Deep traces of the primeval mega-collision? Known density anomalies in the Earth's interior could be remnants of the celestial body that formed the moon after impacting the primordial Earth. This emerges from new simulations of the dynamics and mixing processes in the suspected collision around 4.5 billion years ago. The scientists say the results shed light on the causes of the Earth's internal structures as well as the early development processes in the solar system.
How did our home planet's companion form? This question has puzzled scientists for generations. A bombastic origin story has now become established as the prevailing theory: According to this, the moon was formed from debris that was formed around 4.5 billion years ago when the primordial Earth “Gaia” collided with a Mars-sized protoplanet called “Theia”. The assumptions are based on model simulations of the processes that could have occurred during the gigantic collision and in its aftermath. However, there are still some unclear aspects of this impact theory and remains of Theia are not known. So far, previous research has mostly focused on the formation of the debris disk - the precursor material of the moon. However, the effects of the giant collision on the early Earth have received less attention.
How did the collision shape the Earth?
An international team led by Qian Yuan from Arizona State University in Tempe has now devoted themselves to this topic. The scientists investigated the question of how the collision could have led to certain distributions of the material from both partners in the newly formed Earth. Specifically, their focus was on well-known density anomalies in the Earth's interior, the cause of which is currently considered mysterious. These are the two so-called Large Low Velocity Provinces (LLVPs), which extend over thousands of kilometers at the base of the Earth's mantle. One is under the African tectonic plate and the other under the Pacific tectonic plate. Previous seismic studies have shown that these are accumulations of material that are noticeably denser than the surrounding rock.
The scientists now investigated the assumption that the LLVPs could be remnants of material from the impact celestial body Theia. To do this, they developed complex model simulations that show dynamic processes after the impact. The aim was to make it understandable what could have happened to certain components of the collision partners. The researchers implemented evidence that the mantle material of Theia was slightly richer in iron and therefore heavier than that of the ancient Earth Gaia.
Sunken remnants of Theia
As the team reports, their model simulations now show that a significant amount of Theia material could actually have reached the lower mantle of Gaia. Specifically, it appears that the collision resulted in many molten Theia clumps with a diameter of several dozen kilometers. Due to their comparatively high density, they then slowly sank into the solid mantle made of Gaia material. In the end, they united in the area of the transition zone to the Earth's core and thereby formed the long-range LLVPs, the results of the geodynamic model simulations suggest.
These structures were largely spared from further geological developments on Earth and have therefore survived to this day, the scientists explain. In other words: The previously mysterious LLVPs could be remnants of Theia and thus concrete traces of the massive collision that led to the formation of our moon.
The results shed light on the evolutionary history of our home system - and beyond, the team concludes: "Since giant impacts occur frequently in the final stages of planet formation, similar mantle heterogeneities caused by collisions could also have arisen in the interior of other planetary bodies," write Yuan and his Colleagues.
Source: Chinese Academy of Sciences, specialist article: Nature, doi: 10.1038/s41586-023-06589-1