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Science

Astronomers were nervous as they trained their instruments on the Comet C/LINEAR last summer. They had staked out lots of valuable observation time on telescopes and satellites, based on the early prediction that the comet would become bright enough to be seen with the naked eye. As the year went on, however, the iceball showed signs that it might fizzle out as it neared Earth. When C/LINEAR finally arrived, the scientists witnessed a show far more exciting than they’d dared to hope for.

As if waiting for an audience, the comet unexpectedly burst apart, exposing its fragile heart as it died a most unusual death.

A special collection of reports in Friday’s issue of the journal Science reveals for the first time that C/LINEAR — formally known as Comet C/1999 S4 (LINEAR) — led an unusual life as well.

The findings may lead scientists to rethink some of their theories about where comets come from and their role in providing some of the essential ingredients for life on Earth.

Comets split apart occasionally, but no other comet on record has thoroughly disintegrated, as C/LINEAR did.

“Most times, we’re just seeing the outer skin boiling off. With Comet C/LINEAR, we were given an opportunity to look inside, break the egg open, and check it out,” said Science author Hal Weaver of Johns Hopkins University.

Comets are conglomerations of ice, dust and rock from the early solar system, whose cores have remained physically and chemically unchanged ever since. They offer some of the best evidence available about the environment in which planets formed.

“Comets are building stones of the planetary system that we can still observe in their almost original state. ... The breakup of the comet’s nucleus releases the fresh material, which everybody in cometary science wants to study,” said Hermann Boehnhardt of the European Southern Observatory, who wrote a commentary accompanying the Science research.

Rewinding planet formation?
The solar system began as a disk of dust and gas swirling around the young sun. In the colder outer regions of the disk, some of the gaseous materials were frozen into icy shards. Over time, the particles clumped together, growing faster as they collided and stuck together in larger clumps called planetesimals.

Continued collisions eventually produced planets, and the leftover comets and other debris were scattered outward toward the periphery of the solar system. Most of the comets congregated either in the Kuiper Belt, which starts around Neptune, or farther out in the so-called “Oort cloud.”

Studying images from the Hubble Space Telescope and the Very Large Telescope in Chile, Weaver and his colleagues found that C/LINEAR broke up into 16 fragments, which then disintegrated. The fragments may well have been primordial planetesimals, according to Weaver.

“We’re very interested in how the planets formed, and figuring out how comets are put together is a very important step in that process. By watching the comet come apart, we were hoping it was like hitting the rewind button and allowing us the opportunity to see how it formed,” Weaver said.

He cautioned, however, that they couldn’t be absolutely sure this is what they saw until they figured out what caused the comet’s breakup.

Did comets deliever life's components?
Curiously, several groups of scientists, each using different instruments to study C/LINEAR’s chemistry, found that the comet had few carbon-containing molecules. Based in large part on the discovery of such molecules on other well-known comets, scientists have proposed that comets delivered to Earth the carbon-based components necessary to the emergence of life.

I

Image: Comet breakup
The Hubble Space Telescope captured this close-up of Comet C/LINEAR's breakup in July 2000. The comet disintegrated into a swarm of "mini-comets" seen here, each with its own tail.
f more comets turn out to be like C/LINEAR, however, that hypothesis may require revision.

“The heavy bombardment of the inner planets by comets may have brought less prebiotic material to the inner solar system than we thought,” said Boehnhardt.

The carbon-bearing molecules found on other comets, such as methane and certain other hydrocarbons, are highly volatile, meaning they only stay solid in extremely cold temperatures. In another of the Science papers, Michael Mumma of NASA’s Goddard Space Flight Center and his colleagues propose that the carbon-depleted C/LINEAR may have formed in a relatively warm environment. The organic compounds would have been in the gas form, too ethereal to stay packed inside a comet.

The Jupiter-Saturn region would have been just the right temperature for such a comet, which is much closer to the sun than the Kuiper Belt or Oort Cloud where most comets probably formed.

Cause of death unknown
Perhaps the weirdest thing of all about C/LINEAR is that it didn’t explode all at once, as scientists would have predicted.

A comet's birth and death
The leading scenario for a comet’s breakup is that all the ice vaporizes quickly as the comet is heated by the sun, and the expansion from solid to gas breaks the nucleus apart. Researchers have also proposed that gravitational pull from a nearby object might be another possible cause.

Several of the Science studies, however, found evidence that the breakup happened progressively, with the comet shedding material during the weeks leading up to its final demise.

Why else might the comet have come undone? Weaver says it may have been an initial collision with an asteroid, or perhaps the comet’s rapid rotation caused it to unravel. Tony Farnham of the University of Texas and his colleagues calculated, however, that the comet may have been rotating relatively slowly.

Image: Comet views
X-ray and optical images show Comet C/LINEAR on July 14, 2000. A: Chandra X-ray image. B: EUVE ultraviolet image. C: Optical ground based image. While the comet had an extensive optical tail, the X-ray and ultraviolet emissions are confined to the hemisphere facing the sun.
The best guess for now is that C/LINEAR’s unusual delicacy was its downfall. The pieces of the comet’s nucleus appear to have been loosely assembled, so that they broke apart soon after the icy “glue” in between began to vaporize.

To learn more, NASA and the European Space Agency are planning a number of visits to comets over the next decade. One mission, called Deep Impact, will probe the interior of a comet by shooting a 770-pound (350-kilogram) copper bullet through its heart.

In the meantime, Weaver and his colleagues are grateful they got a chance to see their comet perform on its own. “C/LINEAR was terrific natural experiment,” Weaver said.

© 2013 American Association for the Advancement of Science

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