by Peter Tyson
Whence our moon? Was it a chunk of Earth flung off in our planet's early
history? Did the Earth capture a small, roaming planet in its gravity grip? Or
did the moon fashion itself alongside our world from the same planetary batter?
One of the Apollo program's chief scientific goals was to give lunar
researchers the means to decide, once and for all, between these three main
theories of how the moon formed.
What transpired in this "battle of the Big Three" after the last Apollo
mission flew in 1972 surprised just about everyone. The story provides a
revealing glimpse of the workings of the scientific process, while at the same
time opening a window on the origins of what one lunar researcher has called
"one of the most peculiar bodies in the solar system"—the moon.
The Big Three
Human beings have surely wondered about the moon since they had brains big
enough to do so. Many cultures, from ancient times to the present day, have
even worshipped it as a deity. The Greeks were perhaps the first to study our
satellite scientifically. Using Earth's shadow on the moon during lunar
eclipses as a guide, the third-century B.C. astronomer Aristarchus estimated it
lay 60 Earth radii away. (It was a remarkable guess: in fact, the distance
varies between 55 and 63 Earth radii, or 220,000 and 250,000 miles.) The
biographer Plutarch went so far as to posit that people lived on the moon,
whose dark regions, the Greeks thought, marked oceans and the bright areas
land. Their belief survives in the Latin names—maria (seas) and
terrae (lands)—by which we know these dark and light regions.
Modern scientific study of our neighbor began in 1610, when Galileo, training
his spyglass on the moon, became the first person to see the dark and light
regions for what they really were: vast plains and rugged mountains,
respectively. Galileo's famous trial for heresy—for insisting that the Earth
revolved around the sun rather than vice verse—apparently kept Descartes
from publishing one of the first theories about the origin of the moon until
1664, long after his own death. (His theory was essentially an early version of
the planet-capture theory.) Descartes left a fuller explanation for others,
admitting "I have not undertaken to explain everything."
In the 1870s, Charles Darwin's son proposed that the Earth flung off a
portion of itself that became the moon.
The first moon-origin theory to gain a solid foothold was put forth in 1878.
That year, George Howard Darwin, son of the famous evolutionist, proposed that
Earth spun so rapidly in its early years that the sun's gravity eventually
yanked off a chunk of an increasingly elongated Earth; that chunk became the
moon. Four years later, the geologist Osmond Fisher added a juicy addendum: The
Pacific ocean basin marks the scar left behind where our future satellite
ripped away. The so-called "fission" theory became the accepted wisdom well
into the 20th century, as this quirky, 1936 U.S. Office of Education script for
a children's radio program attests:
FRIENDLY GUIDE: Have you heard that the moon once occupied the space now filled
by the Pacific Ocean? Once upon a time—a billion or so years ago—when the
Earth was still young—a remarkable romance developed between the Earth and
the sun—according to some of our ablest scientists . . . In those days the
Earth was a spirited maiden who danced about the princely sun—was charmed by
him—yielded to his attraction, and became his bride . . . The sun's
attraction raised great tides upon the Earth's surface . . . the huge crest of
a bulge broke away with such momentum that it could not return to the body of
mother Earth. And this is the way the moon was born!
The Darwin-Fisher model eventually met with competition from two other
theories. In 1909, an astronomer with the all-American name of Thomas Jefferson
Jackson See proposed that the moon was a wandering planet that had been snared
by Earth's gravity, like a fly in a spider web. The third theory, advocated by
the astronomer Edouard Roche among others, was coaccretion. In this model, the
Earth and the moon formed independently, side by side as it were, from the same
material that formed all the planets of our solar system.
GIRL: How exciting!
Some clever scientist eventually dubbed the Big Three "daughter" (fission),
"spouse" (capture), and "sister" (coaccretion). Which family member would win
In the years after Gene Cernan, shown here
driving the Apollo 17 Lunar Rover, brought back a final batch of moon rocks
(see Last Man on the Moon), our understanding of the moon grew by leaps
By the end of the Apollo program, lunar scientists had elucidated many aspects
of the moon's history, giving them clues unavailable to the likes of Darwin or
See. Selenology, the study of the origin of the moon, had taken off. Most of
the new evidence came from the more than 800 pounds of moon rocks retrieved by
the American and Russian lunar missions.
In many ways, the moon turned out to be quite different from Mother Earth.
Anybody can see that, of course: It's airless, colorless, lifeless. But the
differences run deeper. It is compositionally different, with fewer volatile
elements—those that tend to boil off at high temperature. The moon might
have inherited such differences—maria rocks contain no water, for instance,
unlike volcanic rocks on our planet—from the impactor. The lunar samples
also suggest that much of the moon may have once been molten; no definitive
evidence exists that the Earth ever melted to such a degree. And while
one-quarter its size, the moon has but one percent of our planet's mass, and
its density more closely resembles that of Earth's mantle rather than the
planet as a whole. Lunar scientists in the immediate post-Apollo years
explained these discrepancies by postulating that the moon had but a tiny core.
In 1998, the Lunar Prospector, NASA's first mission to the moon since Apollo,
confirmed that the moon's core indeed comprises less than three percent of its
mass. (By contrast, Earth's core represents 30 percent of its mass.)
In other ways, the Earth and moon have remarkably similar characteristics.
Studies of radiogenic elements and isotopes in lunar rocks reveal that the two
bodies are roughly the same age, 4.5 billion years old. They also came from the
same neighborhood: Unlike those in all meteorites ever analyzed, the
nonradioactive, stable isotopes of oxygen in moon and Earth rocks match like
blood types, implying the two spheres formed at the same radial distance from
the sun. Indeed, results from Apollo showed the pair to be more intimately
connected than previously thought. "Apollo tied together for the first time the
history of the moon with the history of the Earth," says William Hartmann of
the Planetary Science Institute in Tucson, Arizona. "It showed us that we live
in a system, the Earth-moon system."
In many ways, the Earth is remarkably similar to its lifeless
In fact, it's a pairing unlike any other in the solar system. Our moon is far
more massive relative to Earth, for example, than the satellites of all other
planets save Pluto (whose moon, Charon, is half its size). The Earth-moon
system also has an unusually high angular momentum—that is, the sum of the
our planet's rotational velocity and the speed at which the moon orbits the
So how do the Big Three stand up in the face of all the new evidence? Not
well, it turns out. The fission theory might explain the moon's lack of a large
core and the oxygen-isotope similarity, astronomers say, but calculations show
that the Earth would have to have had four times its present angular momentum—a lightning-fast rotational speed that astronomers cannot square in their
models. Add to that the understanding reached decades ago that the Pacific
basin formed less than 70 million years ago and therefore could not possibly
have spawned the moon, and the Darwin-Fisher model suddenly comes up short.
See's capture theory suffers as well. The idea that Earth's gravity caught a
rogue planet might explain the compositional differences between the two
bodies. But, then, why doesn't the moon have its own regular-sized core? And
why the oxygen-isotope similarity if the two formed in different parts of the
solar system? Finally, most modelers deem the chance that a speeding planet
would gracefully ease into Earth's embrace rather than slam into it or career
off into space too remote for consideration.
Coaccretion led the pack through the 1970s, because, for one thing, it doesn't
require a low-probability event like capture. But today it faces the same
problem regarding the core. As Hartmann says, "It's very hard to imagine the
two bodies growing together but somehow the Earth magically gets all the stuff
with the iron in it and the moon doesn't get any." Even more troublesome,
experts say, the theory cannot account for the enormous angular momentum we see
in the Earth-moon system today.
Continue: The Big Whack
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