About 4.5 billion Years ago, a primordial version of Earth covered in molten lava orbited the sun. Barely in its newfound existence, it was struck by a smaller Mars-sized object dubbed Theia in an explosive event. Theia was blown to pieces by the impact while a huge chunk of Earth was thrown into space.
The gravitational pull of the remaining majority of our planet caused this material to swirl around the earth. In a surprisingly short amount of time, perhaps less than 100 years, some of this material stuck together to form the moon.
At least that’s how a popular theory of the origin of the moon goes. But now there’s new evidence that the moon did in fact form from the debris of that cosmic impact billions of years ago. The discovery of certain gases in the moon’s interior supports the idea and also gives us important new details about how it might have happened.
During her doctorate at the Swiss Federal Institute of Technology (ETH) in Zurich, Patrizia Will examined six lunar meteorites recovered from Antarctica by NASA in the early 2000s. In these rocks, she and her colleagues found helium and neon trapped in tiny glass beads formed during volcanic eruptions on the moon’s surface as magma was pulled up from the moon’s interior. These gases, known as noble gases because they are relatively inert, appear to have originated on Earth and were likely inherited from the moon “during its formation,” Will says. The research was published in the journal scientific advances.
Previous work has pointed to the giant impact hypothesis. Moon rocks show a striking resemblance to terrestrial rocks, suggesting a common origin. Still, there are key differences: moon rocks, for example, have a lighter version of chlorine, pointing to a dramatic event early in the history of our two worlds that separated some material.
Most scientists now agree that the event was a gigantic collision. “We’re pretty confident in the giant impact hypothesis,” says Sujoy Mukhopadhyay, a geochemist at the University of California, Davis who was not involved in Will’s study. “That’s still the best hypothesis on the table.”
After the impact, a disc of material may have formed around our planet, displaced by the collision — possibly a donut of vaporized rock known as synestia, with a temperature in the thousands of degrees. The amount of neon and helium detected in the lunar samples supports the theory that the moon formed in this synest, as the relative abundance of these gases suggests they came out of the Earth’s mantle and were ejected into space by the impact before they were merged with the interior of our satellite. Had these gases instead been carried across space to the moon by solar winds, we would expect much, much smaller amounts to be present in the analyzed meteorites.
“It’s really interesting work,” says Mukhopadhyay, noting that no study has found evidence of such native gases in lunar rocks. “The concentrations are very low, so it’s very difficult to detect,” says Ray Burgess, a geochemist at the University of Manchester and reviewer of Will’s study. “This is a big step forward.”