Boom rescaled: ‘Extremely giant impact’ may be behind making of Earth’s moon

© Leonhard Foeger
It lights up our sky and has hosted astronauts, but there’s still much to learn about the moon. Astronomers have long believed it was formed from a space rock’s low-contact collision with Earth, but new research says the impact was far more violent.

Since the 1970s, researchers have settled on a theory which assumes the moon was created when a space rock the size of Mars “lightly” grazed Earth around 4.5 billion years ago. The impact would have scattered debris from both bodies into space, and formed a disk orbiting early Earth. That disk, according to the theory, eventually became the modern-day moon.

However, a new study published in Nature says the collision was far stronger and far less friendly than previously believed.

The new information comes after researchers studied Apollo-era moon rocks with new technology.

“We’re still re-measuring the old Apollo samples from the ‘70s, because the tech has been developing in recent years. We can measure much smaller differences between Earth and the moon, so we found a lot of things we didn’t find in the 1970s,” lead author Kun Wang, an assistant professor at Washington University, told Gizmodo. “The old models just could not explain the new observations.”

If the theory that the space rock lightly collided with Earth were true, evidence should be available showing that the chemical composition of the moon is distinct from that of the Earth, as the odds that the space rock that struck Earth had the same chemical make-up would be incredibly small, as scientists expected.

However, the studied samples of potassium isotopes failed to produce such evidence. Instead, chemical analyses showed isotopic compound readings that were nearly identical.

Intrigued, the researchers conducted increasingly advanced tests in an effort to identify any differences in the signatures. They eventually found one, but it suggested that the samples’ origins were even more connected than previously thought.

The isotope signatures were the same, except for more of a heavy potassium isotope in the moon samples, which would have required extremely hot temperatures to separate out. Those extremely high temperatures could have been caused by an extremely violent collision between the space rock and Earth.

Based on that assumption, the researchers hypothesized that the giant space rock slammed into early Earth with such force that it vaporized much of the Earth and the space rock. That vapor then expanded over an area 500 times the size of the Earth, before finally cooling and condensing into the moon.

“We need a much, much bigger impact to form a moon according to our study,” Wang said. “The giant impact itself should be called extremely giant impact. The amount of energy required isn’t even close.”

Wang went on to state that the new information has “changed our understanding of the early solar system, it’s much more violent than we thought.”

However, not everyone is ready to jump on that train. “That is definitely a tall claim,” Munir Humayun, a geologist at Florida State University not involved in the study, told The Verge. “It’s a little too early with their data to tell that.”

Wang and his colleagues will now continue to study the Apollo lunar samples and try to gather even more clues from them.