Sept. 18, 2008 -- Scientists dispatched the Stardust science probe to get a look at material they thought had formed in the far outskirts of the solar system. Instead, they uncovered proof that the same type of particles forged near the sun ended up in a frozen comet's body.
"We still think comets formed far from the sun, but the material that goes into comets didn't," said NASA's Michael Zolensky, who studies cosmic dust and grains at the Johnson Space Center in Houston.
In this week's issue of Science, a team of scientists headed by Tomoki Nakamura of Japan's Kyushu University, reports they have found fragments of molten mineral droplets called chondrules, which are common in meteorites, in samples retrieved during a 2004 encounter with Comet Wild 2 ("Vilt 2").
Finding similar flash-heated material in a comet indicates that particles from the inner solar system were somehow booted out to the nether regions before solid bodies started forming.
"This definitely kills one idea -- that comets are formed of just interstellar material," Zolensky told Discovery News. "That theory is just dead."
University of Wisconsin's Noriko Kita and colleagues used a unique ion microscope to probe the tiny grains, the largest of which is about one-thousandth of an inch across. They found oxygen isotope ratios that closely resemble materials in asteroids, meteorites and even the sun itself.
Chrondules are silicates, similar to rocks on Earth. They typically need temperatures of 1,500 to 1,800 degrees Celsius to form. The grains melted and cooled quickly, with no crystallization, Kita added.
Physicists have a few ideas to explain how the early solar system became a blender. One theory credits the mix to a pair of strongly interactive magnetic fields which were generated by the infant sun and the disk of gas and dust that would give rise to its family.
"We do not know the exact process yet," said Kita. "This observation motivates us to figure out the real condition of the early solar system."
The discovery builds on early results from the Stardust science mission, which visited a comet and collected interstellar particles during a seven-year cruise that extended halfway to Jupiter.
Zolensky says it would be a good idea to get samples from another comet to make sure Wild 2 isn't just a fluke.
"We thought this was going to be a mission about what you'd find in the outer solar system but it turns out it's a mission about what's in the entire solar system," he said. "It makes it much more complicated, but it's more exciting."
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