Comet Dust Suggests Solar System Mixed

Sept. 18, 2008 -- Scientists dispatched the Stardust science probe to get a look at material they thought had formed in the far outskirts of the solar system. Instead, they uncovered proof that the same type of particles forged near the sun ended up in a frozen comet's body.

"We still think comets formed far from the sun, but the material that goes into comets didn't," said NASA's Michael Zolensky, who studies cosmic dust and grains at the Johnson Space Center in Houston.

In this week's issue of Science, a team of scientists headed by Tomoki Nakamura of Japan's Kyushu University, reports they have found fragments of molten mineral droplets called chondrules, which are common in meteorites, in samples retrieved during a 2004 encounter with Comet Wild 2 ("Vilt 2").

Finding similar flash-heated material in a comet indicates that particles from the inner solar system were somehow booted out to the nether regions before solid bodies started forming.

"This definitely kills one idea -- that comets are formed of just interstellar material," Zolensky told Discovery News. "That theory is just dead."

University of Wisconsin's Noriko Kita and colleagues used a unique ion microscope to probe the tiny grains, the largest of which is about one-thousandth of an inch across. They found oxygen isotope ratios that closely resemble materials in asteroids, meteorites and even the sun itself.

Chrondules are silicates, similar to rocks on Earth. They typically need temperatures of 1,500 to 1,800 degrees Celsius to form. The grains melted and cooled quickly, with no crystallization, Kita added.