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 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, University of Washington astronomer and principal scientist for the $212 million Stardust mission.
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A series of papers detailing the first scientific results from the Stardust mission were to be published today 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.
Analysis also found Wild 2 appeared to differ from comet Tempel 1, which was studied in NASA's Deep Impact mission and focused on the dust and ice spewing from its belly, not from Tempel 1's surface.
