The Moon formed in a catastrophic impact between the proto-Earth (Gaia) and a Mars-sized object we call Theia. The collision happened around 4.5 billion years ago but its effects are still with us. And not just on the Moon, but also in structures buried deep in the Earth’s mantle.

The traditional scenario sees Gaia being affected, but not dramatically, by the impact, while most of Theia was thrown into space around the soon-to-be Earth, eventually coalescing into the Moon. If this were the case, the Earth and the Moon would have different chemical compositions. But this is not the case. Now, new simulations suggest how Theia affected our planet, with peculiar features that survive to this day.

The simulation uses a technique called Meshless Finite Mass (MFM). MFM is very good at accurately modeling turbulence and material-mixing. Applied to the Theia collision, it reveals that the mixing between the primordial worlds might have been more extensive than previously thought.

“Previous research had placed excessive emphasis on the structure of the debris disk (the precursor to the Moon) and had overlooked the impact of the giant collision on the early Earth,” Professor Deng Hongping of the Shanghai Astronomical Observatory said in a statement.

“Our findings challenge the traditional notion that the giant impact led to the homogenization of the early Earth,” said Deng. “Instead, the Moon-forming giant impact appears to be the origin of the early mantle’s heterogeneity and marks the starting point for the Earth’s geological evolution over the course of 4.5 billion years.”

The simulations show that the upper and lower mantles have different compositions and states after the collision. The lower mantle is mostly solid and with little contamination from Theia; roughly two percent of material from the colliding object penetrated that deep. The upper mantle, however, is a mix between the Gaian and Theian material.

The Theian material in the lower mantle could be responsible for the formation of the Large Low-Velocity Provinces (LLVPs), the anomalous structures in the mantle that sit beneath the African Tectonic plate and the Pacific tectonic plate, respectively. The material from Theia might have sunk to the bottom because it is richer in iron, so heavier than the surrounding material.

This is not the first time researchers have proposed this scenario to explain the LLVPs. The fact that different simulations and scenarios are converging on the idea is very intriguing, but more data is needed. 

Material from the deepest mantle and even the core can be brought to the surface by plumes and studying that material could provide experimental evidence for the scenario the simulation suggests, with implications for not only Earth, but also for how other rocky planets form. 

“Through precise analysis of a wider range of rock samples, combined with more refined giant impact models and Earth evolution models, we can infer the material composition and orbital dynamics of the primordial Earth, Gaia, and Theia. This allows us to constrain the entire history of the formation of the inner Solar System,” lead author Dr Qian Yuan, from the California Institute of Technology, explained.

“This research even provides inspiration for understanding the formation and habitability of exoplanets beyond our Solar System,” Deng added.

The study is published in the journal Nature.