Title: Deconstructing the moon.
Subject(s): MOON; DARWIN, Charles; CAMERON, Alastair
Source: Astronomy, Sep98, Vol. 26 Issue 9, p40, 6p, 8c
Author(s): Jayawardhana, Jay
Abstract: Looks at the historical background of the moon. How Charles
Darwin formulated a model for the origin of the moon in 1878; Theories
formulated by Alastair Cameron about the origin of the moon; Other
scientific explanations about the origin of the moon.
AN: 913098
ISSN: 0091-6358
Note: Tucson-Pima Public Library subscribes to this magazine.
Database: MasterFILE Elite

DECONSTRUCTING THE MOON

If astronomers are right, the moon was formed in a catastrophic impact
some 4.5 billion years ago.

Imagine you have stepped into a time machine and gone back in time 4.5
billion years to see the solar system just 50 million years after the
sun formed. Chunks of rock that haven't been swept up into planets are
still chaotically flying about. You see that one of the largest
chunks, about the size and mass of Mars, is on a collision course with
the still-forming Earth. Suddenly, this rogue planet slams into Earth
at some 25,000 miles per hour, violently launching a huge plume of
material into space. In the throes of the collision, Earth's
primordial atmosphere boils off into space, and its mantle melts into
an ocean of magma. Within hours, the material blown into space forms a
beautiful ring around young Earth, and debris in the ring starts
clumping together. From a catastrophic event -- the most violent
impact Earth has ever suffered -- the moon is born.

This is the emerging consensus among scientists attempting to unravel
the genesis of Earth's cosmic companion. Although recent research in a
variety of fields has bolstered and refined this picture, scientists
still lack a complete understanding of how the moon formed. "The past
several years of work have told us that it's not as simple as we
thought 10 years ago. For instance, maybe more than one giant impact
is needed," says Robin Canup of the Southwest Research Institute in
Boulder, Colorado, who models the moon's formation on computers.

Scientists have mulled over different ways of making the moon for more
than a century. George Darwin, son of the famous naturalist Charles
Darwin, was among the first to put forth a model for the moon's
origin. In 1878, he suggested that a newly-born, still-molten Earth
started spinning faster and faster until it threw off a piece of
itself as big as the moon, sort of like a merry-go-round spinning out
of control and sending a rider flying. But Darwin's model fails a
critical test; it can't explain the total spin rate -- a quantity
physicists call "angular momentum" -- of the Earth-moon system. As
Harvard University planetary scientist Alastair Cameron points out, if
Darwin's model were true, "Both the Earth and moon would have to be
spinning four times faster than they actually are."

A second theory suggested that the moon assembled itself independent
of Earth from primitive rocks and dust, just as other planets in the
solar system did. "If that happened, both would have about the same
percentage of metallic iron in their bodies," Cameron explains. But
the model didn't hold up. Data from seismic instruments left behind by
astronauts suggest that the moon's core has an iron deficit.

A third possibility is that the moon formed elsewhere in the solar
system and was later captured by Earth's gravity. Cameron's
calculations show this scenario to be virtually impossible; it's much
more likely that the moon would have either hit Earth directly or
received a gravity kick that set it flying off into deep space. What's
more, according to Cameron, the capture scenario "still leaves you
with the iron problem. A body as big as the moon that formed from the
same material as other planets would have an iron core like Venus,
Earth, and Mars."

By the early 1970s, it was clear that all three of the most popular
existing theories for the moon's origin had serious flaws. Thus, along
with William Ward, a former Harvard University colleague who now works
alongside Canup at the Southwest Research Institute in Boulder,
Cameron came up with a new idea that could explain both the moon's
composition and the total spin rate of the Earth-moon system. They
reckoned that the moon formed from the debris of a giant impact
between Earth and a roaming planet roughly the size of Mars.

At about the same time, William K. Hartmann and Donald Davis of the
Planetary Science Institute in Tucson, Arizona, had come to the same
conclusion from an entirely different direction. Following up on the
ideas of Russian scientist Victor Safronov, they estimated that there
had been bodies near the newly formed Earth large enough to blast out
enough mantle to make the moon. The two groups learned about each
other's work at a planetary science conference in 1974.

For a decade, both groups' models received scant attention, because,
as Hartmann explains, "Other researchers had been taught to abhor
catastrophes as a mode of explanation in geo- physics. They felt that
the giant impactor had literally been conjured up out of the blue."
Finally, at a 1984 conference on the origin of the moon, the "giant
impact model" -- more popularly known as the "Big Whack" -- took
center stage. By then, most researchers had come to believe that
collisions among hundreds of planetesimals -- some as big as the moon
-- were required to build up the planets to their present sizes. In
addition, it was clear that none of the other theories for the moon's
origin could account for its composition and the total spin of the
Earth-moon system.

Over the years, Cameron and his collaborators, as well as a team led
by Jay Melosh of the University of Arizona in Tucson, have performed
dozens of computer simulations of the impact and its aftermath. Their
simulations require an impactor about the size of Mars with roughly a
tenth of Earth's mass to leave the Earth-moon system with the right
amount of spin. In each simulation, the impactor is destroyed, and a
plume of rock, magma, and vapor is boosted into Earth orbit.
Occasionally, a fairly large rocky moon is formed. The impactor's iron
core falls onto the deformed proto-Earth and sinks to its center. That
explains why the moon has very little iron; after all, the debris that
went into it came from the rocky mantle material of the impactor and
Earth. The model can also account for the lack of water and other
volatile compounds on the moon. Because the giant impact heated the
ejecta to high temperatures, the volatiles would have escaped into
space as gases.

Computer simulations of the giant impact that Cameron and others have
been doing usually end once a hot disk of material has just formed
around Earth, only hours after the impact occurred. So, how does all
that stuff come together to make the moon? That's the question Robin
Canup set out to answer. In two papers, one with Larry Esposito of the
University of Colorado, and the second published in the September 25,
1997, Nature with Shigeru Ida of the Tokyo Institute of Technology and
Glen Stewart of the University of Colorado, Canup presented the first
simulations of how that debris disk coalesced into the moon.

Canup and her colleagues started their simulations where Cameron's
simulations ended, once the debris has cooled down and formed swarms
of individual particles of varying sizes, a process that could take up
to 100 years after the impact. In 27 different models, the researchers
varied the number and the sizes of the particles, and followed them up
to see what happened. In every case, the particles invariably clumped
together to form one or two moons in less than a year at a distance of
about 14,000 miles (22,500 km) from Earth. The particles in the outer
disk clumped together pretty easily, but those in the inner regions
could not form big clumps because Earth's gravity pulled them apart
much like Saturn's gravity pulls apart ring particles when they
collide. "Once the particles in the outer disk accreted to form the
moon, its gravitational forces likely scattered the inner disk
material back on to Earth," explains Canup. "That means an initial
disk mass of two to five lunar masses is required to yield the moon."

That, in turn, implies a larger impactor than previously suggested
--one with about three times the mass of Mars. "The problem with this
requirement is that such an impact also produces an Earth-moon system
with two to two and a half times the angular momentum of the current
Earth-moon system," Canup points out. "The laws of physics tell us
that the angular momentum of the Earth-moon system has been very
nearly conserved over the past 4.5 billion years."

In one third of the simulations, two moons form instead of one. "That
would have been quite a sight," notes Canup. But two moons would grace
Earth's sky for only a brief time. In every one of the trials, either
the inner moon crashes back to Earth or the two moons collide in only
1,000 to 10,000 years.

At the time of its birth, the moon was much closer to Earth than it is
today. Canup's simulations suggest it formed about 14,000 miles from
Earth, whereas it is now orbiting at an average distance of 239,000
miles (380,000 km). Gravitational interactions between Earth and the
moon give rise to tidal forces that push the moon outward. These tidal
forces were much stronger when the two bodies were closer together, so
the moon receded at a faster rate. Within a few hundred million years
of its birth, the moon had already moved out to half its present
distance. It is still receding, as confirmed by radar reflectors left
by astronauts, but at a slower-than-snail's pace of about 1.5 inches
(4 cm) per year.

While these simulations can form a moon of the right size and
composition, the nagging angular momentum problem remains. One way
around this problem is to invoke a second large impactor, about the
size of Mars, smashing into Earth millions of years after the first
one. Big Whack II could have altered Earth's rotation rate enough to
reset the angular momentum of the Earth-moon system. While this
scenario could potentially explain all the characteristics of the
Earth-moon system, both Cameron and Canup consider it too ad hoc for
their tastes.

To get around the angular momentum problem, Cameron and Canup went
back to the drawing board. In Cameron's most recent simulations, which
he presented this past March at a planetary science conference in
Houston, Texas, the impactor delivers a glancing blow. After
side-swiping Earth, part of the impactor survives. It slows down and
swings halfway around Earth and hits a second time. This double
whammy, it seems, is necessary to blast enough material into Earth
orbit to form the moon. The collision ejects a long tail of material
into Earth orbit, with a sizeable blob at the tail's far end (the blob
is an intact piece of the impactor that's thrown into Earth orbit).
"That blob is the principal seed for growing the moon," Cameron
explains.

Cameron's new results suggest that the impact occurred fairly early in
Earth's history, before our planet was fully assembled. If Earth were
more than about half its current mass at the time of the collision, an
impactor that could yield our moon would have left the Earth-moon
system with too much angular momentum. An impactor with twice the mass
of Mars and a proto-Earth with half its present mass are the
ingredients Cameron's recipe needs to provide enough mass for forming
the moon while still leaving the Earth-moon pair with the right amount
of angular momentum.

This scenario does raise one concern, however: If Earth were much
smaller back then, how did it grow to its present mass? The simple
answer is that Earth accreted much of its mass after the moon formed.
But then the moon would have accreted quite a bit of mass as well and
should have more iron than it actually does. "That's an issue we need
to address in greater detail," Cameron concedes.

When was the moon born? To make an estimate of the moon's birthdate,
University of Michigan geochemist Alex Halliday and colleagues studied
the amounts of two elements -- hafnium and tungsten -- in 21 lunar
samples brought back by Apollo astronauts. Hafnium radioactively
decays into tungsten with a half-life of nine million years. By
measuring the ratio of the two elements in moon rocks and comparing
that to their ratio in primitive meteorites, the researchers could
estimate the time that elapsed between the formation of the solar
system and the birth of the moon. In a paper published in Science last
November, Halliday and coworkers reported that the moon was assembled
a mere 50 million years after the solar system itself was born 4.6
billion years ago. Previous estimates had placed this event 40 million
years earlier.

Canup and Craig Agnor of the University of Colorado have conducted
several simulations of planetary growth that have shown that large
impacts tended to occur about 50 to 90 million years after the solar
system formed. "All the pieces are fitting together. The timing from
the computer simulations seems to agree fairly well with the
geochemical evidence," says Canup.

A giant impact also implies a melted early Earth. The energy released
when a Mars-sized body collided with Earth must have been enormous.
The resulting heat was more than enough to blow away Earth's
primordial atmosphere into space. In addition, Earth's mantle must
have been heated to several thousand degrees Celsius, melting it into
a so-called magma ocean. But, as Michael Drake of the University of
Arizona's Lunar and Planetary Laboratory points out, "Earth doesn't
show any credible evidence of ever being melted." If there once was a
magma ocean, as it cooled, different minerals would have frozen out
first and either risen to the top or sunk to the bottom. That should
have altered the composition of the mantle. Geochemists like Drake
have found no signs of such alterations. However, Drake is quick to
add that "absence of evidence is not evidence of absence."

According to Halliday, "It's not clear that any evidence of the impact
would have been preserved in Earth's mantle . . . All that memory
might have been erased by four billion years of geologic activity."
Cameron agrees: "Earth is a very dynamic body, and it may have wiped
out any traces of the impact." Still, Earth's geochemistry remains a
potential stumbling block for the giant impact origin of the moon.

Although the computer simulations still can't completely explain all
the characteristics of the Earth-moon system, the giant impact model
is now the runaway favorite scenario for the birth of the moon. After
all, it can account nicely for the moon's makeup and the spin of the
Earth-moon system -- two of the most important criteria that any model
must meet. As Drake puts it, "Not all the details have yet been worked
out, but the giant impact model explains the main features of both the
Earth and moon." And that means, according to Cameron, "giant impact
is pretty much the only game in town right now" for those who worry
about the moon's origin.

PHOTO (COLOR): Before Earth was fully formed, a large body smashed
into it at a grazing angle, leading to the moon's origin.

PHOTO (COLOR): Due to geological activity and wind and water erosion,
Mother Earth shows no trace of the catastrophic impact that gave birth
to the moon 4.5 billion years ago. Galileo took this portrait of our
home planet during its December 1990 flyby.

PHOTO (COLOR): The moon appears tranquil today, but its origin was ...

The moon appears tranquil today, but its origin was anything but.
Galileo took this image of the moon's Western Hemisphere in December
1990, showing parts of the near side and far side. The 600-mile-wide
Orientale Basin is the bull's-eye in the middle.

PHOTO (COLOR): A computer simulation models how a giant impact
sideswiped ...

A computer simulation models how a giant impact sideswiped Earth,
blasting enough material into orbit to form the moon.

PHOTOS (COLOR): Immediately after the giant impact, a disk of debris
forms around Earth (top). In just one year, the material blasted into
orbit accretes to form the moon at a distance of about 14,000 miles
from Earth (center); most of the remaining debris falls back to Earth.
After the moon forms, tidal interactions with Earth cause the moon to
slowly spiral away (bottom). By 100,000 years after the impact, the
moon is about 40,000 miles from Earth.

PHOTO (COLOR): Cosmic companions whose origins are inextricably linked
together: Galileo ...

Cosmic companions whose origins are inextricably linked together:
Galileo snapped this image of Earth and the moon en route to Jupiter.
The moon's brightness has been artificially enhanced.

~~~~~~~~

by Ray Jayawardhana

Contributing editor Ray Jayawardhana, a graduate student at Harvard
University, studies star formation. He was the leader of a team that
recently imaged a possible planet-forming disk around the young star
HR 4796A. His previous feature article for Astronomy, "NASA's Next
Space Observatories," appeared in the January 1998 issue.
_________________

Copyright of Astronomy is the property of Kalmbach Publishing Co. and
its content may not be copied or emailed to multiple sites or posted
to a listserv without the copyright holder's express written
permission. However, users may print, download, or email articles for
individual use.
Source: Astronomy, Sep98, Vol. 26 Issue 9, p40, 6p, 8c.
Item Number: 913098
_________________________________________________________________