The Earth in its infantile years had once collided with a Mars-sized planet called Theia; and from this violent collision, the moon was born.

Scientists are already aware of this high-speed collision which took place about 4.5 billion years ago, but many believe that the Earth crashed into Theia at an angle of 45 degrees or more. This suggests a powerful sideswipe that resulted to the formation of the moon.

Now, a new study conducted by a team of researchers from the University of California, Los Angeles shows that the collision was a case of head-on assault.

Detecting 'Fingerprints'

UCLA geoscientists analyzed rocks from the moon and the Earth: seven rocks brought to our planet from the moon by the Apollo 12, 15 and 17 missions, and six volcanic rocks from the mantle of the Earth, with five from Hawaii and one from Arizona.

To reconstruct the giant impact that resulted from the collision, scientists looked into a specific chemical signature that was revealed in the rocks' oxygen atoms. Incidentally, oxygen comprises 90 percent of the volume of rocks and 50 percent of their weight.

Scientists said more than 99.9 percent of the planet's oxygen is O-16, which takes its name from the sum of the eight neutrons and eight protons in its atoms. There are also small quantities of O-17, a heavier oxygen isotope. Earth, Mars and other celestial bodies in our own solar system have a unique ratio of O-16 to O-17, and this is their distinctive fingerprint.

A 2014 report conducted by German scientists even suggested that the moon also has its own unique fingerprint, supposedly different from that of the Earth. The UCLA study, however, revealed that this is not the case.

"We don't see any difference between the Earth's and the moon's oxygen isotopes," said Cosmochemistry and Geochemistry Professor Edward Young, the study's lead author. "They're indistinguishable."

Young and his colleagues used advanced techniques and technology to make extraordinarily careful and precise measurements, and then verified these calculations with the university's new mass spectometer.

He said the fact that oxygen in Earth and moon rocks share chemical signatures was very suggestive. Had Theia and Earth crashed in a sideswipe, the vast majority of the moon would have been made entirely of Theia, and the moon and Earth would have different oxygen isotopes.

A head-on collision, on the other hand, would have most likely resulted in the Earth having a similar chemical composition with the moon.

Part-Theia, Part-Earth

What's more fascinating is the fact that there is no distinguishable signature of Theia in the Earth versus the moon. Theia did not survive the crash, but it now makes up huge chunks of the moon and the Earth.

"Theia was thoroughly mixed into both the Earth and the moon," said Young, adding that this "planetary embryo" was evenly distributed between the Earth and the moon.

Had the crash not occurred, Theia would have even become a planet because it was also growing. Young and his colleagues believe Theia was nearly the same size of the Earth, while some believe it was smaller and approximately similar in size to Mars.

Lastly, another question that the team looked into was whether the head-on collision with Theia removed any water that early Earth might have contained. Young and his team postulates that perhaps 10 million years after the collision, small asteroids likely hit Earth. Some of the asteroids may have been rich in water.

Collisions of large bodies were definitely a thing back then, but lucky Mars always managed to avoid large collisions.

The team's findings, which are featured in the journal Science, was first proposed in 2012 by several other scientists.

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