SPACE.com -- A Comet's Life: Icy Adventure From Birth to Death


	A Comet's Life: Icy Adventure From Birth to Death By Robert Roy Britt Senior Science Writer posted: 02:00 pm ET 17 May 2001

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A detailed analysis of a comet that broke apart last summer has provided direct evidence to support the idea that the ingredients for life on Earth arrived in a comet.

Chaos, Quietude, Chaos

The Adventure
The tale this comet tells has enthralled the dozens of researchers involved in the new studies. Here, then, is the life of a comet, from its chaotic beginnings around an insignificant young star to its frigid wanderings through a solar system's hinterlands, to its ultimate return and fiery breakup close enough to a planet where curious scientists could observe the whole show. More >

Fragments of Comet LINEAR seen as mini-comets by the Hubble Space Telescope on August 5, 2000. Part of a dust tail is visible at top right. Click to enlarge

THE FINDINGS

New Model of Comet Formation
Not all comets formed near the orbits of Uranus and Neptune, as previous studies had shown. Comet LINEAR almost certainly formed near the orbit of Jupiter, NASA's Michael Mumma says in a telephone interview. Other comets that formed near Jupiter would have likely been booted clear out of the solar system by Jupiter's strong gravity or they might have been tossed in toward the Sun, increasing the chances of an Earth impact. More >

Seeding Life on Earth
Comets like LINEAR, therefore, may well have brought to Earth the chemical building blocks of life, as well as enough water to create the oceans. No one disputes that Earth experienced a period of heavy bombardment right up to the time life appeared, some 3.8 billion years ago. But previous research has suggested that most of these objects were asteroids. More >

Snowy Dirtball
Not all comets fit a 50-year-old popular description of their composition. "C/LINEAR was more like a "snowy dirtball" than a "dirty snowball," Hal Weaver tells SPACE.com. Further, the inner layers of the nucleus were composed of the same stuff as the outer layers, indicating the comet gathered itself together all in one orbital region, the same distance from the Sun, and then remained relatively pristine throughout its life. More >

Rubble Pile Led to Breakup
LINEAR reveals a composition less like a solid rock and more like a pile of rubble, which has already been shown to be typical of asteroids. Researchers say this collective makeup, which lacks expected amounts of ice, may have contributed to the comet's demise. More >

The breakup has also acted somewhat like a videotape viewed in rewind mode, allowing scientists to study when, how and where the comet was put together.

As Comet 1999 S4 LINEAR unraveled into more than a dozen chunks larger than a football field and countless tiny bits, researchers got an unprecedented view of a comet's nucleus, where primordial puzzles unfolded before a host of ground-based telescopes and orbiting observatories.

In six separate studies in the May 18 issue of the journal Science, researchers detail a 4.5-billion-year icy adventure, a tale quite different from what they expected.

"The idea that comets seeded life on Earth with water and essential molecular building blocks is hotly debated," said Michael Mumma of NASA's Goddard Space Flight Center and lead researcher in one of the studies. "And for the first time, we have seen a comet with the right composition to do the job."

The findings might also eventually improve models of planet formation. Planets are thought to have built themselves from humble beginnings as asteroid or comets, which collided and stuck together to form larger objects.

"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," said Hal Weaver of Johns Hopkins University, lead author on one of the papers and a co-author on two others.

Weaver, Mumma and others say the comet, popularly called LINEAR, might just be an oddball. But their suspicion is that it represents a whole newly realized class of comets that formed in abundance back when the solar system was developing, but were then swallowed by the Sun, slammed into Earth or lost to interstellar space.

In this scenario, LINEAR represents a rare survivor.

And its rarity makes its observation all the more fortuitous. Other comets have been studied as they swing around the Sun, which burns off their surface and creates a halo of gas and dust whose composition scientists can measure. But never has a comet performed a death dance in front of so many telescopes.

"Normally we can see only the surface of a comet, and the dust particles that are small enough to be lifted off the surface by vaporized ices," said Tony Farnham of the University of Texas at Austin. "In the case of LINEAR, however, the breakup occurred near the time that the comet was at its closest point to both the Earth and the Sun."

Hermann Boehnhardt, a European Southern Observatory researcher who was not involved in the studies, said they collectively represent a glimpse into pristine material, uncontaminated since the solar system formed, and are the first real measurements that can be used to test models of solar system formation.

"It is a step forward," Boehnhardt said, "and an important step."

Next Page: The Adventure

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