Head-on collision between Earth, Theia likely created the moon

The collision between Earth and the “planetary embryo” known as Theia that took place roughly 100 million years after our planet formed was most likely responsible for forming the moon, according to new research published Friday in the journal Science.

While the high-velocity impact between Earth and Theia has been well documented by scientists, previous studies had suggested that the two objects simply side-swiped one another. However, in their new paper, Edward Young, a professor of geochemistry and cosmochemistry at UCLA, and his colleagues have found new evidence suggesting that it was a head-on impact.

Young’s team analyzed seven rocks collected from the moon and brought back to Earth by the astronauts of the Apollo 12, 15, and 17 missions, as well as six volcanic rocks from the planet’s mantle, and conducted a chemical analysis and comparison of each set of samples. They found no distinguishable difference between the oxygen isotopes between the Earth and moon rocks.

The findings contradict a 2014 German study that the lunar rocks would have a unique ratio of oxygen isotopes, different from those on Earth. The discovery suggests that a glancing collision between Earth and Theia was unlikely, as such an impact would have caused the moon to have been made primarily of material from Theia that had a non-Earth-like chemical composition.

Oxygen isotopes reveal striking similarities between Earth, moon rocks

During their analysis, Young and his colleagues focused on oxygen atoms, which they noted makes up 50 percent of the weight and 90 percent of the volume of a rock. The overwhelming majority of Earth’s oxygen is referred to as O-16 oxygen, because every atom contains eight protons and eight neutrons, but there are trace amounts of heavier oxygen isotopes.

Those isotopes, O-17 and O-18, have one and two additional neutrons, respectively, the study authors said. Every planet in the solar system has a unique ratio of O-17 to O-16, and by using high-tech equipment to measure the isotope signatures of both the Earth and moon rocks, they found that the ratio of oxygen isotopes in each were nearly identical.

This indicates that the collision between Earth and Theia was no little fender-bender, Young explained. “Theia was thoroughly mixed into both the Earth and the moon, and evenly dispersed between them,” he said, noting that the forming planet did not survive the collision in-tact. “This explains why we don’t see a different signature of Theia in the moon versus the Earth.”

Had Theia survived the impact, it probably would have become a planet similar in size to either Mars or the Earth, the researchers said. Furthermore, they believe that the impact with Theia may have removed any water that might have been found on the young Earth, only for a plethora of small asteroids rich in water to restore the planet’s H2O million of years after the collision.


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