Earth's head-on collision with Mars-sized planet formed the Moon

When a "planetary embryo" called Theia collided with the early Earth approximately 100 million years after the Earth was formed, the moon span off into the orbit around the nascent planet, according to a new study.

Scientists had already known about this high-speed crash, which occurred almost 4.5 billion years ago, but many thought the Earth collided with Theia (pronounced THAY-eh) at an angle of 45 degrees or more, a powerful side-swipe (simulated in this 2012 YouTube video).

The UCLA geochemists and colleagues analysed seven rocks brought to the Earth 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.

"We don't see any difference between the Earth's and the moon's oxygen isotopes; they're indistinguishable," said lead author Edward Young. The fact that oxygen in rocks on the Earth and our moon share chemical signatures was very telling, Young said. Had Earth and Theia collided in a glancing side blow, the vast majority of the moon would have been made mainly of Theia, and the Earth and moon should have different oxygen isotopes. A head-on collision, however, likely would have resulted in similar chemical composition of both Earth and the moon.

"Theia was thoroughly mixed into both the Earth and the moon, and evenly dispersed between them," Young said. "This explains why we don't see a different signature of Theia in the moon versus the Earth."

Theia, which did not survive the collision (except that it now makes up large parts of Earth and the moon) was growing and probably would have become a planet if the crash had not occurred, Young said.

Young and some other scientists believe the planet was approximately the same size as the Earth; others believe it was smaller, perhaps more similar in size to Mars. The study is reported in the journal Science.