New Evidence Points to the Moon Once Being Part of Earth

Gases trapped in lunar meteorites hint that the moon was formed out of material displaced from Earth after a planetary collision.
moon
Photograph: NASA

Roughly 4.5 billion years ago, a primordial version of Earth covered in molten lava orbited the sun. Barely into its newfound existence, it was struck by a smaller object the size of Mars, referred to as Theia, in an explosive event. Theia was blown to pieces by the impact, while a huge chunk of Earth was sent careening into space.

The gravitational pull of the remaining bulk of our planet saw this material swirl around Earth. In a surprisingly short span of time, perhaps less than 100 years, some of that material stuck together and formed the moon.

Or at least, this is how one popular moon origin theory goes. Now, though, there’s fresh evidence to suggest that the moon was indeed created from the debris of this cosmic impact billions of years ago. The discovery of certain gases inside the moon supports the idea, and also gives us important new details on how it might have happened.

While completing her PhD at the Swiss Federal Institute of Technology (ETH) in Zurich, Patrizia Will studied six lunar meteorites recovered by NASA from Antarctica in the early 2000s. In these rocks, she and her colleagues found helium and neon trapped in tiny glass beads, which were formed in volcanic eruptions on the lunar surface as magma was pulled up from the moon’s interior. These gases, known as noble gases because they are relatively unreactive, appear to have originated on Earth, and were likely inherited by the moon “during its formation,” says Will. The research was published in the journal Science Advances.

Previous work has hinted at the giant impact hypothesis. Lunar rocks show a striking similarity to Earth rocks, suggesting a common origin. Yet there are key differences: Lunar rocks have a lighter version of chlorine, for example, pointing to a dramatic event early in the history of our two worlds that separated some material.

Most scientists now agree this event was a gigantic collision. “We are pretty set on the giant impact hypothesis,” says Sujoy Mukhopadhyay, a geochemist from the University of California, Davis, who was not involved in Will’s study. “That’s still the best hypothesis on the table.”

Following the impact, a disk of material displaced by the collision—possibly a donut of vaporized rock known as a synestia, measuring thousands of degrees in temperature—may have formed around our planet. The amount of neon and helium discovered in the lunar samples supports the theory that the moon formed in this synestia, as the relative abundance of these gases suggests they came from Earth’s mantle and were blasted into space by the impact before being fused into the interior of our satellite. Had these gases instead been transported across space into the moon by solar winds, we’d expect there to be much much lower quantities present in the meteorites analyzed.

“It’s really interesting work,” says Mukhopadhyay, noting that no study has been able to find evidence for such indigenous gases in lunar rocks before. “The concentrations are very low, so it’s very hard to detect,” says Ray Burgess, a geochemist from the University of Manchester and a reviewer of Will’s study. “It’s a big step forward.”

Will and her colleagues were able to make the discovery using an advanced mass spectrometer at the Noble Gas Laboratory at ETH Zurich—an instrument that can determine what’s in a chemical substance by measuring the weight of its individual molecules. The instrument at ETH Zurich “has the highest sensitivity for studying helium and neon” in the world, says Will. The machine enabled the researchers to study the composition of the glass beads in the meteorites—separated using small tweezers under a microscope—and find the tiny traces of helium and neon trapped inside. The glass beads themselves were just millionths of a meter in size, “really tiny, tiny grains,” says Will.

The next step is to understand how Earth got its noble gases. There are two main possibilities: that they were delivered on comets and asteroids that crashed into our protoplanet, or that Earth quite literally sucked them into its atmosphere from the nebula of gas and dust that surrounded our young sun. To find out, scientists want to look for more noble gases—namely krypton and xenon—in lunar meteorites.

We find krypton and xenon in other meteorites that have crashed into our planet: pieces of asteroids that may have been the building blocks of planets like Earth. If we can also find those gases in lunar meteorites, we can compare their compositions “and see the correspondence,” says Burgess. The reason for looking at lunar meteorites, and not just rocks here on Earth, is that they offer a better record of the solar system’s early history.

If krypton and xenon found in lunar meteorites is similar to that found in meteorites from elsewhere, it would support the theory that our noble gases originated from asteroids and comets; if not, it would support the nebula idea. On the other hand, if we find no krypton or xenon, that would be an “interesting puzzle that we’d have to sort out,” Burgess adds.

Henner Busemann from ETH Zurich, a coauthor on Will’s study, says the team saw evidence of krypton and xenon in the lunar meteorite samples they looked at, but they couldn’t be sure of their results. “We cannot make the case yet,” he says. “We will try now to get better precision.”

Finding noble gases on the moon may tell us about its water content, too. If hydrogen and neon managed to survive its turbulent formation, then water could also have done so in the moon’s interior—something we have seen evidence for, as with the water frozen as ice at the moon’s poles. Such water could be an invaluable resource for future human missions. “If the moon is wetter than we thought, it adds further possibilities to finding resources that we might want to use,” says Burgess.

This might suggest that a wide variety of life-forming material can survive giant impacts early in a planet’s life. “We could produce new models about this planetary formation process in the solar system and beyond,” says Will, adding that this could be one piece of the puzzle of how life originated on Earth—and maybe other planets, too.