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New Information about Hale-Bopp Surprises Astronomers

Story: A team of scientists, led by Johns Hopkins astrophysicist Harold Weaver, will detail new findings about comet Hale-Bopp in a paper to be published in the journal Science on March 28. The findings are based on an unprecedented year-long series of cometary observations with the Hubble Space Telescope.

After studying the comet in detail as it got closer and closer to the sun, astronomers found that components in the frozen nucleus may be arranged in a more complicated manner than previously thought. Some models for a comet's nucleus suggest that trace components, such as carbon disulfide ice, are contained inside of the most abundant ice, frozen water. Therefore, as water sublimates, or turns directly from a frozen solid into a gas, trace components and dust should be released at similar rates. But, to their surprise, astronomers found that water ice sublimated at a different rate than the trace ices, implying that those components are not contained within the water. The astronomers also observed that the amount of dust leaving the nucleus was much different than the sublimation rate of water, providing further support for the finding.

During the course of long-term observations of Hale-Bopp with the Hubble Space Telescope in September 1996, astronomers unexpectedly caught the comet going through a sudden brief outburst; in little more than an hour the amount of dust being spewed from the nucleus increased at least eight-fold. "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," Weaver said.

The comet's nucleus is apparently huge. By studying Hubble Space Telescope images, the astronomers have estimated that its nucleus may be 30-40 kilometers (about 19-25 miles) in diameter. In comparison, the potato-shaped nucleus of Halley's Comet, considered to be a large one, had an equivalent diameter of about 10 kilometers (6 miles). The average comet is thought to have a nucleus of about 5 kilometers (3 miles) in diameter. The comet or asteroid that struck the Earth 65 million years ago, possibly causing the extinction of the dinosaurs, was probably about 10-15 kilometers (6-9 miles) across.

Background: Astronomers observed ultraviolet light from Hale-Bopp, using the Hubble telescope and the International Ultraviolet Explorer satellite over a one-year period beginning late August 1995, when Hale-Bopp was about 586 million miles from Earth and 633 million miles from the sun. It was a sort of "last gasp" for the IUE, which was then shut down after 18 years of operation. The scientific paper, which was based on findings from those observations, was co-authored by Weaver, Johns Hopkins astrophysicist Paul Feldman, and eight scientists from other institutions in the United States and Europe.

Because Hale-Bopp was unusually bright when it was still a great distance away, well outside the orbit of Jupiter, it has given scientists their best view ever of the changes in a comet's nucleus as it gets closer to, and is progressively heated by, the sun. Those changes, in turn, provide information about the composition and structure of comets, which 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.

"This is a unique opportunity," Weaver said. "We have never had the chance to examine a comet in this much detail over this large a range of distance from the sun."

The astronomers were surprised by the way in which the comet would suddenly grow brighter and then return to its usual brightness within an hour or so. Also surprising is the way in which various types of ices are being vaporized. Well-accepted models suggest that dust particles and various 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.

But their observations have astronomers wondering about a certain aspect of that picture. They found that various 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 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. "Hale-Bopp will probably provide the most revealing portrait of the workings of a cometary nucleus since the spacecraft missions to comet Halley" in 1986, he said.


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