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Comet dust sparks scientific intrigue

The top scientist for NASA's Stardust mission says the dust samples returned to Earth bear the signature of an ancient comet, including sulfides and apparently organic compounds.
A tiny bit of aerogel contains a carrot-shaped track carved by comet dust particles, as seen in this cross-section. The particles themselves are at the very tips of the "carrot."
A tiny bit of aerogel contains a carrot-shaped track carved by comet dust particles, as seen in this cross-section. The particles themselves are at the very tips of the "carrot."NASA

Giving a sneak peek of results to come, a top mission scientist said flecks of material collected during the Stardust spacecraft's seven-year journey bear the unmistakable signature of an ancient comet, including sulfides, crystalline silicates and probably organic compounds as well.

"We're seeing a variety of things that we know absolutely come from a comet," University of Washington astronomer Donald Brownlee, Stardust's principal investigator, told reporters here Monday at the annual meeting of the American Association for the Advancement of Science.

Brownlee heads an international team of about 150 scientists who are getting their first looks at the specks captured by NASA's Stardust spacecraft as it flew close to Comet Wild 2 back in 2004. At the end of a 2.9-billion-mile (4.6-billion-kilometer) round trip, the spacecraft successfully delivered a capsule containing the samples back to Earth on Jan. 15.

The flecks of dust and grit are contained within 132 ice-cube-sized tiles of aerogel, an ultra-light, porous material that has been compared to "solid smoke." As the bits entered the tiles, they carved carrot-shaped or turnip-shaped tracks in the transparent aerogel.

Brownlee said six of aerogel blocks have been pulled out for inspection so far. "All of the tiles are in good shape — which is amazing," he said.

Meeting expectations
What's even more amazing is how well the first round of analysis is matching expectations. Brownlee and other Stardust scientists are holding back their first formal reports for a scientific meeting in Texas next month — but during Monday's news conference, Brownlee said the samples studied so far contain iron sulfides and glassy material such as crystalline silicates. Those ingredients are found in meteorites as well.

Later, Brownlee told that there were preliminary indications of organic compounds, based on telltale infrared readings. He cautioned that the initial indications were tentative and could still be traced to contaminants. "The spacecraft is made of plastic, for example," he said. But Brownlee also said it wouldn't be surprising to find organics in comet dust.

"I would suspect that somewhere around 10 percent would be organic particles," he said.

Scientists have known for a long time that organic compounds exist in space, and the term shouldn't be understood to refer to a biological source for the compounds. However, some scientists suggest organics could have arrived on Earth from space to serve as the building blocks for primitive life.

Checking the compounds
A co-investigator for the Stardust mission, Scott Sandford of NASA's Ames Research Center, agreed that organic readings could be due to contamination. "Just because we detect a compound doesn't mean it's a cometary compound," he told But so far, he said, "it looks like we brought back a pretty clean capsule."

Sandford said researchers are finding "little facts here and there, but it's not falling all together," simply because it's too early to get a comprehensive sense of the composition of the cometary dust.

Nevertheless, he said, "everybody on the team is really pleased — things could have been a lot more complicated."

In the weeks and months ahead, Sandford and his team will be analyzing the types of carbon found in the samples — not only to trace the organics, but also to determine whether such compounds predated the formation of the solar system.

Scientists say comets represent the cold leftovers from the solar system's beginnings, 4.5 billion years ago. The Stardust samples confirm that the material thrown off by Comet Wild 2 has not undergone chemical change for billions of years, Brownlee said: "It's never been hot."

So far, the Stardust team has not seen firm evidence that the cometary samples contained water — and that ingredient should be abundant in comets, which are popularly called "dirty snowballs." Brownlee emphasized that the water could not be detected directly. Rather, scientists would look for the presence of hydrated minerals created by interaction with water. "We do not see hydrated silicates, at least so far," Brownlee said.

Solving the puzzles
Scientists at the AAAS meeting said that Stardust could eventually shed light on many of the puzzles surrounding comets and the solar system's formation. "Dust, lowly dust, plays a very important role in the birth of solar systems and the death of solar systems," said Lee Anne Willson, an Iowa State University astronomer who studies dust conditions around dying stars.

Joe Nuth of NASA's Goddard Space Flight Center said that "Stardust represents a very great challenge for us," and that the mission's findings will likely rule in or rule out a variety of theories about the nature of comets and the genesis of the solar system.

That won't all happen at once, however. Ernst Zinner, an astrophysicist at Washington University of St. Louis, said his team was just beginning the work of analyzing the Stardust samples for the presence of interstellar dust. Brownlee, meanwhile, cautioned reporters that the full findings from the $212 million Stardust mission won't sink in for a long time.

"It will be years and years before a reasonable understanding of this comes out," he said.