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How did the moon form?

The Earth and Moon.
[NASA/courtesy of nasaimages.org]

A giant impact on Earth could have produced a Moon chemically similar to Earth, two new studies appearing online in the October 18 issue of Science Express report.

The findings challenge a long-standing theory that the Moon was produced primarily from the material of a Mars-sized planet after a giant collision with Earth about 4.5 billion years ago.

First appearing in the 1970s, the giant impact model gained traction in the 1980s as computer simulations of the collision event suggested that the Moon was produced primarily from foreign material. This idea became a problem when better analytical techniques revealed that the Moon's chemical make-up is actually quite similar to Earth's.

These previous models also assumed that the giant collision preserved the same spin or angular momentum that the Earth-Moon system has today. In a series of computer simulations, researchers found that a giant impact on an early Earth with a fast spin can produce a Moon-forming disk made from Earth material.

It turns out that a faster-spinning Earth--Moon system initially can lose angular momentum, reaching its present state through the Sun's gravitational influence.

In a separate study, Robin Canup and colleagues performed computer simulations involving lower-velocity giant collisions by planets with a mass similar to Earth's.

The results produced a Moon with the same chemical composition as the Earth mantle, adding to evidence that the moon could have formed from the Earth.