Moon was produced by a Head-on Collision between Earth and a Forming Planet

This image shows from left Paul Warren, Edward Young and Issaku Kohl. Young is holding a sample of a rock from the moon. Credit: Christelle Snow/UCLA

The moon was formed from a violent, head-on collision between early Earth and ‘planetary embryo’ called Theia ~100 million years after the Earth formed, almost 4.5 billion years ago. Many thought Earth collided with Theia at 45 degrees or more. New evidence substantially strengthens the case for a head-on assault.

The researchers analyzed seven rocks brought to the Earth from the moon by the Apollo 12, 15 and 17 missions, as well as 6 volcanic rocks from the Earth’s mantle – 5 from Hawaii and 1 from Arizona. The key to reconstructing the giant impact was a chemical signature in the rocks’ O atoms. (Oxygen makes up 90% of rocks’ vol and 50% of their wt.)
>99.9% of Earth’s oxygen is O16 (each atom contains 8 protons, 8 neutrons. But there also are small quantities of heavier oxygen isotopes: O17, which have one extra neutron, and O18, with 2 extra neutrons. Earth, Mars and other planetary bodies in our solar system each has a unique ratio of O-17 to O-16 – each one a distinctive “fingerprint.”

In 2014, German scientists reported the moon also has its own 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 Prof. E Young.
Had Earth and Theia collided in a glancing side blow, the vast majority of the moon would have been made mainly of Theia, and Earth and moon should have different oxygen isotopes. A head-on collision, however, likely would have resulted in similar chemical composition.

Theia, which did not survive the collision probably would have become a planet if the crash had not occurred, Young said. It was likely the same size as the Earth; others believe it was smaller, similar in size to Mars. Another interesting question is whether the collision with Theia removed any water that early Earth may have contained. After the collision – perhaps tens of millions of year later – small asteroids likely hit the Earth, including ones that may have been rich in water.

A head-on collision was initially proposed in 2012 by Matija, now a research scientist with the SETI Institute, and Sarah Stewart, now a professor at UC Davis; and, separately during the same year by Robin Canup of the Southwest Research Institute. http://newsroom.ucla.edu/releases/moon-was-produced-by-a-head-on-collision-between-earth-and-a-forming-planet