Astronomers: Multiple Cosmic Collisions May Have Created Our Moon

Hagai Perets
Hagai Perets / Hagai Perets

It looks so tranquil up there. So still. But we know space is neither tranquil nor still. Our own Moon is no exception. New research suggests the Moon was the product of some very, very violent moshing: Around 4.5 billion years ago, a rowdy gang of stellar objects smashed into our young planet, creating the debris that would one day become the Moon. The astronomers published their report in the journal Nature Geoscience.

We’ve known for a while now that the Moon is made at least partially out of pieces from a banged-up Earth. But we’re still trying to sort out the details of the collision itself. At first, scientists thought Earth had been hit by one big object, like another planet. Then they thought there must have been a bunch of objects all striking around the same time. As we learned more, the single-impactor theory returned to prominence and stayed there for decades.

One 2016 study named the object—the planet Theia—and even the angle of impact. The researchers theorized that the extremely similar molecular makeup of Earth and the Moon could only have resulted from a head-on collision.

Other astronomers disagree. The authors of the new paper ran hundreds of simulations, and they argue it’s far more likely that Earth was walloped by a score of different objects called planetesimals. Each of the repeated impacts smashed up a huge amount of the young planet’s matter. That debris then drifted into orbit around Earth, where it settled into disks, which then resolved into little (gargantuan) chunks called moonlets. Over time, the baby moonlets merged into one single, spinning rock.

“Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth,” co-author Hagai Perets said in a statement. “It’s likely that such moonlets were later ejected, or collided with the Earth or with each other to form bigger moons.”

Perets says the moonlets could easily have crossed orbits with one another, smashed together, and been rolled up into larger bodies. “A long series of such moon-moon collisions could gradually build-up a bigger moon—the Moon we see today.”