Was Moon Born From Planet's Crash Into Earth?

By Ben Harder
for National Geographic News
August 20, 2001

The moon is not made of green cheese, as myth suggests. But the real story of the moon's creation may hardly be more probable.

Many scientists have thought for years that the moon was formed during the early days of the solar system when another planet collided with Earth, ejecting fragments of rocky material that condensed into Earth's only satellite.

The effect would have been as though a lousy cosmic golfer tore up a giant chunk of turf and sent it hurtling into orbit.

For more than two decades, scientists have sought to determine how large the mysterious intruder planet must have been and exactly how its cataclysmic crash could have helped form the moon. But none of their models have offered a completely satisfying explanation.

Now, new research offers a scenario that may work. It suggests the impact may have come from a much more modestly sized foreign body than previous research has proposed.

Robin Canup, a researcher at Southwest Research Institute in Boulder, Colorado, and her colleague have fashioned an improved model using a sophisticated computer-modeling technique. It explains the size, composition, and orbital properties of both Earth and the moon.

"We determined that a Mars-sized impactor would work the best," said Canup. She and co-author Erik Asphaug, a scientist at the University of California at Santa Cruz, proposed their scenario in a study that appeared last week in the scientific journal Nature.

Colossal Impact

"Giant impact" theories explaining the moon's formation were first proposed in the mid-1970s. A decade later, researchers ruled out a Mars-sized object as the source of the impact and began to model larger and larger impacts. The two best models that emerged, however, both had inherent problems.

In one model, the mass of the Earth was right, as was the composition of the moon. But the Earth's rotation rate after the collision was unrealistically fast. An improbable second impact would have been required to slow the Earth's spin.

A second scenario suggested that the impact occurred when Earth was only half formed. That idea better explained the Earth's modern rate of rotation and the moon's orbit, but it required Earth to continue accumulating matter after the impact. That material would have been rich in iron, which composes 30 percent of Earth's mass. But the moon, which contains almost no iron, would have simultaneously absorbed similarly iron-rich rock. The model offers no way to explain the moon's confounding dearth of iron.

Canup and Asphaug have proposed that the impact came from an object that was smaller than in the previous models, but was nonetheless substantial. At one-tenth the mass of the Earth, it was about the size of Mars, the two researchers say.

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