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View an animation of the hypothesized Big Whack, in which a planet-sized meteor slams into Earth, creating a debris cloud that later coalesces into the moon.

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Origins
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The Big Whack

Rather than clarifying the issue of the moon's origin, the Apollo data only complicated it. As Hartmann declared in Origin of the Moon, a 1984 book he co-edited with two other researchers, "neither the Apollo astronauts, the Luna vehicles, nor all the king's horses and all the king's men could assemble enough data to explain the circumstances of the moon's birth." Many felt something else was needed.

It came in the mid-1970s, when a new theory of lunar origin began to emerge. It rose phoenix-like from the ashes of constraints not adequately met in tests of the three other models. First, Hartmann, along with Planetary Science Institute colleague Don Davis, determined that a roving planetoid, large enough to blast off enough mantle material to make the moon, could have struck the Earth shortly after its formation. (Previous work had held that the solar system had long since run out of planet-sized meteors.) Working independently, Alastair Cameron and William Ward of Harvard University concluded that an impact from a body at least as large as Mars could have supplied the rough material for the moon and also given our bipartite system its angular momentum.

The "Big Whack" theory was born. The giant-impact hypothesis, as it's more formally known, initially had little impact of its own. If Nature abhors a vacuum, researchers generally abhor catastrophic solutions to geophysical problems, Hartmann says; such solutions are too tidy. So the theory languished for a decade. As Robin Canup, an astrophysicist at the Southwest Research Institute in Boulder, Colorado, put it, "At first it was seen as ad hoc, probably unlikely, possibly ridiculous." But in 1984, a seminal conference devoted to the moon's origin that was held in Kona, Hawaii jumpstarted research.

Some scientists believe a meteor two to three times the size of Mars slammed into Earth, providing the raw material for the moon.

Canup, for one, leapt in with both feet beginning in the mid-1990s. Cameron's models had left off after the giant impact, when a debris cloud from which the moon would arise formed around Earth. She extended the modeling from debris cloud to finished moon. Canup's calculations showed that most debris from the collision would either fall back to Earth or fly off into space, leaving only 20 to 50 percent to make a moon. The Big Whack, she figured, required a much bigger whacker—one two to three times the mass of Mars. But that resulted in an Earth spinning at two to two and a half times its present angular momentum. She addressed that problem by introducing a Big Whack II: a second impactor that hit Earth against the grain of our planet's rotation millions of years after the first, thus slowing its spin.

In contrast to the Big Three, the Big Whack stands up nicely against what we now know of the moon. According to theoretical models, the impact would have destroyed the impactor, sending most of its remains, along with huge amounts of the Earth's mantle, into an Earth-orbiting debris cloud that ultimately coalesced into the moon. This would explain the reduced density of the moon, which is believed to be composed of two-thirds impactor and one-third Earth mantle. And it explains its tiny core: Since the models suggest that all of the impactor's core wound up in the Earth's core, the moon must have got its core iron from later, smaller impacts. By the same token, Earth got its additional volatile elements from later impacts from comets and carbonaceous meteors. Finally, the Big Whack can account well for the Earth-moon's angular momentum and even our planet's odd, 23.5-degree tilt off the ecliptic plane (the invisible platter on which nearly all planets orbit the sun).

While the Big Whack currently leads theories of the moon's origins, our satellite will always remain tantalizingly enigmatic.

While currently the frontrunner, the Big Whack needs more work. Many would like to see it account somehow for the oxygen-isotope similarity, which, by definition, would seem to argue against an impactor formed elsewhere in the solar system. Canup, for her part, has a running list of research questions she'd like to see addressed. These include: Make the Big Whack model work with just a single impact, rather than the more ad hoc multiple. Explain the formation of Charon, Pluto's moon, which scientists have postulated might also have been the offspring of a giant impact. And finally, chemically match the moon's characteristics with what would have happened in the proto-lunar debris cloud.

"If we can get to a point where we can naturally explain with our theoretical models the chemical signatures and elemental abundances in the lunar material," Canup says, "to me that's the nail in the coffins of the other theories." Cameron concurs. "Quite independently of the giant-impact theory," he says, "they were sealed long ago."

Peter Tyson is Online Producer of NOVA.

Photos: NASA

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