July 20, 1999
Moon Landings Rock Earlier Theories About Planets
Science was low on the agenda when two
Americans landed on the Moon 30 years ago
today, beating the Russians in one of the
cold war's most costly and daring contests.
But in time, the six lunar landings from 1969
to 1972, in which a dozen people roamed
another world for the first time, probing its
composition and taking hundreds of rocky
samples, led to a major surprise.
For centuries scientists had assumed that
the forces of nature were generally uniform,
that comets and mountains, cats and molecules, all obeyed laws rooted in steadfastness. Newton and his intellectual heirs often
saw the universe as a vast clockwork in
which precise gears turned with slow deliberateness.
But the 842 pounds of Moon rocks that the
Apollo astronauts painstakingly gathered
and brought back to Earth told a different
Unforeseen clues led scientists to conclude that the Moon was born in cataclysm
and searing heat. In time, evidence of similar violence was seen in relation to Mercury,
Venus, Mars and other worlds that were
oddly built, off kilter or spinning fast. Routine fury from the heavens, scientists eventually argued, had even altered life on
Earth, periodically wiping out families of
plants and animals, including the dinosaurs.
In hindsight, scientists and historians now
say that the lunar landings helped reveal the
secret of cosmic violence, opening a line of
inquiry still advancing today.
"We've learned that big impacts were an
important part of planetary formation,"
said Dr. David Morrison, head of the space
directorate at the Ames Research Center of
the National Aeronautics and Space Administration. "Almost every planet shows a
Apollo evidence of a violent lunar birth,
said Dr. Stephen G. Brush, a science historian at the University of Maryland, who has
written about the episode in "Fruitful Encounters" (Cambridge University Press,
1996), "caused people to suddenly start
thinking about ideas that were out of bounds
"There was a general change in thinking
about catastrophic events," he said.
Throughout history, leaders of science
tended to hunt for uniformity in nature and
to abhor chance events.
As Einstein put it, "God does not play
dice." For instance, the founders of paleontology saw periodic mass extinctions of ancient life as rooted in natural forces acting
over millions of years. Their rivals, the
catastrophists, inspired by religion, laid the
die-offs to supernatural upheavals like Noah's flood.
So too, planetary scientists. Working from
assumptions of uniformity, they envisioned
the Sun and its family of worlds as forming
slowly over eons.
When Dr. Immanuel Velikovsky argued
otherwise in his 1950 book, "Worlds in Collision," taking biblical and archeological evidence to wild extremes, scientists battled
him in print and threatened to stop buying
textbooks from Macmillan, his publisher.
Macmillan quickly sold the book's rights to
Doubleday, which found it had acquired a
Before the Apollo landings, scientists
backed three rival theories of lunar origin.
All were rooted in uniformitarianism, as the
idea was known. And all had nicknames --
the sister, the daughter and the pickup theories.
Sister had the Moon and Earth evolving
separately from primordial rubble but in
the same general neighborhood, as siblings.
The daughter theory had the early Moon
spinning off from Earth by fission, leaving
the Pacific basin as a scar. Pickup saw the
Moon as a small wandering planet that
Earth's gravity had captured.
The theorizing began to unravel on July
20, 1969, when Neil Armstrong and Buzz
Aldrin landed on the Moon.
"It looks like a collection," Col. Aldrin
radioed back to Earth. The moonscape
showed "just about every shape, angularity,
granularity -- about every variety of rock
you could find."
Hundreds of small craters and outcroppings lay before the astronauts. In bulky
spacesuits, they probed the site for 2 hours
21 minutes, at the end hammering coring
tubes through dusty soil to capture deeper
rock. The next day, July 21, the explorers
blasted off for home, bringing back samples
that scientists handled like priceless jewels.
The Moon rocks were full of surprises.
Experts found that the samples had little
iron, water or other elements common on
Earth, like sodium, potassium and sulfur.
That finding diminished the sister theory,
because no scientist could see how such
stark differences might arise.
Yet the inquiries also found that the Moon
and Earth were similar in terms of their
mixes of oxygen isotopes, or light and heavy
oxygen atoms. That virtually ruled out the
pickup theory, because studies of meteorites had shown that extraterrestrial objects
tended to have unique isotopic signatures.
The daughter theory died on its own without the aid of the Apollo findings. Studies of
planetary dynamics showed that Earth
could never have spun fast enough to give
As the mystery of the Moon's origin deepened, scientists were not only confused and
frustrated, but dwindling in number as Apollo money in the early 1970's dried up. Still, a
Dr. William K. Hartmann was a young
astronomer who in the 1960's at the University of Arizona had peered through telescopes to scrutinize the pockmarked Moon.
Like other scientists, he joined the debate
over whether the origin of the craters was
slow and volcanic or quick and violent.
The Apollo program settled the issue in
favor of meteorite crashes, as Dr. Hartmann had suspected. The astronauts found
that the craters bore no signs of flowing lava
and much evidence of great impact forces
that had welded soil and rubble into solid
In 1972, as the Apollo landings ended, Dr.
Hartmann was inspired to extend his impact reasoning after reading the translation
of a book by a Soviet scientist, Victor S.
Safronov. He had speculated that cosmic
dust and rubble spinning around the primordial Sun had repeatedly collided and melded
together, forming ever larger aggregations,
like swarms of asteroids, from which the
planets eventually arose.
Dr. Hartmann went further. Working at
the Planetary Science Institute in Tucson,
Ariz., with Dr. Donald R. Davis, he reasoned
that even larger bodies had formed, perhaps
dozens of them.
One of these might have begot the Moon,
Dr. Hartmann and Dr. Davis found in their
analyses. Some four and a half billion years
ago, they theorized, the speeding wanderer
collided with the early Earth and shattered
to smithereens. The planetoid's heavy iron
core burrowed into the Earth. Its light
crustal remains, as well as bits of Earth's
crust, were blown into space and then
formed an orbiting ring of debris, from
which the Moon in turn coalesced.
The searing heat of such a collision, the
Tucson scientists reasoned, could explain
why the Moon held no water and few volatile
elements. All had boiled away. And the
Moon's relative lack of iron was explained
by the lost core.
In 1974, at a meeting at Cornell University, Dr. Hartmann presented the radical idea
publicly for the first time. He was nervous.
Afterward, a hand in the audience went up.
It was Dr. Alastair G. W. Cameron, a top
astrophysicist at Harvard, a man some
young scientists greatly admired.
The junior scientist braced for the worst,
ready for a blast of criticism. "I was quaking in my boots," Dr. Hartmann recalled.
Instead, Dr. Cameron agreed, saying that
he and a young colleague, Dr. William R.
Ward, had arrived at similar conclusions by
a different route.
Moreover, the intruder,
Dr. Cameron and Dr. Ward had calculated,
was even bigger than the Tucson team had
"I said it was at least as big as Mars," Dr.
Cameron recalled. So in theory at least, the
hypothetical impactor was a blockbuster
about half the diameter of Earth.
Greatly encouraged, Dr. Hartmann, who
paints in his spare time, began illustrating
the great-impact idea. Eventually, his colorful visualizations appeared in many science
publications, including one reproduction on
the cover of Science Times.
Despite agreement among the four experts, most scientists at first ignored or
disparaged the great-impact idea. It was too
radical, too much like the discredited catastrophism of Velikovsky.
But slowly it caught on, especially as
computer simulations of that distant era of
rocky creation showed that the collision idea
was not crazy.
In 1984, a dozen years after the last Apollo
mission, a conference was held to discuss
the Moon's origins. To attract wide participation, it was held in Hawaii, scientists
liking paradise as much as anybody else. A
hundred or so planetary specialists showed
Surprisingly, a strong consensus emerged
that the radical theory was probably right.
"The conference was incredible," Don E.
Wilhelms, a lunar geologist at the United
States Geological Survey, recalled in his
book, "To a Rocky Moon" (University of
Arizona Press, 1993). Beaches and gentle
breezes were ignored as meeting participants grew increasingly excited over the
idea that "our satellite is a daughter of not
one but two parents."
Dr. George W. Wetherill of the Carnegie
Institution of Washington told meeting participants of his supportive calculations,
which showed that four and a half billion
years ago perhaps 20 or so small planet-sized bodies sped through the inner solar
system, making a large impact quite likely.
His work, Dr. Wetherill said in an interview, showed that sketchy ideas of planetary upheaval were "clearly correct."
The watershed event encouraged scientists to look for signs of violence among
other planets, and led to a flurry of papers.
In time, cosmic tumult was tied to Mercury's lack of a thick crust, to Venus's spinning backward, to Mars' fast rotation and to
Uranus' bizarre orientation. Most planets
have an axis of rotation that is more or less
perpendicular to the plane of the solar system. But the gas giant is tilted, its axis
listing at 97 degrees.
Today, debate on the Moon's origin is still
led by the great-impact theory, with scientists continuing to probe its nuances. In 1997,
Dr. Robin M. Canup, a scientist at the University of Colorado at Boulder, working with
Dr. Cameron of Harvard, suggested the
impactor's mass might have been as large
as three times that of Mars.
In an interview, Dr. Canup, who now
works at the Southwest Research Institute
in Boulder, said her newest computer models show not only that large impacts "were
ubiquitous" in the early days of the solar
system but that moon-forming smashups
were common. Few of these ancient moons
survived, she added, because most were in
Skeptics remain, especially among geochemists. Many find insufficient chemical
evidence to convince them of the cataclysm.
Some would like to find chemical clues of
blistering heats on Earth.
"Although a general picture may be
emerging, many issues remain hotly debated," Dr. Alex N. Halliday, an Earth scientist
in Switzerland, and Dr. Michael J. Drake, a
planetary scientist at the University of Arizona, wrote in the March 19 issue of the
But suggestive new clues are also emerging. NASA's Lunar Prospector spacecraft,
which cost $63 million and arrived in lunar
orbit last year, recently confirmed that the
Moon has a very small core, bolstering the
"This is a critical finding," said Dr. Alan
Binder of the Lunar Research Institute in
Tucson, the lead investigator for Lunar
The spacecraft found that the lunar core
holds less than 4 percent of the Moon's total
mass, and probably 2 percent or less. In
contrast, the Earth's iron core is about 30
percent of the planetary mass. Dr. Binder
said further analysis of Lunar Prospector
data might help "pin down for good" whether the giant-impact model is right.
Dr. Hartmann of the Planetary Science
Institute in Tucson, who helped get the ball
rolling and is now 60, said he hoped astronauts one day returned to the Moon, greatly
expanding the number of rocky samples and
perhaps aiding the idea of the colossal
"I'm always interested in new data," he
said. "There are many places you might