 "planets")
A violent, head-on collision between the Earth and a forming planet 4.5 billion years ago created the Moon, claim researchers from the University of California-Los Angeles (UCLA), suggesting this massive crash likely resulted in similar chemical composition of both the Earth and the Moon.
Scientists know about this high-speed crash but many thought the Earth collided with the forming planet called "Theia" at an angle of 45 degrees or more -- a powerful side-swipe.
In 2014, a team of German scientists reported that the Moon also has its unique ratio of oxygen isotopes, different from Earth's. The new research finds that is not the case.
"We don't see any difference between the Earth's and the moon's oxygen isotopes; they're indistinguishable," said Edward Young, lead author and a UCLA professor of geochemistry and cosmochemistry.
The researchers 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.
The key to reconstructing the giant impact was a chemical signature revealed in the rocks' oxygen atoms.
The team used state-of-the-art technology and techniques to make extraordinarily precise and careful measurements, and verified them with UCLA's new mass spectrometer.
"The fact that oxygen in rocks on the Earth and Moon share chemical signatures was very telling," Young said.
Had the 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, would have resulted in similar chemical composition of both the 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 the Earth and the Moon) was growing and probably would have become a planet if the crash had not occurred," Young added.
Young and some other scientists believe the planet was approximately the same size as the Earth's; others believe it was smaller, perhaps similar in size to Mars.
