New Model Suggests Moon Formed Faster Than Previously Expected

A new supercomputer model of the moon’s formation indicates that it could have formed within hours after a Mars-sized proto-planet called Theia collided with a proto-Earth. Previously, theory had it that the Moon could have taken several months or years to form after the collision.

Jacob Kegerreis is a postdoctoral researcher at NASA’s Ames Research Center and the lead author of the paper describing the new model of the Moon’s formation published in The Astrophysical Journal Letters.

“This opens up a whole new range of possible starting places for the Moon’s evolution,” he said in a NASA press release announcing the new model.

Previous models of the collision between early Earth and Theia suggest that Theia disintegrated on impact, which would explain why scientists haven’t found a remaining piece of Theia floating around – unless the Moon is heavily made up of debris from Theia. However, isotopic signatures indicate that the Moon’s composition is too similar to Earth to support that possible explanation. Theia could have been very much like Earth, but is unlikely to have been close enough to explain the Moon’s similarities to Earth’s composition.

Collisions were very common during the early formation of the solar system. Larger objects could use their gravity and size to attract and absorb smaller ones and occasionally punt things around – and, often, had a better chance of surviving high-speed collisions. Planet-sized objects could have been thrown out of the solar system or thrown into the sun by gravitational forces from other planets. It’s possible that Earth survived the collision with a Mars-sized object with no worse than losing a bunch of material to the Moon because it was already bigger.

Most of the Moon’s isotopic signatures studied by scientists come from rock samples brought back by the Apollo missions. Although the sampling of rocks taken from six sites on the “near side” of the Moon is unlikely to give a complete picture, the available moon rocks are more similar to rocks found on the surface on Earth than to other planets in the solar system like Mars.

What happened to Theia if its remains didn’t become the Moon? Previous models suggest that its material, plus material from a young and still cooling Earth, could have sprayed into Earth orbit. A lot of the sprayed-off material could have fallen back to Earth. Most of the rest became the Moon.

However, according to NASA’s press release, the synthesia model – which says that the moon formed inside a swirl of vaporized material from the collision – fails to explain the Moon’s current orbit. Theia could still have disintegrated. It just didn’t provide as much material for the Moon as previously thought.

The “fast formation” model provides a better explanation for its orbit and some lunar properties. The Moon doesn’t have a fully molten core. It has a thin crust and a tilted orbit.

According to NASA, theories of the Moon’s formation can be refined with future lunar exploration missions. Artemis will be able to provide more rock samples from more locations for scientists to work with.

“The more we learn about how the Moon came to be, the more we discover about the evolution of our own Earth,” said Vincent Eke, a researcher at Durham University and a co-author of the paper on this new model of the Moon’s formation.