New research by UNM scientists provides possible insights into the formation of Earth: UNM Newsroom

A new study conducted by University of New Mexico scientists found ancient, primordial helium-3 emanating from Earth’s core, suggesting the planet formed in a solar nebula, prompting further debate among scientists .

Each year, about 2 kg of the rare isotope gas helium-3 escapes from the Earth’s interior, mainly along the mid-ocean ridge system, a series of underwater volcanoes around the globe. Helium-3 was originally formed shortly after the Big Bang and was extracted from the solar nebula during the formation of the earth. Geochemical evidence suggests that Earth has deep reservoirs of helium-3, but their locations and abundances are uncertain.

The earth’s helium inventory consists of two stable isotopes, the more common helium-4 and the rarer helium-3. In contrast to terrestrial helium-4, which is mainly produced by the decay of uranium and thorium, terrestrial helium-3 is mostly of primeval origin, having been synthesized after the Big Bang and mainly incorporated into the earth during its formation.

Now, scientific models of volatile exchange during Earth’s formation and evolution implicate the metallic core as a leaky reservoir that supplies helium-3 to the rest of the Earth. The results also suggest that other volatiles could be leaking from the core into the mantle. Helium-3 is primarily formed in nebulae, a vast cloud of dust and other basic elements such as hydrogen and other ionized gases. One of the earliest elements produced in the universe, most of 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, that is, within a few seconds after the Big Bang,” said Peter Olson, UNM geophysicist and lead author of the paper. Original helium-3 exchange between the earth’s core and mantle, recently published in the American Geophysical Union journal Geochemistry, Geophysics, Geosystems. “This study helps identify the core as the source of the leak and not the mantle. It’s more than 13 billion years old and is measured to come from the Earth’s interior, and the place where it appears to be escaping fastest is the mid-ocean ridge propagation centers. These are the plate boundaries where new ocean crust is formed.

“Two things are important, even if it’s just a small amount. First, it hasn’t gotten there lately. It’s a primordial element and some of the places where it comes out have to do with the core. For example, the source of the lava that makes up Hawaii and Iceland is thought to be from plumes rising through the mantle from the core-mantle interface. Earth’s helium loss is global. It’s not just in a few places. It is concentrated in distribution centers at the mid-ocean ridges. These centers of propagation are global and cover the entire earth. Helium also leaks from other environments. So it’s global and it’s coming from deep within the earth, and those are two conclusions that I think are really solid.”

The study, which also included Zach 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 first got to the depths of the Earth, the detection process, and second, how it comes out. Previous studies have shown how helium-3 gets in, but none have done both sourcing helium-3 and the process of getting it out. Both are fundamentally different mechanisms and occur on different time scales of the Earth’s history.

“The acquisition process, or the gas that makes up the solar system, is actually the gas that makes up the Sun, Jupiter and Saturn and is about 15 percent helium,” Olson said. “It 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 put lots of helium-3 into Earth is to build Earth while the solar nebula was around it. If the Earth was enveloped in fog gas and the Earth’s surface has melted, the gas can dissolve into the molten Earth as it forms because gases easily dissolve in melts.”

“There are many small comets, or small pebbles, which we call snowballs, within the solar nebula that are slowly falling towards the Sun simply because of the Sun’s gravitational pull,” Sharp said. “It’s a physical certainty – it has to happen. Well, if you have planet bodies that aren’t fully grown yet, and the pebbles are coming toward the sun, then a significant portion of the pebbles will be gravitationally trapped by the growing Earth. With this process, you can make something the size of the Earth in 2 million years, while previous models took more than 10 million years to make a body the size of the Earth.”

Scientists used a model consisting of a nebula atmosphere composed of the same composition as the solar nebula and gassing this material into the melt that provides the environment needed to separate the helium from the mantle and core.

“You find out very quickly that under these conditions the surface would be so hot that it would be a magma ocean, the very environment where you could dissolve helium losses,” Olson said. “That gets the helium into the earth, but not into the core, so you have to dissolve it in the iron that makes up the core. There have been laboratory measurements measuring the solubility of helium in free metals such as molten iron. This allowed us to estimate how much helium was able to dissolve in the core of the Earth when it was formed. This is the modeling process for the first step, which told us that this is how you get one or more petagrams (1,000,000,000,000,000 grams) of helium-3 into the core.”

“It’s very nice where we’re going with this. The question is, “How do we get that much helium in the mantle?” This has always been an issue that has never been fully addressed,” Sharp said. “It was like, yeah, it’s in there, and maybe it came from these late comets or asteroids, but the problem is helium isn’t dense. It wants to “swim” on the surface. It’s like taking a beach ball and trying to push it to the bottom of a swimming pool. It will reappear. How do you get helium down into the deep mantle? It’s really a problem.

“It’s not generally discussed in the case of nebula ingassing, but 15 percent of the nebula is helium. Most of the rest is hydrogen, so there you go, that’s the bulk of the nebula gas. When you have that high pressure, the helium, like the CO2 that dissolves in your water in a can of soda, dissolves all the way to the interior of the planet.”

The second step in this process is tricky because you have to strip the mantle of helium-3 before it exits the core. Numerous studies have suggested that helium was lost from the Earth’s mantle as the Earth solidified after the Giant Impact. The Giant Impact is the supposed formation of the moon during a collision between proto-Earth and a large planet the size of Mars.

“The Giant Impact was such a destructive event that the Earth’s mantle would have lost many of its gases, including its helium three. This is a critical step, otherwise helium will not leak out of the core,” Olson said. “Once those two were in place, the process of leakage we modeled was just ordinary diffusion plus convection in the Earth’s mantle driving plate tectonics. That would transport mantle material down to the core-mantle interface, where it would entrain helium-3 from the core and transport it back to the surface at the ocean ridges and volcanic hotspots and maybe, for example, the Rio Grande Rift here in New Mexico.”

“The amount of helium that escapes is somewhere near four pounds a year, maybe enough to fill 50 balloons, depending on the size of the balloons,” Sharp said. “It’s not much, but the fact that it’s constantly coming out of the earth, with the idea that the core is a major source, it’s all viable.” Nobody cares about a bit of helium leaking from Earth into space, but we think it’s a fingerprint of important early events in our planet’s history. It is evidence that the nebular outgassing idea is valid. If the helium were later supplied by asteroids and comets that slammed into the Earth millions of years after Earth’s formation, we wouldn’t expect to see so much helium in the deep mantle and core. It is essentially a proxy for the delivery of life-giving water to Earth. It provides a mechanism to create a habitable planet.”