SAN FRANCISCO -- Detailed observations from the first comet samples returned to Earth are debunking some of science's long-held beliefs on how the icy, celestial bodies form.
Scientists expected the minute grains retrieved from a comet Wild 2 to be made up mostly of interstellar dust _ tiny particles that flow through the solar system thought to be from ancient stars that exploded and died.
Instead, they found an unusual mix of primordial material as if the solar system had turned itself inside out. Hot particles from the inner solar system migrated out to the cold, outer fringes beyond Pluto where they intermingled and congealed to form a comet.
"People imagine that comets form in total isolation, which is definitely not true," said Don Brownlee, a University of Washington astronomer who is the principal scientist for the $212 million Stardust mission.
Brownlee estimated that up to 10 percent of materials in comets may come from the inner solar system.
A series of papers detailing the first scientific results from the Stardust mission were to be published Friday in the journal Science and presented at an American Geophysical Union meeting Thursday.
A capsule carrying thousands of minuscule samples from comet Wild 2 returned to Earth last January after looping around the sun to capture the interstellar and comet debris and swooping past Wild 2 to scoop up dust.
Scientists had dubbed Wild 2 a frozen time capsule because it contained material preserved from the aftermath of the solar system's birth more than 4.5 billion years ago.
How material from the inner solar system could have ended up in comets is still a mystery. Brownlee said the solar system-forming process was probably chaotic and unstable, allowing high-temperature particles to loft billions of miles out to the edge of the solar system.
Many of the grains contained high-temperature minerals that likely formed in the hottest part of the solar nebula. At least one grain was made of a rare mineral seen in some meteorites, which are among the oldest samples in the solar system.
An analysis also found Wild 2 appeared to differ from comet Tempel 1, which was studied in NASA's Deep Impact mission. Last July, the space agency crashed a probe into Tempel 1 and studied the dust and ice spewing from its belly. It did not retrieve any samples from the surface.
In an accompanying editorial, Michael A'Hearn of the University of Maryland, who is also the Deep Impact chief scientist, said the Stardust results have gotten scientists thinking about their original views.
"Stardust has certainly brought us plenty of food for thought," A'Hearn wrote.