Dying Comet Gives Rare View of Space

May 18 2001 @ 12:09

A comet falling toward the sun last summer peeled apart like an onion, giving astronomers a close look at one of the icy balls of dust that may have played a role in forming life on Earth.

Comet LinearWASHINGTON (AP) - Powerful telescopes were trained on Comet Linear as it shed mountain-sized chunks of rock and vaporized tons of ice during its disintegration.

Eventually, the half-mile-wide comet was reduced to a cloud-like formation of football field-sized pieces, a spray of dust, water vapor and fragments too small to be seen, said Hal Weaver, an astronomer at Johns Hopkins University in Baltimore.

``We think watching it come apart was a lot like seeing how the comet was put together in the first place, played in reverse,'' said Weaver, the leader of a team of observers. He also is co-author of a study appearing Friday in the journal Science.

The astronomer said an analysis of Linear's breakup supports the idea that comets may have supplied a primitive Earth with the water and organic chemicals to form life.

He said study of three other comets, Halley, Hale-Bopp and Hyakutake, had cast doubt on the comets' role because the chemistry of water on those objects was different from that of the Earth's oceans.

But the characteristics of Linear's breakup suggest it had a different water chemistry, Weaver said. ``This is the type of comet that could have done the job'' of providing Earth with the water and organic chemicals needed for life, he said.

Linear's breakup originally was estimated to be about a half-mile across in size and contain some 660 billion pounds of ice. Measurements of the debris, however, can account for only small percentage of the rocky material and for only about 7.2 billion pounds of the ice, Weaver said.

``If you add up the many pieces that broke off, you come out way short of the original mass estimate,'' he said. ``So there is some missing matter here.

While astronomers do not know what cause the breakup, Weaver said the comet's fragility surprised them.

``The forces that took it apart were gentle,'' he said. ``It was not like there was a stick of dynamite that blew it apart. The forces were not that powerful.''

This supports the traditional view that comets may be ``cosmic rubble pile'' that are only loosely held together, he said.

Researchers also were surprised at the ratio of ice to dust and rock in Linear. Since the 1950s, astronomers have believed comets were ``dirty snowballs,'' objects that were about half ice and half dusty rocks.

Weaver said the analysis of Linear suggests that it had about 100 times more solid rock and dust than ice.

``Comet Linear was more like an icy dirtball than a dirty snowball,'' he said.

Astronomers also noted that the ice vaporized at much higher temperatures than expected. This suggests the comet formed closer to the sun, perhaps around the orbit of Jupiter, Weaver said. Most comets are thought to have formed on the distant fringes of the solar system, where temperatures are much colder.

Weaver said that comets formed nearer the sun are more apt to contain frozen water whose chemical composition more closely resembles that of the Earth's oceans. This would support the theory that comets supplied much of the planet's water, he said.
 

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