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Scientists believe that like many other comets, Wild-2 originated in the Kuiper belt, a region extending from beyond Neptune's orbit into deep space, containing icy debris left over from the formation of the solar system. "The sulfide minerals formed between 50 and 200 degrees Celsius (122 and 392 degrees Fahrenheit), much warmer than the sub-zero temperatures predicted for the interior of a comet."ĭiscovered in 1978 by Swiss astronomer Paul Wild, Wild-2 (pronounced "Vilt") had traveled the outer reaches of the solar system for most of its 4.5 billion year history, until a close encounter with Jupiter's field of gravity sent the 3.4 mile-wide comet onto a new, highly elliptical orbit bringing it closer to the sun and the inner planets. "When the ice melted on Wild-2, the resulting warm water dissolved minerals that were present at the time and precipitated the iron and copper sulfide minerals we observed in our study," Lauretta said. Unlike asteroids, extraterrestrial chunks made up of rock and minerals, comets sport a tail â jets of gas and vapor that the high-energy particle stream coming from the sun flushes out of their frozen bodies.
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The discovery is to be published in an upcoming online edition of the journal Geochimica et Cosmochimica Acta.Ĭomets are frequently called dirty snowballs because they consist of mostly water ice, peppered with rocky debris and frozen gases. "At some point in its history, the comet must have harbored pockets of water." "In our samples, we found minerals that formed in the presence of liquid water," Berger said. Launched in 1999, the Stardust spacecraft scooped up tiny particles released from the comet's surface in 2004 and brought them back to Earth in a capsule that landed in Utah two years later. UA graduate student Eve Berger, who led the study, and her colleagues, Lindsay Keller from Johnson Space Center and Thomas Zega from the Naval Research Laboratory, made the discovery analyzing dust grains brought back to Earth from comet Wild-2 as part of the Stardust mission. Lauretta is the principal investigator of the UA team involved in analysis of samples returned by NASA's Stardust mission. "Current thinking suggests that it is impossible to form liquid water inside of a comet," said Dante Lauretta, an associate professor of cosmochemistry and planet formation at the UA's Lunar and Planetary Laboratory.
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For the first time, scientists have found convincing evidence for the presence of liquid water in a comet, shattering the current paradigm that comets never get warm enough to melt the ice that makes up the bulk of their material.