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Frozen Water Discovered on "Deep Impact" Comet

Elizabeth Svoboda
for National Geographic News
February 2, 2006
 
Astronomers have tracked comets since before Galileo's time. But their exact composition has long remained a mystery. The thick, dusty clouds that trail comets make them difficult to observe with ground-based telescopes.

New images and data from NASA's Deep Impact spacecraft, however, have revealed the first confirmed deposits of water ice on the comet Tempel 1.

The discovery is likely to add to the debate over theories that comets seeded Earth with water and the organic compounds necessary for life.

Last July the NASA mission slammed an 820-pound (370-kilogram) projectile into Tempel-1, producing a crater and cloud of dust to analyze. (See photo.)

"Dirty Skating Rink"

At the same time, the Deep Impact spacecraft took close-up photos of the dust-covered surface of Tempel 1, a Manhattan-size comet that orbits between the orbits of Mars and Jupiter.

Jessica Sunshine of the Science Applications International Corporation in Chantilly, Virginia, says that when she and her colleagues looked at these surface photos, they noticed something unusual.

"There were some bright areas that we were suspicious about—brighter than the rest of the comet, which was darker than charcoal," she said.

Infrared imaging of the comet helped clarify what the researchers were seeing, allowing them to chart the reflectivity of different areas of the comet. This made it easier to confirm the presence of ice and pinpoint its location.

"We mapped the data and saw absorption-spectrum bands that are very characteristic of the structure of water ice," Sunshine said. "The surface is about 5 percent ice, and the rest is just dark dirt. So it's like a very dirty skating rink."

Microcrystals

The ice crystals the team detected are extremely fine, measuring tens of microns across, or narrower than a human hair.

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."

Comet Dust

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 space—more than a ton every day—constantly passes through Earth's atmosphere and settles on its crust.

Scientists think this dust contains life-facilitating organic compounds, such as carbon dioxide and ethanol—a 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 water—the most essential ingredient for life—along 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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