Earth’s moon was created by a head-on collision between Earth and a “planetary embryo” called Theia approximately 4.5 billion years ago, according to a statement by researchers from the University of California, Los Angeles.
The finding challenges a long-held theory that Earth collided with Theia at a glancing angle of 45 degrees or more.
The study is detailed in the latest issue of the journal Science.
The research team analyzed seven rocks collected from the moon by the Apollo 12, 15 and 17 missions, as well as six volcanic rocks from the Earth’s mantle — five from Hawaii, and one from Arizona.
A chemical signature uncovered in the rocks’ oxygen atoms was the key to reconstructing the Earth-Theia impact event.
Oxygen makes up 90 percent of a rock’s volume and 50 percent of its weight. Over 99.9 percent of Earth’s oxygen is an isotope called O-16, of which each atom contains eight protons and eight neutrons.
However, small quantities of heavier oxygen isotopes also can be found, including O-17, which has one extra neutron and O-18, which has two extra neutrons. In our solar system, Earth, Mars and other planetary bodies each has a unique ratio of O-17 to O-16, resulting in a distinctive “fingerprint.”
The research team also found evidence challenging a 2014 finding by German scientists that the moon possesses a unique ratio of oxygen isotopes, different from Earth’s.
“We don’t see any difference between the Earth’s and the moon’s oxygen isotopes; they’re indistinguishable,” said Edward Young, a UCLA professor of geochemistry and cosmochemistry, and lead author on the new study, in the statement. “Theia was thoroughly mixed into both the Earth and the moon, and evenly dispersed between them. This explains why we don’t see a different signature of Theia in the moon versus the Earth.”
According to Young, if Earth and Theia had collided in a glancing side blow, Theia’s remnants would make up most of the moon. As a result, the Earth and moon would yield different oxygen isotopes. But a head-on collision likely would have produced similar chemical compositions on both Earth and the moon.