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HALE-BOPP- STARDUST MEMORIES
By Sean Henahan, Access Excellence 
WASHINGTON, DC (3/27/97) New studies of the fast-moving Hale-Bopp
comet are providing valuable information on everything from how comets
move, to the origins of the solar system and life on Earth.
Cover Caption: Comet Hale-Bopp on 20 August 1996 observed from the European Southern Observatory, Chile, during the monitoring campaign to study its activity evolution. "The increased understanding afforded by this very bright comet [Hale-Bopp] bears not only upon the origin of these icy transients from beyond the planetary region of the solar system but upon our own origins as well," said Dale .P. Cruikshank, NASA Ames Research Center, Moffett Field, CA, in a commentary of a special comet issue of the journal Science. The new findings contribute to an emerging paradigm involving the role of comets and the origins of life on Earth. The paradigm suggests that comets - rich in the chemicals and organic molecules essential to life- may have contributed to critical prebiotic conditions on Earth some 4 billion years ago. These compounds would include water, carbon dioxide and nitrogen, along with more complex organic molecules. At the other extreme, the new studies may help explain the destructive effects of comets, for example, the catastrophic extinction of the dinosaurs. Hale-Bopp is unusually bright and unusually large. This has given astronomers the best opportunity to study a comet's nucleus they have ever had. Comets are believed to be remnants from the formation of the solar system, about 4.6 billion years ago. Therefore, learning more about comets can provide important information about the materials and processes that formed the solar system. Studies of the nucleus can also help explain the composition of the comet and the nature of its ever-changing tail. "This is a unique opportunity. We have never had the chance to examine a comet in this much detail over this large a range of distance from the sun. Hale-Bopp will probably provide the most revealing portrait of the workings of a cometary nucleus since the spacecraft missions to comet Halley" said Cornell University astrophysicist Harold Weaver said Astronomers were puzzled by the comet's changing appearance, as it would suddenly grow brighter and then return to its usual brightness within an hour. Another challenge was to determine how the various types of ice in the comet are vaporized. The standard model suggests that dust particles and chemical compounds, such as carbon dioxide and carbon disulfide, are all contained inside frozen water. As the comet nears the sun, it heats up, vaporizing the water ice and releasing other material and dust particles that are contained in the ice. The dust is driven off in a huge tail extending millions of miles, reflecting sunlight and brightening the comet. However, Cornell scientists discovered that these chemicals have been vaporizing independently of water. While the vaporization rate of water ice increased more than 13-fold between April and October 1996, there was only a two-fold increase in the rate of dust being released. The vaporization of carbon disulfide ice increased by less than three-fold. If the standard model were correct, water, dust and the other components should be released at the same relative rates. The new information suggests the components are contained in separate regions of the nucleus. "The surface of Hale-Bopp's nucleus must be an incredibly dynamic place, with `vents' being turned on and off as new patches of icy material are rotated into sunlight for the first time," said Weaver. Another group of scientists has been analyzing the comet with an infrared spectrometer and camera attached to 200-inch telescope at Palomar Observatory. Their studies show that Hale-Bopp has an abundance of tiny (sub-micrometer size) silicate grains. Some of these grains are crystalline, in contrast to the more amorphous structure of the rest. This means that the grains were subjected to strong heating sometime in their history, before they were incorporated into the frozen comet nucleus about 4.5 billion years ago. "Did the heating occur in the solar nebula, or did it occur in an interstellar cloud prior to the formation of the solar nebula? We can't say," notes Thomas L. Hayward, Cornell senior research associate in astronomy at the Center for Radiophysics and Space Research. Hale-Bopp's birthplace was in the cold outer region of the solar nebula where intact interstellar grains could have been incorporated. "But we were surprised to see such grains even when the comet was over four astronomical units from the sun. We thought the grains would be icy at that distance." (An astronomical unit is the average distance from the Earth to the sun, about 93 million miles.) Most comets can be studied only when they are within 1 or 2 astronomical units, when dust grains are warm. But Hale-Bopp was unusually active and could be detected easily in the infrared when it was still far from the sun. The new information gives astronomers a basis for comparison to other comets. This in turn could yield clues to the origins of the solar system. The ultimate goal is to understand what the grains are made of and their processing history. That would indicate the history of this comet, which in turn tells us what the early solar system was like. "Our hope is that these dust grains, from under the surface of the comet's nucleus, represents what the nucleus was like billions of years ago when it was formed," Hayward said. "That could help tell us what the solar system was like as it was forming. This is just another piece of the puzzle." Other new findings:
Hale-Bopp Magazine
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