Earth Is Made Up of Two Planets

February 3, 2016 | Joanne Kennell

Two planets colliding
Photo credit: NASA/JPL-Caltech

The violent collision that occurred nearly 4.5 billion years ago also formed the moon.

It turns out that the moon was created from an extremely violent, head-on collision between early Earth and a planetary embryo called Theia approximately 100 million years after the Earth first formed.

Scientists already knew that a high-speed collision had to have occurred almost 4.5 billion years ago, however many thought Earth collided with Theia (pronounced THAY-eh) at an angle of at least 45 degrees — more of a side-swipe as simulated in this 2012 YouTube video.

But new evidence from a report written by UCLA geologists and published in the journal Science strengthens the theory of a head-on collision.

For the study, the team analyzed seven rocks brought to the Earth from the moon by Apollo 12, 15 and 17 missions, as well as six volcanic rocks that originated in the Earth’s mantle — five from Hawaii and one from Arizona.

To reconstruct the collision, the team had to identify chemical signatures in the rocks’ oxygen atoms.  Oxygen makes up 90 percent of a rock's volume and 50 percent of its weight, and more than 99.9 percent of Earth’s oxygen is O-16, a version or “isotope” of oxygen that has eight protons and eight neutrons.  

SEE ALSO: Our Solar System is Missing One Giant Planet

However, there are also small amounts of heavier oxygen isotopes, O-17 and O-18, which have one and two extra neutrons, respectively.  In fact, Earth, Mars and other objects within our solar system each have a unique ratio of O-17 to O-16 — much like how humans have distinct fingerprints.

Back in 2014, a team of scientists from Germany reported in Science that the moon has its own unique ratio of oxygen isotopes — different from Earth’s.  However, this new study out of UCLA disagreed with those findings.  “We don't see any difference between the Earth's and the moon's oxygen isotopes; they're indistinguishable,” Edward Young, lead author of the new study and a UCLA professor of geochemistry and cosmochemistry said in a press release.

In order to determine the oxygen ratios, the team used state-of-the-art technology and techniques and verified them with UCLA’s new mass spectrometer.  The results of their study showed that the oxygen found in the rocks on both Earth and the moon share chemical signatures.  

If Earth and Theia collided on a 45-degree side-swipe as previously thought, the majority of the moon would be made mainly of Theia, meaning the moon would have a different oxygen isotope ratio.  A head on collision, however, would have resulted in a similar chemical composition of both the Earth and 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.”

Unfortunately, Theia did not survive the collision and now makes up large parts of Earth and the moon.  Theia probably would have become an Earth- or Mars-sized planet if the crash did not happen.  Planet nine (or ten), perhaps?

It is really hard to imagine what would have happened if the collision never occurred!

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