May 18, 2001 Astronomers from the United States, France, and elsewhere may have seen a snippet of the solar system's formation in reverse, while taking the first-ever detailed look inside the disintegrating heart of Comet C/1999 S4 (LINEAR) or "Comet C/LINEAR."
A special collection of reports in the 18 May issue of the international journal, Science reveals that the comet, which exploded while at its prime viewing distance from Earth last summer, had a number of unexpected quirks that should improve scientists' understanding of how comets are born, how they live, and how they die.
Comets are balls of ice and dust and rock that are some of the most pristine objects in the solar system, physically and chemically unaltered since the time of their formation. While some have been known to split into a few pieces, the breakup of a comet's entire nucleus has never been seen before.
Scientists believe that comets may have been some of the building blocks or "planetesimals" that clumped together to form planets. Thus, the 16 chunks observed after the nucleus of Comet C/LINEAR broke apart may have been primordial planetesimals, according to Science author Hal Weaver of Johns Hopkins University.
"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.
The timing of the breakup was particularly serendipitous for astronomers, who had prepared to catch a glimpse of the bright comet. In Science's special research package, six teams of scientists report their findings on the comet's chemical composition, structure, and breakup process, using data from a variety of ground-based and spacecraft instruments.
One of the many unusual things about C/LINEAR is that it was surprisingly low in volatile compounds and seemed to break apart in stages, suggesting that heating by the sun didn't make it explode. Instead, fast rotation, collisions with debris from asteroids, or some combination of both may have made the comet unravel, according to Weaver.
The comet was also depleted in carbon-containing compounds, compared to other known comets. Many scientists believe that comets delivered the carbon-based molecules that were the building blocks for life, but this theory may need to be reconsidered if more comets turn out to be like C/LINEAR.
Following are additional findings of each research team:
* Hal Weaver and his colleagues in the United States, France, Hungary, Chile, Belgium, Germany, and Italy, used the Hubble Space Telescope and the Very Large Telescope, to follow the break up. They identified 16 large fragments, approximately 100 meters across, and a large dust tail.
* M. J. Mumma of NASA Goddard Space Flight Center and his American colleagues used infrared spectroscopy to analyze the organic composition of the comet's nucleus. While the composition of other long period comets is consistent with the theory that they formed from interstellar ices in the cold regions of the outer solar system, C/LINEAR seems to have formed closer to Jupiter, the authors propose. In this warmer environment, volatile hydrocarbons would have been gaseous instead of icy, and would not have been incorporated into the comet.
* Dominique Bockelée-Morvan, of the Observatoire de Paris-Meudon, in France, and her French and American colleagues monitored the comet's gaseous activity and composition using radio spectroscopy. They identified several outbursts before the main breakup event and a rapid decrease afterwards, suggesting that the ejected debris consisted of objects smaller than one meter. Their data also indicate that the nucleus had a unexpectedly homogenous composition.
* Tony L. Farnham of the University of Texas and his colleagues in the United States and Australia monitored the event on ground-based telescopes. They estimate that the nucleus had a radius of at least 0.44 kilometers, and was spinning relatively slowly, completing a full rotation in up to 12 hours.
* J. Teemu T. Mäkinen, of the Finnish Meteorological Institute in Finland, and his colleagues in France and the United States, used the SWAN all-sky camera aboard the SOHO spacecraft to measure water production rates in the coma, the "atmosphere" around the comet's nucleus that grows as the sun causes the ice to sublimate. Like others, this team noticed several initial increases in activity, probably caused successive fragmentation combined with ejection of gas and debris from the freshly exposed surfaces.
* Carey M. Lisse, of the University of Maryland, and his colleagues in the United States and Germany monitored the comet's x-ray emissions using the Chandra X-ray Observatory. While several mechanisms for x-ray production by comets have been proposed in the past, the authors determined that the x-rays were created by charge exchange between the ions in the solar wind and the neutral gases in the coma.
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