Have you ever looked up at Earth’s moon and wondered where it came from? Most scientists agree that 4.5 billion years ago a small planet called Theia collided with a young Earth. Most of the debris from this giant impact coalesced and became the moon we see today. But where did the rest of Theia go? Qian Yuan and colleagues hypothesized that Earth absorbed parts of Theia when they collided and that these Theian remains can still be found deep within the Earth.
Scientists have found that mechanical energy waves, called seismic waves, slow down as they pass through specific zones inside Earth, called large low-velocity provinces or LLVPs. Previous researchers suggested these zones are graveyards of ancient ocean floor that sunk into Earth’s interior during plate tectonics. But Yuan’s research team proposed these zones could be the remains of Theia instead.
The team explained that LLVPs contain gases like hydrogen, carbon dioxide, and nitrogen, termed volatiles. Volatiles are most commonly present during the formation of solar systems and planets, as material floating around in space begins to accumulate. Volatiles are light, so they escape into space when given the opportunity. When Earth and Theia collided, volatiles from both planets were caught in the collision. Scientists think most of the volatiles escaped into space, but Yuan and colleagues suggested parts of Theia could have captured these volatiles and sunk deep into the Earth, forming LLVPs.
To test whether LLVPs could be the remains of Theia, the researchers used computer models designed to test how different types of solids interact, called thermal evolution models. They explained that other researchers had shown Theia was made of material much denser than Earth, so they wanted to test whether the denser Theian material would completely mix with Earth or remain separate.
The researchers used the thermal evolution models to scatter blobs of Theia-like material randomly throughout a block of Earth-like material and compute how well they mixed. They ran 8 models with blobs of different sizes, densities, and temperatures. In almost all of these models, they found the Theian material sank deep into the Earth and coalesced to form LLVP-like piles without mixing with the Earth material.
The researchers further explored how Theia interacted with a young Earth by running 7 giant-impact simulations. They used these simulations to slam Theia into Earth and calculate how the impact affected Earth’s interior. In these simulations, they found that after the impact with Theia, the inside of Earth developed into layers as denser and more solid material sank towards the center, while less dense material stayed towards the surface. The researchers explained that these simulations also suggested the dense Theian material would have sunk deep into the Earth without mixing.
The researchers concluded that the Theian remains could have sunk into Earth and coalesced into a region similar to the LLVPs, where they remained for the ensuing 4.5 billion years. They further proposed that if material from Theia was preserved within the Earth over billions of years, then the composition of Earth’s interior could be variable.
They suggested future researchers test their hypothesis by comparing the composition of the LLVPs to basalts found on the moon to see if they match. They also suggested researchers use newer and more accurate models of Earth’s thermal evolution to further investigate how Theia’s impact with Earth affected the evolution of the inside of Earth and the creation of the LLVPs.
