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Celestial dust challenges basic view of comets
Seattle Times staff reporter
At first, Don Brownlee thought he was looking at a bit of debris from the spacecraft.
The crystals he saw in his microscope were so unexpected, the University of Washington astronomer didn't think they could have possibly come from a comet.
"It was truly astounding," he said Monday at a briefing in Houston to unveil the first scientific results from NASA's Stardust mission. The robotic probe flew by the comet Wild 2 in 2004, grabbed dust from its halo and brought it back to Earth in January.
Tiny grains embedded in the capsule's collector contain minerals such as olivine, found on Hawaii's green sand beaches, and spinel, a rubylike gemstone used in jewelry.
Both form at temperatures higher than 2,000 degrees Fahrenheit.
But that doesn't jibe with the standard view that comets are made up only of materials from the distant fringe of the solar system, where temperatures hover around minus 400 degrees.
"Remarkably enough, we have found fire and ice," said Brownlee, principal investigator for the $212 million mission. "We have found samples in the coldest part of the solar system that formed at extremely high temperatures."
Unraveling the mystery will reveal much about the creation of the solar system, which scientists believe coalesced about 4.5 billion years ago from a spinning disc of gas and dust. The center of that disc was a turbulent inferno that eventually gave birth to the sun and the inner planets.
The new findings from Stardust suggest high-temperature materials like olivine were somehow hurled from the blistering center of the vortex to the icy edges where comets were born, said Mike Zolensky, of NASA's Johnson Space Center in Houston.
"They would have been ejected ballistically all the way out across the solar system ... like a conveyor belt," he said.
It's also possible the high-temperature minerals in the comet dust originated in the fiery environs of far-flung stars, not our own solar system.
Scientists will be able to tell the difference once they have time to analyze the comet particles in greater detail, Brownlee said. Grains that form on other stars differ from those formed in our solar system.
In the two months since the Stardust capsule parachuted to the Utah desert, researchers have extracted hundreds of bits of comet dust from the collector, made of an extremely light-weight material called aerogel. Averaging less than one-fifth the diameter of a human hair, the particles have been distributed to 150 researchers around the world.
Stardust marks the first time a NASA mission has delivered extraterrestrial material to Earth since the Apollo moon missions in the 1970s.
Brownlee has been studying two particles in his Seattle lab. With diamond blades called microtomes, he can carve one speck into a hundred slivers. His electron microscopes are powerful enough to resolve individual molecules.
"For us these are actually quite large rocks," he said.
One of the first particles extracted from the aerogel — on Valentine's Day — was shaped like a heart. Others fractured into dozens of even tinier particles.
While the early results are exciting, there's much more to come, Brownlee said.
Comets almost certainly contain organic material. Some scientists believe comets may have delivered the ingredients of life to Earth. There are already some hints of organic compounds in the Stardust grains, but it's a laborious process to rule out any possibility of contamination from Earth.
"It's a very exciting mystery story," Brownlee said. "So stay tuned."
Sandi Doughton: 206-464-2491 or firstname.lastname@example.org
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