Comet Dust Loses Luster in New Analysis : Astronomy: UCSD research team reports that Halley's comet is not composed entirely of pristine solar system materials.

TIMES STAFF WRITER

San Diego scientists have analyzed comet dust and report today that--like so much in life--things are not what they appear.

The analysis of comet dust, measured by spacecraft while Halley's comet passed near Earth in 1986, shows that comets are not composed of pristine material as scientists have believed, according to a report released today in a scientific journal.

Instead, using data collected by the Soviet spacecraft Vega 1 and 2, the research team from UC San Diego found that some chemical components in the dust contain minerals that could only result from more recent times.

"Comets are widely believed to be the the most pristine objects in the solar system. The minerals we discovered . . . are thought to be the result of some alteration," said Marina N. Fomenkova, an author of the study in Science and a visiting scholar at the California Space Institute at UCSD's Scripps Institution of Oceanography.

"Our discovery is that cometary dust contains particles highly enriched in certain compounds that could not have been formed during the condensation of the solar system, and that are likely the result of later processes," said Fomenkova, a visiting scholar from the Space Research Institute of the Soviet Academy of Sciences.

Scientists have long believed that comets were formed during an early stage of the solar system's formation. Since that time, they say, the stuff of comets has not been altered.

"That is not true," Fomenkova said. "There have been some kind of processes, and alteration and chemical reaction. . . . It shows that comets are subject to change."

The studies were based on data gathered from the Soviet spacecraft that came within 500 miles of Halley's comet. A tiny portion of the comet dust--about one-billionth of a gram--was measured.

In its analysis, the team found grains of magnesium-rich particles, which they identified as magnesium carbonate. These would not have formed under the conditions that were present when material in the solar nebula condensed into the planets, asteroids and comets of the solar system.

The particles, Fomenkova and her colleagues maintain, must have been formed as a result of processes involving water on the comet that caused materials to become hydrated. Most space scientists acknowledge that such hydrated particles existed only on planets and asteroids--not comets.

One reason comets were always thought to be composed of unprocessed materials, according to Fomenkova, is that their orbital pattern brought them to the outer reaches of the solar system, where temperatures are low.

Fomenkova maintains that the results of an analysis can be extrapolated to other comets as well.

"There is no reason to believe this comet is in some principal different from others, it is just one that is representative of the whole class," she said. "Certainly, we can't be sure until we have investigated another comet and another. But now that is what we have."

Fomenkova co-authored the study with Lucy McFadden of the California Space Institute and John F. Kerridge and Kurt Marti of UCSD's department of chemistry.

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