Dirty snowballs frozen in time no more.

• Position: Comets

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 a bulk of their material. "Current thinking suggests that it is impossible to form liquid water inside of a comet," confirms Dante Lauretta, associate professor of cosmochemistry and planet formation at the University of Arizona's Lunar and Planetary Laboratory, Tucson.

Graduate student Eve Berger, who led the study, and her colleagues from Johnson Space Center, Houston, Tex., and the Naval Research Laboratory, Washington, D.C., made the discovery analyzing dust grains brought back to Earth from the comet Wild-2 as part of the NASA Stardust mission. 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 2006. "In our samples, we found minerals that formed in the presence of liquid water," Berger indicates. "At some point in its history, the comet must have harbored pockets of water."

Comets frequently are called dirty snowballs because they consist mostly of water ice, peppered with rocky debris and frozen gases. Unlike asteroids (extraterrestrial chunks made up of rock and minerals), comets sport a tail--jets of gas and vapor that the high-energy particle stream coming from the sun flushes out of their frozen bodies.

"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," says Lauretta. "The sulfide minerals formed between 122[degrees] and 392[degrees], much warmer than the subzero temperatures predicted for the interior of a comet."

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