Remains of planet that formed the Moon may be hiding near Earth’s core

Seismic waves created by earthquakes as they travel through the planet's interior change speed and direction as they move through different materials. Things like rock type, density, and temperature all alter the travel of these waves, allowing scientists to gradually build up a picture of the Earth's crust and mantle, spotting things like the rise of plumes of hot mantle material, as well as the colder remains of tectonic plates that dropped off the surface of the Earth long ago.

There are some things that show up in these images, however, that aren't easy to explain. Deep in the Earth's mantle there are two regions where seismic waves slow down, termed large low-velocity provinces. This slowdown is consistent with the materials being higher density, so it's not really a surprise that they're sitting near the core. But that doesn't explain why there are two distinct regions of them or why they appear to contain material that has been there since the formation of the Solar System.

Now, a team of scientists has tied the two regions' existence back to a catastrophic event that happened early in our Solar System's history: a giant collision with a Mars-sized planet that ultimately created our Moon.

Hard to explain

A number of explanations for these large, low-velocity provinces have been offered, but none of them are entirely satisfactory. One idea is that they're leftovers from the process by which the Earth's interior separated into its crust-mantle-core structure. But that material should have been thoroughly churned up when a Mars-sized object, which has picked up the name Theia, smashed into the early Earth, leaving enough debris in orbit to form the Moon.

Other suggestions include the idea that these might be the remains of tectonic plates that sank to unusual depths in the mantle. But this doesn't account for what this material looks like when mantle plumes bring some of it to the surface via volcanism. When sampled, the ratios of isotopes in gasses trapped in this material look like those that were expected to be present in the early years of the Solar System, and not like those found in the crust today.

The team behind the new paper suggests that a completely different source could explain the odd properties of these large low-velocity provinces. Relative to the Earth, the Moon has a lot more iron oxide, which suggests that Theia also had a lot of this material. Since iron oxide is more dense than a lot of other mantle material, it could explain that property of the large low-velocity provinces. In addition, the collision would have taken place early in the Solar System's history, which could explain why the isotope ratios look primordial.

The big problem with this idea is that the material from Theia would also have been churned up in the wake of the collision, so it's hard to understand how it could form discrete layers inside the Earth. So, the researchers modeled the Earth's interior, both during and after the collision, to better understand how things might work.

Model after model

There have been many models of the giant collision between the early Earth and Theia—it's how we know it could have pushed enough material into orbit to form the Moon. But the team here took advantage of advances in computing power to perform these simulations at a higher resolution than those done previously and focused on the interior of the Earth. (They also used two different modeling methods to ensure any results weren't specific to one approach or another.)

These models showed that the post-collision melting of the Earth would extend over halfway through the mantle, but there would be a fraction of the deep mantle that remains solid. Above the melt line, any material from Theia would end up thoroughly mixed with bits of Earth. But fragments of Theia would also end up injected below the melt line and, therefore, remain largely intact. These wouldn't be large pieces, and they would be scattered throughout the Earth's interior, but they would maintain Theia-like properties despite residing in a different planet. Collectively, about 1 percent of the Earth's mass would be derived from these deep Theia fragments.

Obviously, this doesn't look like the two large, low-velocity provinces we see today. But the Earth's interior has had over 4 billion years to evolve since then. To see what would happen with Theia fragments over that time, the researchers turned to yet another model of the viscous solid found in the Earth's mantle.

By testing different conditions, the researchers showed that these fragments would sink close to the core as long as their density was at least 2.5 percent higher than Earth's (1.25 percent more dense didn't work). In addition, the chunks needed to be above a size threshold—chunks that were 25 km across didn't sink, 50 km across did.

The other clear outcome of this model is that the material sinks quickly. That's important, because convection within the mantle would ultimately keep these chunks from pooling up. But convection would start in the upper mantle and gradually work its way down toward the core. The quick sinking allows the pieces of Theia to stay below where convection is happening, creating large pools of this material at the core-mantle boundary in advance of convection reaching those depths.

What stands out from the models is that most of their results would seem to apply if there were regions of high-density material in the pre-collision Earth. Chunks of those could have been injected into the deep mantle during the collision, and they would be difficult to distinguish from chunks of Theia. In any case, the idea is provocative enough that it's likely to send a number of other planetary scientists back to their models to see if they produce anything similar.

Nature, 2023. DOI: 10.1038/s41586-023-06589-1  (About DOIs).