Comets, the lonely wanderers credited with seeding Earth with the ingredients for life, are proving to be far more complex than anyone thought, containing a "zoo" of materials including some that may predate the sun, scientists reported Thursday.
And enough mixing was happening in the early solar system to send material from the proto-sun into deep space, where it could be picked up by comets, the scientists reported, speaking at an American Geophysical Union meeting.
That discovery could alter theories about how stars and the surrounding planets form from primordial dust and gas, according to the researchers.
The team's findings, published today in the journal Science, are the first scientific results from NASA's Stardust mission -- a $200-million project that sent a spacecraft through the tail of the comet Wild 2 to scoop up dust, minerals and organic compounds. The mission was conceived and managed by the Jet Propulsion Laboratory in La Canada Flintridge.
After about seven years in space, Stardust zoomed close to Earth and ejected a capsule containing the precious samples. The capsule safely landed in the Utah desert Jan. 15.
"These are the best-preserved samples we have yet of what the solar system was formed from," said University of Washington astronomer Donald Brownlee, a principal investigator for Stardust.
Michael Zolensky, a researcher from NASA's Johnson Space Center in Houston, said: "There were a lot of preconceived ideas of comets, and a lot of them are now gone."
Wild 2 is the second comet to be examined by scientists since last year, when NASA sent a probe into comet Tempel 1, finding an array of water-bearing minerals.
Stardust didn't see any of that, Brownlee said. That could indicate that despite their glittering similarity as they streak through the night sky, comets can be very different in composition.
Comets are considered an important source of information about the solar system's formation. Unlike planets, which have undergone significant changes since forming 4.6 billion years ago, comets are thought to be ancient repositories of conditions during the sun's infancy.
Until now, the conventional wisdom held that comets formed in isolation on the fringes of the solar system.
But the Stardust results show that some particles were subjected to high heating. The scientists said they weren't yet sure of the mechanism involved. When the sun was forming, the solar disk was much larger. But that doesn't completely explain how superheated material ended up on a comet at the far edge of the system beyond Neptune, where comets formed.
The scientists also found several unusual compounds in the comet samples whose presence they could not explain.
"We're doing things no one ever imagined we could do, even at the time we launched the mission," Brownlee said. "We've taken a pinch of comet dust and are learning incredible things."