Earth’s past collision with Theia

The Earth’s shape has evolved quite dramatically ever since it was formed. Initially, the Earth was a tiny protoplanet without a moon around 4.5 billion years ago. That proto-Earth (called Gaia) clashed severely with another early planet, Theia, while tracing its orbit around the sun. Due to Gaia’s large size, it dominated the collision and began to form the shape of Earth as we know it today. Meanwhile, much of the debris from the collision consolidated and produced the Moon.

Though this has been the dominant idea for the Moon’s formation, no evidence of Theia has ever been discovered in surrounding asteroids or meteorites, leading astronomers to suspect the planet’s leftover components may have been incorporated into early Earth. A multinational team of scientists claims to have discovered evidence that substantial portions of Theia were left behind, forming a part of the Earth’s core and makeup. The team’s results were published in Naturel on the 1st of November, 2023.

The prevalent explanation for how and why the Moon originated early in our planet’s history is a huge collision with a Mars-sized planetary body, but tangible evidence for that impact elsewhere in the Solar System has proven difficult to uncover. However, there were traces left behind on Earth. Seismic measurements reveal two massive clusters known as large low-velocity provinces (LLVPs) deep beneath the Earth’s crust; one beneath the African continent and one beneath the Pacific Ocean. These LLVPs differ in chemical composition from the mantle around it; it has a higher iron content. 

The Moon also has a higher concentration of iron. Because of the similarities in patterns seen between these massive subsurface chunks of material and the Moon, experts hypothesised that both may have resulted from the same giant collision with Theia. The study team ran simulations of what we know about the collision to see if both the LLVPs and the Moon might have come from the Theia impact. The simulations discovered that it is plausible and that it implies that the Earth’s core was significantly hotter at that time in its history than previously estimated. Otherwise, the shards of Theia would have likely blended more evenly into the mantle.

The possibility that Theia had such a big impact on the formation of the Earth inspires new research into the Earth’s early history.

“A logical consequence of the idea that the LLVPs are remnants of Theia is that they are very ancient,” said Paul Asimow, a Caltech researcher and author of the study, in a release. “It makes sense, therefore, to investigate next what consequences they had for Earth’s earliest evolution, such as the onset of subduction before conditions were suitable for modern-style plate tectonics, the formation of the first continents, and the origin of the very oldest surviving terrestrial minerals.”

Image description: Artistic impression of Gaia’s collision with Theia

Image credit: Dana Berry via National Geographic