Artist's impression of a collision between Earth and a protoplanet, thought to have led to the formation of the Moon

When Earth’s surface was awash with magma oceans, a collision with a protoplanet (artist’s impression pictured) could have ejected enough material to form the Moon. Credit: Mark Garlick/SPL

Planetary science

Moon might have formed from blobs of a molten Earth

Magma-ocean theory of Moon’s origin could explain why Earth and its satellite are chemically so similar.

The Moon might have formed after an enormous protoplanet slammed into a partially molten Earth.

The leading theory of Moon formation holds that an object the size of a small planet slammed into Earth early in the Solar System’s history, kicking up debris that coalesced to become the Moon. But scientists haven’t been able to explain some details of this scenario — notably, why the material that makes up Earth is chemically so similar to that which makes up the Moon. Simulations of the giant impact suggest that most of the Moon’s material would have come from the impacting object, not from Earth.

The answer to this conundrum could lie in an ancient magma ocean, says a team led by Natsuki Hosono at the Yokohama Institute for Earth Sciences in Japan. Using computer simulations that explored how molten particles behave when they collide, the researchers studied what would happen if the impacting object slammed into an Earth that was covered in molten rock. They found that molten material is more easily flung into space than is solid rock, meaning that much of Earth’s magma could have been incorporated into the Moon.