Was Moon Born From Planet's Crash Into Earth?

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The collision occurred 4.5 billion years ago, only 50 million years after the solar system formed. The colossal impact must have nearly rent the young Earth apart.

"It didn't break the Earth up, but it came pretty close," Canup said.

"The Earth was distorted into an oblong shape before it gravitationally rebounded" over the course of several hours or a day, she said. Some of the material flung into space settled into orbit and eventually clumped together to form the moon.

Better Modeling

Canup and Asphaug were able to re-test the discredited mid-1980s hypothesis of impact by a Mars-size object thanks to greater computing power. They used a technique called smooth particle hydrodynamics to simulate interactions among the many rocky fragments that would have been created by the impact.

Using several powerful computers, the two scientists produced simulations involving 20,000 virtual fragments of the Earth and of the smaller foreign planet that collided with the Earth. Earlier simulations of similar impacts had been done with only 3,000 particles, which limited the realism of the simulations.

The researchers ran many simulations, adjusting the key variables—the size of the object that caused the impact, the angle of its course, and the mass of the Earth—to see which combination produced the best result.

The scenario involving a Mars-size object won out. That was when the researchers realized "the resolution makes a big difference," said Canup, referring to the number of particles that were used in the simulations. Three thousand particles, it turns out, is not enough realistically to simulate a collision between planet-sized objects.

In a companion article in Nature addressing Canup and Asphaug's study, planetary scientist Jay Melosh of the University of Arizona in Tucson noted, "Encouraging as these new results are, they are not the final word."

One major question is the accuracy of the mathematical equation underlying the new impact model. That equation, developed in 1962, doesn't distinguish well the behavior of ejected solids, liquids, and gases in the hours following the impact.

Treating these states of matter differently in the simulation could explain another peculiar aspect of the moon's composition: its dearth of easily vaporized "volatile" compounds such as water.

A newer, more sophisticated modeling equation has been developed, but Canup and Asphaug did not use it because it was known to have some imperfections. Since their study, Melosh has reworked that equation. Now he is teaming with Canup and Asphaug to test their new model with the more sophisticated equation to see if the results are consistent with their present findings.

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