Rubble Pile Led to Breakup

Researchers suspect that the primordial building blocks of which LINEAR was composed led directly to the comet's demise.

Weaver, the Johns Hopkins researcher, described the comet as being somewhat like a collection of multi-sized marbles, each covered in ice, all stuffed in a bag. Such rubble piles have been found to be the basis for asteroids, including Eros. But while it is also a popular theory for describing comets, there had been only indirect supporting evidence.

Now the evidence is clear. And Weaver suspects that the building blocks of Comet LINEAR would have been structurally weak.

As the outer surface of the comet burned off on its approach to the Sun, these building blocks were exposed and peeled off, one by one. And because the comet contained far less ice than researchers expected, it lacked the glue that might hold other comets together.

"It is believed that ices hold the [building blocks] together, so a lack of ices could have been a contributing factor in why the comet broke up," said Farnham, the University of Texas researcher whose group examined the asteroid's rotation and the mass of its many remains.

In the SOHO study of the LINEAR's water, researchers ruled out melting as the sole reason for breakup. Instead, they agree that the delicate assembly of the comet's original building blocks is to blame.

The upshot is that the disassembly of Comet LINEAR was far less violent than early reports imagined last summer.

"If someone had placed a stick of dynamite in the nucleus and blew it apart, then the pieces would have nothing to do with primordial building blocks," said Weaver. "However, we think that the process, or processes, that caused C/LINEAR to come apart were not violent enough to destroy its original building blocks."

A handful of other scenarios have been devised to account for why comets break apart, including a frenetic rotation rate or the gravity of planet or other massive object.

But LINEAR was nowhere near a massive planet.

And Farnham's team used ground-based telescopes to study how LINEAR moved about its axis as it sped through space. They determined that the comet rotated at least once every 12 hours, not rapid enough to have been responsible for the breakup.

But there is other evidence, not yet published, indicating that the rotation rate may be as fast as 2.5 hours. If correct, then rotation might have played a partial role in the breakup, Weaver said.

Imre Toth, a Hungarian researcher and co-author with Weaver, suggested in another recently published paper that the comet might have collided with other space debris in late 1999 or early 2000, disrupting the nucleus. This would mean that the pieces observed after the breakup were not primordial building blocks.

Weaver called Toth's hypothesis interesting but improbable.

And Mumma agrees, saying that "everything we've learned in recent years supports the view" that comets are made of individual building blocks.

"I envision the nucleus as a pile of large cotton balls," Mumma said. "Each ball is hard to pull apart because of interlocking fibers, but the pile itself is very weakly bound and so is easily dissembled."

He added that this view is supported by detailed numerical modeling previously done by Stuart Weidenschilling of the Planetary Science Institute in Tucson.

Still, Comet LINEAR has not revealed all. Scientists remain puzzled over some apparently missing mass.

The mass of the large fragments studied by Weaver and his colleagues was to be about 100 times smaller than the estimated mass of the original nucleus in another study. Estimates of ice in the nucleus prior to its undoing, along with estimates of material in the tail, fall well short of solving this "missing mass" problem, Weaver said.

Farnham's team suspects that most of the comet's original mass is now hidden in pieces ranging from the size of a building to grains of sand. These bits would be too small to seen by telescopes.