New View of Comet Formation Forged in Study of Tiny Particle
This image of comet Wild 2 shows one hemisphere in sunlight and the other in shadow, like a view of the quarter Moon. Several large depressed regions can be seen. The comet is about 3.1 miles (5 kilometers) in diameter.
Credit: NASA/JPL/Stardust

Comets have a reputation of being outer solar system natives that were long thought to be made up of pristine remnants of the building blocks of our sun?s planets and moons.

But new analysis of a tiny piece of one comet shows that some of its material actually formed in the inner solar system ? just like that of meteorites ? before it was kicked outward and incorporated into the comet. The study provides the first information on when comet material formed, in this case almost 2 million years after the first material in the solar system was forged, about 4.6 billion years ago along with the birth of the sun.

The particle under study, dubbed "Coki," was picked up from Comet Wild 2 (pronounced "Vilt 2") by NASA's Stardust mission.

To figure out just when and where the material in Coki formed, Jennifer Matzel, a postdoctoral researcher at Lawrence Livermore National Laboratory in California, and her colleagues looked to see if the particle contained any of a particular radioactive isotope of aluminum, which would have been long gone after the first few million years of the solar system's history.

This isotope is signature of the oldest solids thought to have formed in the early solar system, called Calcium-Aluminum-rich inclusions (CAIs).

But Coki seemed to contain little, if any, of the magnesium isotope that the radioactive aluminum decays into. The lack of these isotopes suggests that the material in Coki didn't form until at least 1.7 million years after the first material in the solar system formed.

The research is detailed in the Feb. 26 issue of the journal Science.

The material in Coki also seems to have undergone periods of melting, suggesting that it formed in the inner solar system, closer to the sun. Some meteorite particles also show these signs of heating and melting, which suggest that material for both of these solar system bodies formed in similar environments.

How the material in Coki got transported to the outer solar system, whether by lofting above the solar system disk or mixing processes within it, isn't yet known, but it likely occurred during a time period of more than two million years, the researchers say.

The information gleaned from Coki adds to the picture being formed by the particles collected from Wild2 by Stardust, though there are many more to analyze. Whether more of these particles show signs of coming from the inner solar system or look like more pristine original solar system materials remains to be seen.

"I think we're still learning a lot about the diversity of particles in the Stardust collection," Matzel told

Stardust is currently on its way toward another comet, Tempel 1, where it should arrive in 2011.

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