for National Geographic News
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 aboutbrighter 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."
The ice crystals the team detected are extremely fine, measuring tens of microns across, or narrower than a human hair.
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