While the amount of water ice is minuscule, it hints at greater concentrations below Tempel 1's surface.
Sunshine thinks the clouds of water vapor the comet produces are too large to be emerging from the tiny ice deposits found so far. Initial mission data confirms her hunch, she says.
"When we compared the comet's atmosphere before the impact and after the impact, the organic compounds we detected went up by a factor of 20 afterward," she said. "So the surface is not representative of what's on the inside."
Dale Cruikshank, an astronomer at NASA's Ames Research Center in Moffet Field, California, has analyzed other data from Deep Impact.
He believes Sunshine's investigation of Tempel-1 provides important clues about the composition of comets in general.
"We've been expecting to see water ice in comets for a long time. But it's not easy to detect, since they're surrounded by layers of gas and dust," he said. "This finding gives new insight into what comets are made of and how they behave."
Astrobiologists like Scott Sandford, who is also based at the Ames Research Center, have speculated for years that comets might have kick-started the rise of life on Earth.
Comet dust from deep spacemore than a ton every dayconstantly passes through Earth's atmosphere and settles on its crust.
Scientists think this dust contains life-facilitating organic compounds, such as carbon dioxide and ethanola theory that will be tested this spring as NASA analyzes the results of its Stardust mission.
(The Stardust craft collected dust surrounding the Wild 2 comet in the Kuiper belt beyond Neptune and jettisoned samples to Earth last month.)
Sunshine's findings suggest that comets could have easily delivered waterthe most essential ingredient for lifealong with other organic compounds to Earth's surface. Though Sunshine is excited that her work could help inform the scientific dialogue about the origins of life on Earth, she says it is premature to draw any conclusions.
"Water and organic molecules are necessary but not sufficient to create life," she said. "It's possible comets resupplied Earth with water after its own water was vaporized, for instance."
"But right now we just don't know the details of how everything fits together."
The study will appear tomorrow in the journal Science.
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