The Moon Is Different Than Earth at Its Core

Similarities between lunar samples and Earth’s makeup were throwing off a leading theory of the moon’s origin

Earth moon impact
This artist’s rendition shows a giant impact similar to the one 4.5 billion years ago that scientists think created the Earth-Moon system. Courtesy of NASA/JPL-Caltech

Every hero needs an origin story, and the moon—which helps regulate Earth’s climate and gives the ocean its tides—is no exception. For decades, researchers have known that the Earth and the moon are largely made of the same stuff. But now, new research shows they are more different than previously thought, giving weight to a controversial theory that the moon's origin.

Previous research had found that the moon and Earth had strangely similar levels of two kinds of oxygen atoms: a lighter version, and a heavier one with two more sub-atomic particles. (The variations are called isotopes.) The similarity was a wrench in the leading theory of the moon’s origin, but the new study, published on Monday in the journal Nature Geoscience, gives evidence that the moon’s makeup changes in layers.

"Going into this project, it was expected that our results would likely mirror that of previous studies," University of New Mexico geochemist Erick Cano tells Charles Choi at Space. "The most surprising part of our results was finding the amount of variation that we did between the individual lunar samples."

For decades, the most likely theories about where the moon came from have involved a collision between a young Earth and another proto-planet dubbed Theia after the mother of the moon in Greek mythology. This giant impact hypothesis was able to explain the relationship between the Earth and the moon, including the moon’s relatively large size and its orbit. But lunar samples brought to Earth by the Apollo missions initially showed strange similarities.

Most planets in the solar system each have unique chemical makeups, making it unlikely that Earth and Theia, which was about half the diameter of Earth, were chemical twins. And for the two embryonic worlds to perfectly mix and create two objects with matching ingredients would take a much bigger collision than experts think happened. Instead, computer models show that the moon should still be 70 to 90 percent Theia, per Space.

The new research gives more weight to that possibility. Previous studies recorded and analyzed data without regard to rock type, but the researchers instead grouped their samples by source. Volcanic glass, for example, formed deeper under the moon’s surface. The researchers analyzed oxygen in lunar samples from both the moon’s surface and underground. They found that the deeper below the moon's surface, the more dissimilar the moon’s oxygen makeup is from Earth's.

“This is important. The crust is where mixed debris [from the impact] would have ended up, whereas the deep interior would have more bits of Theia,” writes University of Stirling planetary scientist Christian Schroeder, who wasn’t involved in the research, in the Conversation. “The study shows that there is a small difference between the Earth and the Moon in their oxygen isotope composition – their profiles aren’t identical after all.”

When Earth and Theia collided, they would have created oceans of magma from the heat and thrown tons of debris from both planets into space, where it could mix. This mixing event also would have led to "lava raining down on the moon for hundreds of years," co-author and planetary scientist Zachary Sharp tells Space, creating a lunar crust that resembles Earth’s, in terms of oxygen.

This finding doesn’t mean that a chunk of Theia is preserved at the moon’s core, Cano tells Eos' Javier Barbuzano. Instead, it means that future models of the giant impact between Earth and Theia don’t have to provide an explanation for two unusually-well-mixed planetary bodies, because it turns out that they’re not so similar.

“It’s an important contribution,” Purdue University planetary scientist Jay Melosh tells Eos, adding that the finding “is not something that completely turns the models on their heads, but it might make more difficult to accept the extreme mixing models.”