The team reports its findings in a series of studies appearing in this week's issue of the journal Science.
Surprise Differences
Wild 2, which is billions of years old, has spent most of its lifetime circling far beyond Pluto—preserving it in frozen isolation from the rest of the solar system.
(Explore a virtual solar system.)
But although the comet contained a great deal of ice, none of the comet's minerals appear to have interacted with liquid water, the scientists said.
That means the ice was always frozen, the researchers explained.
This was not the case for comet Tempel 1, which scientists examined in 2005 when the probe Deep Impact was deliberately crashed into it.
Materials analyzed after that collision showed that Tempel 1 appeared to contain clay and other minerals that can only form in conjunction with liquid water.
(Read "'Deep Impact' Comet Spewed Tons of Water, Study Finds" [April 4, 2006].)
"But we've not seen any evidence of any of those [minerals] in this comet," Zolensky said.
The scientists cannot yet explain this disparity between the two comets.
"Probably some comets are different from other comets," Zolensky said.
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