Comet's Formation Appears Gradual

Collision Provides Data on Its Origin

LOS ANGELES -- The plume of debris that spilled from a comet after it collided with a space probe is as fine as talcum powder, suggesting the comet formed gradually, scientists said.

Scientists continue to study data gathered when a copper probe from NASA's Deep Impact spacecraft blasted a crater in comet Tempel 1 on July 4 to expose its primordial core.

Comets are believed to be the frozen leftover building blocks of the solar system, formed when a huge cloud of gas and dust collapsed about 4.5 billion years ago. Studying them could provide clues to the birth of the solar system.

Soon after the 820-pound probe hit Tempel 1, scientists detected evidence of water, carbon dioxide and organic substances spewing from the comet. The high-speed collision produced two flashes of light and hurled a plume of fine, powdery dust from the comet thousands of miles into space.

"This probably means the material in the comet came together very gently," said Michael F. A'Hearn, an astronomer at the University of Maryland and the mission's principal investigator. "If it melted and resolidified, it would have the strength of solid ice."

Scientists are waiting for the dust from the larger-than-expected debris cloud to settle so they can get their first glimpse at the inside of the comet and determine the size and depth of the crater. They said the crater was larger than a house and possibly as big as a football stadium.

Comets are believed to be abundant in water, and astronomers were surprised to find a lack of water vapor after the collision. Preliminary findings by a science instrument aboard a NASA satellite in Earth orbit showed Tempel 1 released about 550 pounds of water per second, similar to the amount before the impact, suggesting the comet contains more dust than ice.

"It's pretty clear that this event did not produce a gusher," said Gary Melnick of the Harvard-Smithsonian Center for Astrophysics.

The findings appear to contradict results by the European Space Agency's XMM-Newton Observatory, which last week found evidence of increased water in the comet's emissions after the impact.