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Strange comet unlike anything known

This composite image was taken by the Stardust probe's navigation camera during its encounter with Comet Wild 2 in January. Several large depressed regions and jets of dust and gas can be seen. Comet Wild 2 is about 3.1 miles (5 kilometers in diameter.
This composite image was taken by the Stardust probe's navigation camera during its encounter with Comet Wild 2 in January. Several large depressed regions and jets of dust and gas can be seen. Comet Wild 2 is about 3.1 miles (5 kilometers in diameter.NASA
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A detailed analysis of the comet Wild 2 (pronounced "Vilt 2") has left astronomers astounded at an object that has no known peers in the solar system.

The comet, examined in a close flyby in January by NASA's Stardust spacecraft, has towering protrusions and steep-walled craters that seem to defy gravity. More than a dozen jets of material shoot out from its insides. Dust swirls around the comet in unexpectedly dense pockets.

Among the bizarre features are two depressions with flat floors and nearly vertical walls that resemble giant footprints. They aren't structured like typical impact craters. The features have been named Left Foot and Right Foot in a new map of the comet, which is roughly 3 miles (5 kilometers) wide.

Only two other comets have been seen up close, but both appeared fairly smooth and were nowhere near so heavily cratered. Nor do the pockmarked surfaces common to asteroids and moons bear much stylistic resemblance to the shapes seen on Wild 2.

"So far, as far as we know Wild 2 is a unique object," said Donald Brownlee, an astronomy professor at the University of Washington and Stardust's principal investigator.

Brownlee told that Wild 2 could represent a unique class of comet. He and his colleagues had expected it to be relatively featureless with a dusty, charcoal-like coating. Instead they found a place riddled with apparently ancient impact craters. Broad mesas and steep canyons stand out clearly.

It is more likely, Brownlee added, that Wild 2 will turn out to be a garden variety comet once more of them are studied up close.

Brownlee's group and other research teams present four analyses of Wild 2 in the June 18 issue of the journal Science, building on preliminary results released in March.

Almost no gravity
Scientists don't know exactly what comets are made of. But they're thought to represent the composition of the outer solar system in its primordial state. They're loaded with frozen water and other ices, plus organic materials and silicates, or rock. Many theorists believe comets delivered the water and other pre-biotic ingredients that led to life on Earth.

Stardust flew to within 147 miles (237 kilometers) of Wild 2 on Jan. 2. The observations -- and dust samples that will be returned to Earth in 2006 for lab study -- should improve understanding of the solar system's formation.

Craters on Wild 2, presumably caused by run-ins with smaller objects, are strangely free of the powder, rocks and other debris commonly seen in impact craters on other bodies. Brownlee thinks this is because the comet is a bit like hard, frozen dirt that takes a hit but is brittle, so material flies out.

And because the comet is so small, the material does not fall back.

"There's almost no gravity at the surface," Brownlee said. "If you were standing on [the surface], you could jump into orbit."

Yet Wild 2 is not a fractured pile of rubble that would all fly apart when hit, as some astronomers expected. Brownlee: "We're sure this is a rigid material because it can support cliffs and spires."

What sort of material can crumble under impact, leave sheer walls and allow its parent body to remain intact?

Previous research in Europe may provide the answer. Scientists made artificial comets of finally powdered organic materials and ice in a vacuum. "They always ended up with a rigid, crusty material," Brownlee said. "If you were standing on it you might go right through it."

Comets 'blow up'
That all assumes the "footprints" and other depressions on Wild 2 are in fact impact craters. Other forces could be at work.

Comets do blow up unexpectedly," Brownlee pointed out, adding that built-up internal pressure and "steam explosions" might be responsible for some of the surface features.

In another baffling surprise, Brownlee said, dozens of photos show no small craters on Wild 2, only the large craters that are presumably billions of years old. Perhaps small craters erode away, he said.

Brownlee is also intrigued by the utter lack of similarities between Wild 2 and Phoebe, a fairly small moon of Saturn recently imaged up close by the Cassini spacecraft. Phoebe is thought to be a captured object, having originated -- like Wild 2 -- beyond Neptune. But Phoebe's gently sloping craters, which are riddled with boulders, resemble those seen on asteroids. And Phoebe has many small craters embedded in larger, older craters.

"It's fascinating that they're so different," Brownlee said in a telephone interview.

The two objects may have started out with the same stuff, he speculates, but then underwent different histories. Perhaps like larger objects -- planets and other moons -- Phoebe was once heated enough to melt its insides, which produces a different chemical and mineral structure.

Phoebe is considerably larger than Wild 2 -- about 137 miles (220 kilometers) wide -- so self-gravity could have something to do with the differences. And Phoebe has likely never traveled inside the orbit of Saturn, so it probably has not been hit with solar radiation to the extent now experienced by Wild 2.

Swarms of particles
Comet Wild 2 probably gathered itself together 4.5 billion years ago, just after the Sun was born, in a region beyond Neptune known as the Kuiper Belt.

In 1974 it had a close encounter with Jupiter and was thrown onto a new orbit that brings it closer to the Sun. A comet loses material when it approaches the Sun, as solar radiation causes ice from its surface to "sublimate" into space, carring dust and larger particles with it. The process creates a cloud of material that reflects sunlight and creates the familiar head of a comet (scientists call it a coma) and sometimes a tail.

Among the new findings: Wild 2 has lost about 3 feet (1 meter) of its surface since 1974.

Stardust flew right through some of that coma material, avoiding boulder-sized objects and grabbing more than 1,000 tiny bits in a specially designed catcher's mitt.

"These things were like a thunderbolt," said Anthony Tuzzolino, a senior scientist at the University of Chicago's Enrico Fermi Institute. "I didn't anticipate running into this kind of show."

Stardust encountered two intense swarms of dust with relative voids in between, suggesting that ejections from Wild 2 come in intense spurts. Comet Halley, studied in a 1986 flyby, exhibited a much smoother distribution of particles.

Large dust clumps are probably ejected from the comet and then fragment into swarms of particles, said Thanasis Economou, another Fermi Institute researcher involved in the study.

More to learn
Comets also lose material when jets of stuff apparently shoot out from inside or, possibly, from surface pockets of especially volatile material. A separate analysis of Stardust data found 20 jets, on both the sunlit side of the comet and, surprisingly, the dark side.

The jets likely emanate from inside the comet, researchers concluded, and they contain water, carbon dioxide and rock particles.

What remains of Wild 2, along with the bits captured by Stardust, is considered pristine. Because this is the comet's first trip to the inner solar system, its composition has been mostly unaltered for the billions of years it spent in the deep freeze. Studying the samples in a lab should provide clues to the state of the solar system at its birth.

But learning whether Wild 2 is common or unique will require visits to other comets, Brownlee said. Two flashy missions are in the works:

The European Rosetta craft, recently launched, will land on a comet in 2014. NASA's Deep Impact will slam a probe into comet Tempel 1 on Independence Day 2005.