About 100 million years after the formation of the Solar System, a Mars-sized object named Theia collided with Earth and formed the Moon. Now we have a better idea of where this disoriented object came from.
In a study published today in the journal Science, researchers examined the isotopic fingerprint of iron – the ratio of isotopes, or versions, of the element in a material – in rocks from the Moon, Earth and meteorites (meteorites that reach the ground). Their results reinforce the theory that the impactor was born in the inner Solar System and was closer to the Sun than where Earth originated.
Their findings suggest that Theia – hidden in the inner Solar System – eventually drifted outward and collided with the proto-Earth about 4.5 billion years ago, a violent impact that broke both bodies apart and led to the formation of the Moon.
Where did Thea come from?
Most computer models simulating the formation of the Moon indicate that our favorite satellite is largely composed of material originally related 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 the origin of a planetary body in the Solar System.
However, previous analyzes of lunar rocks have shown that Earth and the Moon have nearly indistinguishable isotopic compositions of many elements. “The lack of measurable isotopic differences and uncertainty about what process caused them has prevented the determination of Theia’s formation region,” the researchers said. The team included geoscientist Timo Hopp of the Max Planck Institute for Solar System Research.
Thus, the team conducted new high-precision isotope analyzes of iron in lunar samples and terrestrial rocks, as well as analyzing meteorites from isotopic reservoirs (parts of the Solar System with different isotopic compositions) that may have given rise to Theia and Earth. This approach once again revealed that the isotopic composition of our planet and the Moon is similar to each other. However, the researchers also found that this composition 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 probably formed in the inner Solar System. But this evidence in itself was not enough to confidently point to Theia’s origin.
sunnier than earth
“Combining our results with other elements, we performed mass balance calculations for Theia and proto-Earth,” the researchers explain in the study. Simply put, mass balance calculations enabled researchers to use Earth’s current isotopic composition to figure out what the original isotopic signatures of the planet and Theia were before the catastrophic collision.
“We found that all of Theia and most of Earth’s other constituent substances [the ones that aren’t from Theia] Originated from the inner Solar System,” he added. What’s more, “Our calculations suggest that Theia may have formed much closer to the Sun than Earth.”
Although all this may seem far from daily life, the study ultimately sheds light on the astronomical dynamics that likely played a role in making Earth the life-sustaining planet it is today – and placing the Moon in our sky to inspire countless Romantic poets and songwriters.
Earlier this year, related research revealed evidence of a proto-Earth that existed before this cataclysmic impact. Scientists previously believed that the collision and the vast flow of time had erased all traces of this ancient Earth. The discovery sheds light on the building blocks of the early Solar System and the environment that gave rise to our planet.