How two worlds collided to create Earth

Powerful impact of planets 4.5 billion years ago also produced the Moon, say scientists

EARTH is actually made up of two planets which came together in a head-on collision that was so violent it formed the Moon, scientists have concluded.

Initially it was believed that the Moon was created when a smaller planet, called Theia, grazed the Earth at a 45-degree angle and broke up, sending a smaller chunk into space where it was caught in Earth’s gravity.

But if that were the case the Moon would have a different chemical composition to the Earth, because it would be made up, predominantly, of Theia.

However, after studying Moon rocks brought back by astronauts on the Apollo missions, scientists at the University of California have found that their oxygen isotopes are the same as on Earth.

It means that the collision between Theia and the early Earth was so violent that the two planets effectively melded together to form a new planet, a chunk of which was knocked off to form the Moon. “We don’t see any difference between the Earth’s and the Moon’s oxygen isotopes; they’re indistinguishable,” said Edward Young, lead author of the new study and a UCLA professor of geochemistry and cosmochemistry.

“Theia was thoroughly mixed into both the Earth and the Moon, and evenly dispersed between them.

“This explains why we don’t see a different signature of Theia in the Moon versus the Earth.” The crash with Theia happened approximately 100 million years after the Earth formed, almost 4.5 billion years ago.

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 the chemical signature in the rocks’ oxygen atoms. More than 99.9 percent of Earth’s oxygen is O-16, so called because each atom contains eight protons and eight neutrons.

There also are small quantities of heavier oxygen isotopes: O-17, which have one extra neutron, and O-18, which have two extra neutrons.

In 2014, a team of German scientists reported in Science that the Moon also has its own unique ratio of oxygen isotopes, different from Earth’s. The new research finds that is not the case.

Prof Young’s research 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. Theia was growing and prob- ably would have become a planet if the crash had not occurred, added Prof Young.

Prof Young and some other scientists believe the planet was approximately the same size as the Earth; although others believe it was smaller, perhaps more similar in size to Mars.

A head-on collision was initially proposed in 2012 by Matija Cuk, 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.

The new research was published in the journal Science.

‘Theia was thoroughly mixed into both the Earth and the Moon, and evenly dispersed between them’