A striking idea: 2 moons became one

The "big splat": An artist's rendering shows a simulation of a crash between the moon and a smaller companion moon about four billion years ago.
The "big splat": An artist's rendering shows a simulation of a crash between the moon and a smaller companion moon about four billion years ago.
Posted: August 04, 2011

WASHINGTON - Earth once had two moons, astronomers suggested Wednesday. But in a spectacle that might have beguiled poets, lovers, and songwriters if only they had been around to see it billions of years ago, the smaller one smashed into the other in a "big splat."

The result: a single bulked-up and ever-so-slightly lopsided moon.

The astronomers came up with the scenario to explain why the moon's far side is so much hillier than the one that is always facing Earth.

The theory, outlined in the journal Nature, comes complete with computer-model runs showing how it might have happened, 4.4 billion years ago, as a small moon that trailed behind a much larger one got pulled in by gravity at 5,000 m.p.h.

"They're destined to collide. There's no way out. . . . This big splat is a low-velocity collision," said study coauthor Erik Asphaug, a planetary scientist at the University of California, Santa Cruz.

Planetary scientist Alan Stern, a former associate administrator for science at NASA who was not part of the research, said it was a "very clever new idea" but one that was not easily tested to learn whether it was right.

A second moon isn't just an astronomical matter.

Poet Todd Davis, a professor of literature at Pennsylvania State University, suggested that this idea of two moons - one essentially swallowing the other - could capture the literary imagination.

"I'll probably be dreaming about it and trying to work on a poem," Davis said.

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