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Comets: the universal recipe?

POSTED 28 JULY 2005
The young and the restless
In 2001, NASA's Deep Space 1 spaceship, juiced by an experimental ion-propulsion rocket, visited comet Borrelly, and sent back striking snapshots of a spud-shaped object that had survived maybe 4.5 billion years in space. Some parts were smooth, but Lawrence Soderblom, a research geophysicist with the U.S. Geological Survey in Flagstaff, Ariz., and his colleagues (see "Observations of Comet 19P/Borrelly..." in the bibliography) described areas studded by "irregular pits, bumps, troughs, and ridges."

To understand comets, you need to know sublimation -- the process of changing from a solid directly to a gas, without bothering with the intermediate phase, liquid. In the vacuum of space, when comets approach the sun and warm up, the ice inside them sublimates -- turns to water vapor (in a vacuum, most liquids cannot exist due to the lack of pressure). Because ice gains volume fast as it sublimates, sublimation can bring crud to the surface, Soderblom and Co. wrote.

Kinda like an ice-powered geyser.

Borrelly had no for-sure impact craters, maybe because the images were a bit fuzzy. However, comet Tempel 1, which looked quite a bit like Borrelly, did have craters, Soderblom told us. "Both show these very smooth patches, rimmed with a bright edge. Both show these very rugged areas... And Tempel 1 has old impact craters, partly buried, or being re-exhumed, a bunch of different combinations are possible."

Young at heart
Even the lack of obvious impact craters may be significant, Soderblom said, since it gives "the impression that the whole surface is pretty young and active." When you think about activity, remember that this tiny body has scarcely any gravity. You could take a running jump, and wind up in space. So comets can't hold junk arriving from outer space the way the planets do.

If that coating on the surface is not debris from comets and asteroids, what is it? Grut from the comet itself, that's what. "Somehow material is being moved, and laying back down," Soderblom says. "It may be related to the low-velocity material associated with jets... or a gentle impact that fluffs up the surface." Either process would raise a dust cloud that could settle down and slowly cover the craters.

A hard impact, by contrast, might blast away stuff fast enough to escape the comet's gentle gravitational embrace. That could explain the jagged edges of Tempel I and Wild 2, a comet we'll visit shortly.

Live fast, die young. The comet story
Comets are a paradox. They are ancient, but they have nice young skin, Soderblom says. "The notion that it is either old or young is too simple. They are old, but they have some young deposits on the surface."

At this point, you might be asking, didn't you sell us comets as pristine messengers from the early solar system? Sure we did, but a lot can happen in 4.5 billion years, even if it happens real s l o w l y. In space science, Soderblom points out, the harder you look, the weirder things look. "In astronomy, no matter what the object is, as we get better data, it gets more complicated."

The close approach to Borrelly also shed light on the jets that make the coma and tails. Borrelly's major jets come from the part that always points at the sun, Soderblom says. Over a few thousand years, "you could imagine that burning a hole in the object. It will make it thinner and thinner, like a Frisbee." In 1,000 or maybe 10,000 years, he expects, Borrelly "will break into multiple pieces."

Through color-coding, we can see the dust and gas around this comet.

A composite image of comet Borrelly, with the sun shining from the bottom, shows the nucleus, dust jets, and the cloud-like "coma" of dust and gas around the nucleus. The main dust jet is toward the bottom left. False color shows brightness: Red is 10 percent as bright as the nucleus, blue 1 percent, and purple 0.1 percent. The red bumps near the nucleus show three narrow jets, which likely come from separate sources on the surface. Courtesy of NASA

We know water vapor is coming from comets, but what else? The best way to find out is to fetch samples. That's the job of Stardust.

Wild 2: The wild one
In January, 2004, Donald Brownlee, an astronomer at the University of Washington, got up close and personal with a comet, when the spacecraft Stardust visited Wild 2. This comet, like Borrelly and Tempel 1, orbits as far as Jupiter's distance from the sun. Brownlee, who's Stardust's principal investigator, says that he's inching toward an answer for that vexing list-of-ingredients question.

Comet full of shallow craters.

Comet Wild 2, seen by Stardust on Jan. 2, 2004. Scientists used to think comets were 'dirty snowballs,' but this baby shows real topography. Stardust scored some dust from this comet, and is returning it to Earth for analysis. Courtesy of Stardust

The comet is, yes, a "dirty snowball," as scientists have described them, a goulash of water ice and rock. That rock, however, is "not like a chunk of granite," he says. "It's basically a dirt clod, made of very fine-grained material." Stardust's infrared telescope suggests that the comet's jets contain silicate particles, many smaller than a micron across.

Answer a question -- and you get another question. OK, Wild 2 is made of ice and rock dust, but how was that dust formed? The major physical activity on the comet, sublimation, powers those jets, but while the jets may be fast enough to escape the comet's tiny gravity, they are not jackhammers. The jets "are very subtle," Brownlee says, "and they can't possibly bust hard rocks."


Comet Wild 2 has many strange pinnacles, some over 100 meters high, probably formed by the erosion of nearby material. The only other place in space with pinnacles is Earth. Courtesy of Stardust

Soderblom added it up for us. The jets contain "a really fine dust" from inside the comet, and "the comet has to be composed of small dust."

In dust we trust
Now we are making progress on that list of ingredients: A comet, Soderblom says, "Is inherently made of submicron particles; it was made by a process that aggregated them with ices."

Where these grains originated is anybody's guess. Since the objects in the Kuiper Belt and Oort Cloud formed along with the rest of the solar system, they could be interstellar grains, left over from the time before the planets. But the grains could also have been altered after the solar system started forming.

Expect help here from Stardust, which trapped some of Wild 2's dust. In January, 2006, if all goes well, Stardust will return those samples to the desert near Salt Lake City. "When we get the sample back, we'll know" about the source, Brownlee says. Indeed, the sample return mission may be the best chance to decipher the comets' message about the early solar system.

Stay tuned. Right now, Stardust is out near Mars, catching up on Earth, clutching a one-way ticket to Utah.

What do we know of the fate of comets?

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