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