The moon could have formed within hours after the gigantic impact between Earth and a Mars-sized protoplanet early in the solar system’s history. The theory arises from new simulations that take into account how the internal temperature of young protoplanets influenced their geological properties and, therefore, the collisions they suffered. This giant impact scenario has become the leading theory for how the Moon formed, thanks in particular to work done by planetary scientist Robin Canup. Since 2001, he has performed and refined numerous computer simulations describing how a Mars-sized body called Theia collided at a perfect angle to launch debris into orbit around proto-Earth. Now other researchers are getting in on the action. Adeene Denton of the South-west Research Institute led a team that performed some of the most detailed simulations of the collision yet, taking into account more geological properties of the colliding worlds. “The models have evolved to include material strength, something that is really important when studying collisions between smaller bodies like asteroids or for my previous paper on the formation of the Pluto-Charon system,” Denton said in a statement. “We weren’t sure whether it would matter for the Moon or not. When we did the simulations, we found that it actually matters quite a bit.” It turns out that hotter bodies are weaker than colder ones. After the planets formed, they were still hot inside from their formation, which would have affected their material strength. Since the moon-forming impact occurred shortly after the formation of the solar system, this would have affected the collision with Theia, but to what extent depends on exactly when the collision occurred and how much Theia had cooled. You may like When Theia crashed into Earth, it was completely destroyed. While much of what remained of Theia’s shattered iron core sank to Earth, a large ring of debris coiled around Earth to form the moon. The main effect of the temperature being warmer in the outer few hundred miles of Theia was that it would have affected its ability to deform upon impact and absorb the momentum of the collision. This, in turn, would have affected how debris spread around Earth to form the Moon. Therefore, by including different temperatures and material strengths in the simulations, Denton’s team was able to better observe how the collision and the resulting formation of our moon unfolded. Previous simulations have shown two possible scenarios. One is that the ring of debris remained there for a considerable period of time, allowing the moon to gradually accrete. The other scenario, first described in NASA-led simulations in 2022, implies that the Moon formed from this debris in a matter of hours. Now, Denton’s models potentially reinforce this surprising possibility. If Theia were colder, meaning the impact occurred a little later in the history of the solar system, perhaps 100 to 150 million years after the birth of the planets, then the ring of debris would have formed in such a way as to allow the Moon to gradually accrete. And because it was stronger, more of Theia would have survived intact to merge with Earth. On the other hand, a warmer Theia impacting Earth less than 60 million years after the planets formed would create a scenario in which the Moon could form very quickly, while Theia itself would be destroyed. “Depending on how hot Earth and Theia are before the collision, the impact can destroy Theia and produce this huge disk of debris that eventually forms the moon,” Denton said. “But when I used the same parameters as the original impact model, down to equal temperature structures inside both bodies, in about five hours, an intact moon emerged.” That would have been a great day in the history of our planet, acquiring a new moon and an almighty headache at the same time. “These surprising and exciting new results imply a potential connection between the physical properties of the current moon, including perhaps its volatile content, and the thermal state of Earth and Theia at the time of the giant impact,” Southwest said. Robin Canup of the Research Institute, who was not involved in this study. “This, in turn, could help scientists better determine when the moon-forming event occurred.” In both scenarios, the moon appears to be made up primarily of material from Theia’s mantle, with just a little bit of Earth’s mantle mixed in. This latest finding is surprising, given the similarities between the composition of the moon and Earth’s mantle. However, research has shown that there is clearly more to the story because despite the similarities, there are also some puzzling differences in isotope levels that cannot be easily explained by current simulations, including Denton’s. The simulations could have implications for the search for exomoons. If the Moon formed in a matter of hours, then the search for disks that form exomoons around terrestrial exoplanets could be a fruitless endeavor because such disks would be very short-lived. However, moon-forming disks around gas giant exoplanets would be a different matter, because those disks form not from impacts, but from material left over from the assembly of those planets. The results were published September 1 in The Astrophysical Journal Letters.
