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And the Moon Be Still as Bright: How Was Luna Formed?

Some months ago we had a long back-and-forth discussion about the origin of the Moon--the popular theory-of-the-day being that the Moon was created after a Mars-sized planet whacked the Earth and sent a blob of molten stuff into the heavens.  Well here's an interesting article published last year. 

Personally,  I lean toward the idea of the article's closing sentences which states "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... Earth is a very dynamic body, and it may have wiped out any traces of the impact... Earth's geochemistry remains a potential stumbling block for the giant impact origin of the moon."  Perhaps the Moon holds clues?  (I'd have to say that despite the Apollo and Luna samples returned to Earth, the Moon's surface geology has barely been scratched!  And our understanding of its deep interior? Poor!) 

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Full content for this article includes photograph and illustration.
                                                                              
   Source:  Astronomy, Sep 1998 v26 n9 p40(6).
                                                                              
    Title:  Deconstructing the moon.(origin of Earth's moon)(Cover Story)
   Author:  Ray Jayawardhana
                                                                              
 Abstract:  One popular theory of the moon's origin is that 4.5 billion years
ago it was a chunk of rock that collided with the forming earth. There are
several other theories, but those do not account for the Earth-moon spin
system and the moon's lack of an iron core. The collision theory is not yet
perfected.
                                                                              
 Subjects:  Moon - Origin
            Geophysical research - Analysis
                                                                              
  Magazine Collection:  94M3923
Electronic Collection:  A21003241
                   RN:  A21003241
                                                                              

Full Text COPYRIGHT 1998 Kalmbach Publishing Company

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 geophysics. 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 why the moon has very little iron; after 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 sideswiping 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.

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.
                                                                              
                                -- End --



LOUIS VARRICCHIO
 Environmental Information Specialist &
 Producer/Writer, "Our Changing Planet"
  (Visit OCP-TV on the Web at: www.umac.org/ocp)
  Upper Midwest Aerospace Consortium
  Odegard School of Aerospace Sciences
  University of North Dakota
  Grand Forks, N.D. 58202-9007  U.S.A.
    Phone: 701-777-2482
    Fax: 701-777-2940
    E-mail: varricch@umac.org (in N.D.); morbius@together.net (in Vt.)

"Behind every man alive stand thirty ghosts, for that is the ratio by
which the dead outnumber the living. Since the dawn of time, a hundred
billion human beings have walked the planet Earth." -- Arthur C. Clarke

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