancient, primary helium-3 leaked from the Earth’s core, suggesting the planet formed in a solar nebula, sparking further debate among scientists.


Every year, about 2 kg of the rare isotope gas helium-3 comes out of the Earth’s interior, mostly along the system of the mid-ocean ridge, a number of underwater volcanoes around the world. Helium-3 is primary, created shortly after the Big Bang and acquired by the solar nebula during the formation of the Earth. Geochemical data show that the Earth has deep reservoirs of helium-3, but their location and abundance are uncertain.

Earth’s helium inventory consists of two stable isotopes, the more common helium-4 and the rarer helium-3. Unlike terrestrial helium-4, which is produced mainly by the decay of uranium and thorium, terrestrial helium-3 is largely of primary origin, synthesized after the Big Bang and incorporated into the Earth mainly during its formation.

Scientific models of volatile exchange during the formation and evolution of the Earth now suggest the metal core as a permeable reservoir that supplies the rest of the Earth with helium-3. The results also suggest that other volatile substances may leak from the core into the mantle. Helium-3 originates mainly from nebulae, a huge cloud of dust and other basic elements such as hydrogen and other ionized gases. As one of the earliest elements produced in the universe, most helium-3 was created during the early stages of the Big Bang.

“Helium-3 was synthesized very early in the history of the universe, very early, which means within seconds of the Big Bang,” said Peter Olson, a UNM geophysicist and lead author of “In addition to helium-3 exchange between the core and mantle.” on Earth “, published recently in the Journal of the American Geophysical Union Geochemistry, geophysics, geosystems. “This study helps identify the nucleus as a source of leakage, not the mantle. It is 13 billion years old and is measured to emerge from the Earth’s interior, and the place where it apparently flows at the fastest speed is the Mid-Ocean Ridge Distribution Centers. These are the boundaries of the plates where a new oceanic crust forms.

“Two things are important, although few. First, it hasn’t gotten there recently. This is a primary element and some of the places from which it flows are connected to the core. For example, the source It is believed that the lava that makes up Hawaii and Iceland is derived from jets that rise through the mantle from the border region of the core and mantle. The loss of helium from the earth is global. It’s not just in a few places. It is concentrated in distribution centers in the mid-ocean ridges. These distribution centers are global and cover the entire Earth. Helium has been found to leak from other media. So, it’s global and it comes from deep down, and those are two conclusions that are really solid, I think. “

The study, which included Zack Sharp, a UNM geochemist in the Department of Earth and Planetary Sciences, included two aspects as part of the modeling process – first, how helium-3 got into deep earth as a start, the acquisition process, and second, how comes out. Previous studies have shown how helium-3 gets inside, but no one has done both, acquiring helium-3 and the process of removing it. Both are fundamentally different mechanisms and appear on different time scales in earth’s history.

“The process of acquisition, or the gas that makes up the solar system, is actually the gas that makes up the Sun, Jupiter and Saturn, and it’s about 15 percent helium,” Olson said. “This is the second most abundant element in these bodies (after hydrogen), making it the second most abundant element in the solar system. The obvious way to incorporate a lot of helium-3 into the earth is to build up the Earth while the solar nebula was in place around it. When the Earth has been shrouded in nebulae and if the earth’s surface is melted, then the gas can dissolve in the molten Earth as it forms, as the gases dissolve easily in the melt. “

“There are a lot of small comets or small pebbles in the solar nebula, which we call snowballs, that will fall slowly towards the sun simply because of the gravity of the Sun’s attraction,” Sharpe said. “It’s physical security – it has to happen. Now, if you have planetary bodies that are not yet fully grown, and you have pebbles coming to the sun, then a significant portion of the “pebbles” will be gravitationally captured by the growing Earth. You can do in 2 million years, something the size of the Earth through this process, while previous models required more than 10 million years to make a body the size of the Earth. “

Scientists have used a model that consists of an atmosphere nebula made of the same composition as the solar nebula, and the absorption of this material into the molten, which provides the medium needed to separate helium from the mantle and core.

“You discover very quickly that the surface will be so hot under these conditions that it will be an ocean of magma, just an environment in which you can dissolve the loss of helium,” Olson said. “This puts helium in the ground, but not in the nucleus, so you have to dissolve it in the iron that makes up the nucleus. There are laboratory measurements that measure the solubility of helium in free metals, such as molten iron. This gave us an estimate of how much helium you could dissolve in the nucleus during the formation of the Earth. This is the modeling process for the first step, which told us that you get one or more petagrams (1,000,000,000,000,000 grams) of helium-3 in the nucleus this way. “

“It’s very nice where we’re going with this. The question is, ‘how do we get so much helium into the mantle?’ “It’s always been a problem that has never been completely solved,” Sharpe said. He wants to “float” on the surface. It’s like taking a beach ball and trying to push it to the bottom of a swimming pool. It will pop up again. How can you get helium all the way to the deep mantle? This is really a problem.

“The idea of ​​nebulous ingestion is not usually discussed, but 15 percent of the nebula is helium. Most of the rest is hydrogen, so here’s most of the nebular gas. If you have this high pressure, just like CO2 Dissolving in your water in a can of soda, helium will dissolve all the way to the interior of the planet. “

The second step in the process is difficult because you have to deplete the helium-3 mantle before it starts leaking out of the nucleus. Numerous studies suggest that helium was lost from the mantle when the Earth solidified after the “giant shock”. The giant impact is supposed to form the Moon during a collision between Proto-Earth and a large planet equivalent in size to Mars.

“The giant impact was such a devastating event that the Earth’s mantle would lose many of its gases, including helium three. This is a critical step, because otherwise the nucleus will not release helium, “Olson said. “Once these two were in place, the leakage process we modeled was just simple diffusion plus convection in the earth’s mantle that drives plate tectonics. This will transport the mantle material down to the core and mantle boundary, where it will carry helium-3 from the core and transport it back to the surface in ocean ridges and volcanic hotspots and perhaps the Rio Grande fault here in New Mexico, for example. “

“The amount of helium leaking is somewhere around four pounds a year, maybe enough to fill 50 balloons depending on the size of the balloons,” Sharpe said. “It’s not much, but the fact that it keeps coming out of the ground all the time with the idea that the nucleus is an important source is viable. No one is interested in the release of some helium from Earth into space, but we believe that this is a fingerprint of important early events in the history of our planet. This is proof that the idea of ​​gassing the nebula is valid. If helium was later delivered by asteroids and comets that collided with Earth millions of years after Earth formed, we wouldn’t expect to see so much helium in the deep mantle and core. It is essentially a substitute for the supply of life-giving water to Earth. It provides a mechanism for creating a habitable planet. ”


Ancient helium, leaking from the nucleus, offers clues to the formation of the Earth


More information:
Peter L. Olson et al., Primary helium-3 exchange between the Earth’s core and mantle, Geochemistry, geophysics, geosystems (2022). DOI: 10.1029 / 2021GC009985

Provided by the University of New Mexico

Quote: New study provides possible information on the formation of the Earth (2022, May 7), extracted on May 7, 2022 from

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https://phys.org/news/2022-05-insights-formation-earth.html

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