"We have known for a long time that water ice exists in comets, but this is the first evidence of water ice on comets," said Jessica Sunshine, Deep Impact co-investigator and lead author of the Science article.

Sunshine, who is also a chief scientist with Science Applications International Corporation, said the discovery has produced new insight into the composition of comets. "Understanding a comet's water cycle and supply is critical to understanding these bodies as a system, and as a possible source that delivered water to Earth," she said.

"Add the large organic component in comets and you have two of the key ingredients for life."

The findings help satisfy one of the major goals of the Deep Impact mission: Find out what is on the inside � and outside � of a comet.

That is why NASA's Jet Propulsion Laboratory teamed with the University of Maryland to slam a projectile probe into Tempel 1, then analyze materials from the comet's surface and interior. When the spacecraft's copper-tipped probe collided with Tempel 1 on July 4 last year, it created a spectacular extraterrestrial explosion 83 million miles from Earth.

Since then, the Deep Impact team has found an abundance of organic matter in Tempel 1's interior, and has placed its likely origins in the region of the solar system now occupied by Uranus and Neptune. The report in Science reveals that the comet's surface features three pockets of thin ice. The area the ice covers is small. The surface area of Tempel 1 is roughly 45 square miles or 1.2 billion square feet, but the ice covers only about 300,000 square feet, only 6 percent of which consists of pure water ice. The rest is dust.

"It's like a seven-acre skating rink of snowy dirt," said Peter Schultz, professor of geological sciences at Brown, Deep Impact co-investigator and co-author on the Science paper.

Sunshine, Schultz and the rest of the team arrived at their findings by analyzing data captured by an infrared spectrometer, an optical instrument that uses light to determine the composition of matter.

Based on this spectral data, it appears that the surface ice used to be inside Tempel 1, but became exposed over time. The team reported that jets � occasional blasts of dust and vapor � may send this surface ice, as well as interior ice, to the coma, or tail, of Tempel 1.

"So we know we're looking at a geologically active body whose surface is changing over time," Schultz said. "Now we can begin to understand how and why these jets erupt."

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