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University cosmochemists Lawrence Grossman and Steven Simon have studied scores of meteorites during their careers, with a few Apollo lunar samples thrown in for good measure. But until 2006, they had never before examined a verified sample of a comet.
Much to their surprise, what they found looked a lot like components of some of the meteorites in their research collection.

"The thing that strikes me about the sample is how similar the mineral identities and the chemical compositions are to the things that we find in carbonaceous chondrites" - Grossman, Professor in Geophysical Sciences and the College.

These meteorites contain material that has been unaltered since the birth of the solar system 4.5 billion years ago. The cometary grains differ from carbonaceous chondrites in their complete lack of water-bearing minerals, however.
The comet samples that Grossman and Simon examined were among thousands of dust particles that NASAs Stardust spacecraft collected from Comet Wild 2 in January 2004. Two years later, Stardust became the first mission to return samples of a comet to Earth.

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Samples of Comet Wild 2 suggest it is made of rocky material, like an asteroid, rather than the fluffy dust expected of a comet. The object may be a refugee that formed in the asteroid belt before getting kicked to the chilly fringes of the solar system, or it might have formed in that frigid realm from material thrown out of the inner solar system, scientists say.

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Tiny samples of a glowing comet, flown back to Earth by a pioneering spacecraft named Stardust, hold remarkably little dust from any ancient far-off stars, but a lot of the stuff that makes up nearby rocky asteroids, Livermore scientists have found to their surprise.

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Contrary to expectations for a small icy body, much of the comet dust returned by the Stardust mission formed very close to the young sun and was altered from the solar systems early materials.
When the Stardust mission returned to Earth with samples from the comet Wild 2 in 2006, scientists knew the material would provide new clues about the formation of our solar system, but they didnt know exactly how.
New research by scientists at Lawrence Livermore National Laboratory and collaborators reveals that, in addition to containing material that formed very close to the young sun, the dust from Wild 2 also is missing ingredients that would be expected in comet dust.  Surprisingly, the Wild 2 comet sample better resembles a meteorite from the asteroid belt rather than an ancient, unaltered comet.
Comets are expected to contain large amounts of the most primitive material in the solar system, a treasure trove of stardust from other stars and other ancient materials. But in the case of Wild 2, that simply is not the case.
By comparing the Stardust samples to cometary interplanetary dust particles (CP IDPs), the team found that two silicate materials normally found in cometary IDPs, together with other primitive materials including presolar stardust grains from other stars, have not been found in the abundances that might be expected in a Kuiper Belt comet like Wild 2. The high-speed capture of the Stardust particles may be partly responsible; but extra refractory components that formed in the inner solar nebula within a few astronomical units of the sun, indicate that the Stardust material resembles chondritic meteorites from the asteroid belt.

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