Detailed analysis of Comet LINEAR's disintegration last summer has left astronomers still wondering what triggered the breakup and how much of the comet broke into pieces too small for them to see.
But they're not complaining. Most are too busy applauding what, from their point of view, was a fine way for a comet to die.
"Because LINEAR's breakup seemed to preserve the original components of its nucleus, we think watching it come apart was a lot like seeing how the comet was put together in the first place played out in reverse," says Hal Weaver, a researcher in physics and astronomy in the Krieger School of Arts and Sciences.
With support from NASA, Weaver and other astronomers observed the comet's demise with the Hubble Space Telescope and ground-based telescopes. "We were looking at LINEAR so closely, we could really scrutinize what happened," he says.
Data from LINEAR's demise have helped astronomers get a better sense of the comet's component parts and where and how those parts may have come together, according to Weaver, who is an author on three of six LINEAR papers in the May 18 issue of Science.
He says LINEAR has also added fuel to a few old debates among cometary scientists, including: Are comets more like "dirty snowballs" or "snowy dirtballs"?
"We have tended to think of cometary nuclei as being made up of roughly equal parts snow and meteoritic material, or 'dirty snowballs,' an idea proposed in 1950 by Fred Whipple, the father of modern cometary science," Weaver says.
However, astronomers were able to get a fairly good estimate of the amount of water sublimated, or transformed from a solid to a gas, from the nucleus of LINEAR by the sun's heat. And when they compared that to their estimates of the mass of LINEAR's nucleus, a different picture emerged. LINEAR appears to have been mostly meteoritic material with some ice.
According to Weaver, that could mean that LINEAR was initially formed relatively close to the sun, perhaps in the range where Jupiter now orbits, where not as much ice would have condensed upon the meteoritic material.
"This idea that LINEAR formed near Jupiter was first suggested by other investigators in another article in Science," Weaver says. "It was based on their finding that LINEAR was depleted of several 'super volatile' ices that are seen in higher abundance in most other comets."
Cometary scientists have also debated whether the nucleus of a comet is more likely to be a single homogeneous body or a mixed pile of rubble. LINEAR definitely came apart like a pile of rubble, Weaver says, and the rubble they could see from Earth will affect theories about the typical sizes of the chunks of material that go into the rubble pile.
"As we debate these issues, we also have to keep in mind that it's possible that LINEAR was just an odd comet, but to get any sense for that we have to keep looking for evidence from other comets, and try to build up some statistics," Weaver says.
Estimates of the mass of LINEAR's nucleus, based on the amount of icy material on its surface, range as high as 300 billion kilograms. But the estimated total mass in the largest fragments observed after the comet broke up was only about 3 billion kilograms.
That has Weaver and others wondering if a larger-than-expected portion of the comet didn't break up into pieces too small for them to see from Earth.
Another important and unresolved question about LINEAR is why it broke up.
"We know from spectrographic observations taken with Hubble and ground-based telescopes in early July 2000 that LINEAR didn't have a lot of carbon monoxide, a 'super volatile' compound found in some comets," Weaver says. "That would have been high on the list of potential contributors to the breakup."
A paper in the same issue of Science offers evidence that LINEAR had a fairly rapid rate of rotation, Weaver notes, and this could have been a contributing factor.
"It's possible that this rotation, combined with LINEAR's approach to the sun, could have contributed to the comet's demise," Weaver says.
In addition, another astronomer has recently suggested the possibility that LINEAR may have lost some of its structural integrity through a collision with an asteroid.
"The bottom line is that there are still several things that are a bit puzzling about what happened to this comet," Weaver says. "And for scientists, 'puzzling' is a synonym for 'interesting.' "