Skip to main content

Ancient impact that formed Earth's moon was likely a one-two punch

Earth's moon is thought to have formed when a Mars-sized object, called Theia, slammed into the proto-Earth more than 4.4 billion years ago, blasting out material that later coalesced into a large satellite as depicted in this artist's illustration.
Earth's moon is thought to have formed when a Mars-sized object, called Theia, slammed into the proto-Earth more than 4.4 billion years ago, blasting out material that later coalesced into a large satellite as depicted in this artist's illustration. (Image credit: NASA/JPL-Caltech)

The gigantic impact that created the moon was actually a one-two punch, a new study suggests.

Scientists think that the moon, our planet's only natural satellite, was born in violence, coalescing from the material blasted into space after a Mars-size body named Theia slammed into the proto-Earth more than 4.4 billion years ago.

But there are some problems with the canonical collision scenario, which invokes a single catastrophic event, the authors of the new study said.

"The standard model for the moon requires a very slow collision, relatively speaking, and it creates a moon that is composed mostly of the impacting planet, not the proto-Earth, which is a major problem since the moon has an isotopic chemistry almost identical to Earth," lead author Erik Asphaug, a professor at the University of Arizona's Lunar and Planetary Laboratory (LPL), said in a statement.

Related: How was the moon formed?

The moon is thought to be the aftermath of a giant impact. According to a new theory, there were two giant impacts in a row, separated by 100,000 to 1 million years, involving a Mars-sized body called Theia and proto-Earth. In this image, the proposed hit-and-run collision is simulated in 3D, shown about an hour after impact. A cut-away view shows the iron cores. Theia (or most of it) barely escapes, so a follow-on collision is likely.

The moon is thought to be the aftermath of a giant impact. According to a new theory, there were two giant impacts in a row, separated by 100,000 to 1 million years, involving a Mars-sized body called Theia and proto-Earth. In this image, the proposed hit-and-run collision is simulated in 3D, shown about an hour after impact. A cut-away view shows the iron cores. Theia (or most of it) barely escapes, so a follow-on collision is likely. (Image credit: A. Emsenhuber/University of Bern/University of Munich)

Double whammy made the moon

Asphaug and his colleagues performed computer simulations of the long-ago giant impact and came up with what they believe to be a better fit: Theia and the proto-Earth crashed at faster speeds than previously envisioned, producing an initial "hit and run" collision that set the stage for a slower, accretionary encounter between the two battered bodies about 100,000 to 1 million years down the road.

"The double impact mixes things up much more than a single event, which could explain the isotopic similarity of Earth and moon, and also how the second, slow, merging collision would have happened in the first place," Asphaug said.

Hit-and-run collisions weren't restricted to the nascent Earth-moon system in those early days. Indeed, such bouncing smashups were probably about as common as accretionary mergers in the ancient inner solar system, the same research team reports in a second new study.

Blocking space rocks for Venus

In the second paper, the scientists modeled giant impacts in the inner solar system, how those collisions affected planet formation and how the orbits of the involved objects evolved over time. They found that Earth likely acted as a sort of shield for Venus, taking the brunt of hit-and-run first impacts. Those initial collisions slowed the impactors down, setting the stage for accretionary mergers with Venus later.

"The prevailing idea has been that it doesn't really matter if planets collide and don't merge right away, because they are going to run into each other again at some point and merge then," Alexandre Emsenhuber, the lead author of the second study, said in the same statement.

"But that is not what we find," said Emsenhuber, who performed the research during a postdoctoral fellowship in Asphaug's lab at LPL and is now at Ludwig Maximilian University in Munich, Germany. "We find they end up more frequently becoming part of Venus, instead of returning back to Earth. It's easier to go from Earth to Venus than the other way around." (This is because Venus lies closer to the sun, whose powerful gravity draws objects in.)

The results suggest that the compositions of Earth and Venus may differ more than scientists had thought.

"You would think that Earth is made up more of material from the outer system because it is closer to the outer solar system than Venus," Asphaug said. "But actually, with Earth in this vanguard role, it makes it actually more likely for Venus to accrete outer solar system material."

Both new studies — the one led by Asphaug and the one led by Emsenhuber — were published online Thursday (Sept. 24) in The Planetary Science Journal. 

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook. 

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community@space.com.

Mike Wall
SPACE.COM SENIOR SPACE WRITER — Michael has been writing for Space.com since 2010. His book about the search for alien life, "Out There," was published on Nov. 13, 2018. Before becoming a science writer, Michael worked as a herpetologist and wildlife biologist. He has a Ph.D. in evolutionary biology from the University of Sydney, Australia, a bachelor's degree from the University of Arizona, and a graduate certificate in science writing from the University of California, Santa Cruz. To find out what his latest project is, you can follow Michael on Twitter.