Moon Child

This Boulder ballerina took a whack at the Big Whack theory -- and one of science's biggest mysteries.

It came from outer space.

Four and a half billion years ago, when the solar system was a young, violent and messy place, a hot and nasty planet hailing from somewhere between Mars and Venus took a detour. Traveling at 11 kilometers per second, it screamed toward one of its neighbors, another hot and nasty planet much larger than itself: earth.


James Bludworth
When Robin Canup wasn't pursuing her love of ballet, modeling the Big Whack theory kept her on her toes.
When Robin Canup wasn't pursuing her love of ballet, modeling the Big Whack theory kept her on her toes.

The marauder clipped the top of the earth, shearing away layers of mantle and spewing a jet of vaporized rock, liquid metal and pulverized debris. Then, drawn back by the gravitational pull of the now-deformed earth, the planet slammed down again, splashing even more rubble into space and destroying itself. Within thirty hours, the dust began to settle. The earth was left spinning at the accelerated rate of five hours a day, and the remaining chunks of the errant planet slowly merged into its molten surface.

In orbit, a cloud of gas, powder and rubble began to form, like a hotter version of Saturn's rings. In time, much of this debris fell toward the proto-earth, while the rest cooled, bumped around and merged into larger and larger clumps until finally, only a single satellite remained. And this is how the moon was formed.

Or not.

In February 2001, Robin Canup, an intense, enthusiastic and occasionally goofy 33-year-old astrophysicist, sat at her desk in Boulder, contemplating that scenario and preparing to give a talk about the origins of the moon. She'd been working in the field for only eight years, but already she'd become one of its leading theorists. She found the scientific mysteries "romantic" and the complex equations "centering."

For centuries, the world's brightest minds had pondered this puzzle. A mid-'70s theory about a collision made the most sense, but no one could make all of the pieces fit. If something hit the earth, then how big was it? How fast was it moving? What was the angle of impact? Would there be enough debris in orbit to form the moon? And if so, how did the moon actually clump together?

As she sat in her office that winter afternoon, studying results from forty computer simulations of the collision, Canup began to doubt.

"Gosh," she thought. "This is frustrating. None of these is right."

She checked one chart, then another.

"You know, we're looking at this the wrong way."

She ran a finger down a graph.

"If we go by the earth and moon system we have today, what different impactor size would that imply?"

She scribbled an equation on the bottom of her speech notes.

"That's weird."

She checked her calculations for accuracy.

"That can't be right. Surely someone must have looked at that. Something must be off. But I'll just start a simulation to see what happens."

She punched the numbers into her computer, hit "Enter" and launched a massive three-dimensional representation of how the crash may have happened.

A day and a half later, her computer finished the program. Canup reviewed the results.


She rang William Ward, her colleague, friend and trusted sounding board.

"You're not going to believe this."

Taylor's Axiom: "The best models for lunar origin are the testable ones."

Taylor's Corollary: "The testable models for lunar origin are wrong."

-- Scientist paraphrasing fellow scientist Stuart Ross Taylor at 1984 conference (from Origin of the Moon)

Humans have always gazed up at the night sky and contemplated the silvery face of the moon. Ancient cultures ranging from Mesopotamian to Egyptian to Chinese had theories about what it was and where it had come from. Some simply proclaimed it a god.

Through scientific reasoning, philosophical discourse and flat-out guesswork, the Greeks determined that the darker portions of the moon's surface were oceans and the lighter parts were land. They thought people lived there, too.

In 1610, science took a great leap forward when Galileo peered through a telescope, made a few detailed illustrations and pronounced that the darker and lighter regions of the moon weren't land, water or even cheese, but mountains and valleys.

Nearly two centuries later, scientist Pierre-Simon Laplace offered the proposition that the solar system evolved from a rotating nebula of gas and dust, which implied that the earth and moon formed simultaneously and stayed bound by gravity.

That made sense to many scientists until the late 1870s, when George Howard Darwin, son of the evolutionist, came up with another theory: During its formative years, the largely molten earth was spinning so fast that a large, unstable lump formed at the equator; eventually, it was ripped away by the sun's gravity and tossed into space, where it became the moon. Four years later, geophysicist Osmond Fisher bolstered this concept by suggesting that the Pacific Basin was actually a scar left from the errant lump.

In 1909, however, Thomas Jefferson Jackson See rejected the lump idea, suggesting that the moon was a wandering planet from another part of the solar system that had been captured by the earth's gravity.

None of those theories, which came to be known as the Big Three, was entirely convincing, but in the absence of better explanations, all three remained dominant well into the twentieth century.

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