Comet Dust Tells Creation Story

Dec. 14, 2006 — Comets may live most of their lives in the solar system's deep-freeze, but it wasn't always that way.

The results from the first mission to return a comet sample show that mixed in with the ice and interstellar dust grains that many scientists expected to find are particles forged in the nuclear fires of the inner solar nebula.

"We found that the comet is a real grab-bag of stuff that must have formed in many different environments, including very close to the sun," said Michael Zolensky, a NASA scientist at the Johnson Space Center in Houston and a co-author in a series of papers about the Stardust mission that appear in this week's Science.

NASA dispatched a probe in 2004 that, two years later, flew by Comet Wild-2 to trap samples popping off the comet's core. The return capsule parachuted to Earth in January.

The Stardust science team invited experts from around the world to participate in the analysis, resulting in an early and thorough first look at what the comet samples contain.

The prevailing theory had been that comets were made of particles found in the outer regions of the solar system: ices, cold minerals, bits of matter from exploded stars.

Instead the searchers found particles that formed throughout the solar nebula, including the super-heated regions near the sun.

Stardust lead scientist Donald Brownlee, with the University of Washington, estimates about 10 percent of Comet Wild-2 contains material forged in the hottest parts of the solar system.

"We really didn't expect to find anything from the inner solar system. Instead, it showed up in the second particle we looked at," Brownlee said.

Another surprise was the lack of minerals showing interaction with water.

"Some people thought we'd see a lot of evidence for interaction with liquid water, which is important because the comet has all kinds of organics," Zolensky said. "If it spent some millions of years warm with liquid water inside and organics, there's a chance that organic reactions were going on in the comet."

He added: "Some folks have even talked about viruses coming from comets. We saw zero evidence for that."

There were none from Comet Wild 2, anyway.

Though the bits of material retrieved from Comet Wild 2 are the only samples of their kind, other studies — such as the July 2005 Deep Impact mission, which remotely analyzed ejected material from Comet Tempel 1 — have turned up different results.

"All the comets we've visited so far all look different," Zolensky said. "It tells us they all have different histories."