We Finally Know the Birthplace of the Mars-Sized Rock That Spawned Our Moon

Around 100 million years after the formation of the solar system, a Mars-sized object dubbed Theia slammed into the Earth and created the Moon. We now have a better idea as to where this wayward object came from.

In a study published today in the journal Science, researchers investigated the isotopic fingerprints—the ratio of isotopes, or versions, of elements in a material—of iron in rocks from the Moon, Earth, and meteorites (meteoroids that reach the ground). Their results bolster the theory that the impactor was born in the inner solar system and closer to the Sun than where Earth originated.

Their findings suggest that Theia—lurking in the inner solar system—eventually drifted outward and collided with the proto-Earth around 4.5 billion years ago, a violent impact that shattered both bodies and led to the formation of the Moon.

Whence came Theia?

Most computer models simulating the creation of the Moon indicate that our favorite satellite is largely made of material originally belonging to Theia. So if Theia had a different isotopic fingerprint than Earth, the Moon should, too. This matters because isotopic differences can shed light on a planetary body’s origin in the solar system.

Previous analyses of lunar rocks, however, showed that Earth and the Moon have almost indistinguishable isotopic compositions across numerous elements. This “lack of measurable isotopic differences and uncertainty over which process caused this have prevented a determination of Theia’s formation region,†the researchers explained. The team included Timo Hopp, a geoscientist from the Max Planck Institute for Solar System Research.

As such, the team carried out new high-precision isotope analyses of iron in lunar samples and terrestrial rocks, as well as meteorites from isotopic reservoirs (parts of the solar system with distinct isotopic compositions) that may have birthed Theia and Earth. This approach revealed, once again, that the isotopic makeup of our planet and the Moon are identical to each other. However, the researchers also found that this makeup aligns with that of non-carbonaceous meteorites—materials that researchers believe originated in the inner solar system.

In other words, both the Moon and Earth likely formed in the inner solar system. But this evidence on its own wasn’t enough to confidently pinpoint Theia’s origins.

Sunnier than Earth

“Combining our results with those for other elements, we performed mass balance calculations for Theia and proto-­Earth,†the researchers explained in the study. Simply put, mass balance calculations enabled the researchers to use Earth’s current isotopic composition to figure out what the planet and Theia’s original isotopic signatures were before the catastrophic collision.

“We found that all of Theia and most of Earth’s other constituent materials [the ones that aren’t from Theia] originated from the inner Solar System,†they added. What’s more, “our calculations suggest that Theia might have formed closer to the Sun than Earth did.â€

While this might all feel far from daily life, the study ultimately sheds light on astronomical dynamics that likely played a role in making Earth the life-nurturing planet it is today—and put a Moon in our skies to inspire countless romantic poets and songwriters.

Related research from earlier this year uncovered evidence of the proto-Earth that existed prior to this cataclysmic impact. Scientists had previously believed that the collision—and the immense passage of time—had erased traces of this ancient Earth. The discovery shed light on the building blocks and environment of the early solar system that gave rise to our planet.