Dave Joswiak

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The Stardust spacecraft returned minerals from icy comet Wild 2 that formed at more than 1400°C.

Mixing It Up in the Early Solar System

Dick writes about Earth and planetary science for Science magazine.

Comets are supposed to be made of the icy, unaltered remnants of the solar system. So scientists were shocked when last year they began examining the first samples ever returned from a comet. Mixed among the expected minerals that were formed in the cold between the stars were flecks of minerals formed at more than 1400°C, meaning that these minerals must have come from near the sun. A new computer model shows how this may have happened--and the findings could change our understanding of how planets, asteroids, and other solar-system objects formed.

Our solar system started out as a star at the center of a vast, disk-shaped nebula of dust and gas. By conventional thinking, the extreme heat near the sun could melt dust and form new minerals, but all of these high-temperature minerals could only drift inward toward the sun, not outward. That led to different types of minerals in the inner and outer reaches of the nebula. But the Stardust spacecraft threw a wrench in this theory by returning high-temperature minerals to Earth from comet Wild 2 (ScienceNOW, 14 March 2006).

Planetary scientist Fred Ciesla of the Carnegie Institution of Washington's Department of Terrestrial Magnetism in Washington, D.C., thinks he's found a solution to the conundrum. For computational simplicity, past modeling treated the nebula as a one-dimensional object. In such simulations, all gas and dust is assumed to behave the same way, whether it is near the nebula core or far above or below its midplane. But Ciesla performed more computationally demanding, two-dimensional simulations. In these, most nebular material still flows inward toward the sun; but near the midplane, the inward flow ceases or even reverses, letting the outward diffusion of material to the comet-forming region proceed. Ciesla presents his findings in the 26 October issue of Science.

"Fred's modeling is fantastic," says cosmochemist Joseph Nuth of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "It explains a lot of observational results," including the Stardust finding that comet material maintained its sunlike composition despite being diluted by roasted minerals. "My guess is that modeling of nebular chemistry is going to change as [this] model is accepted," Nuth adds. For example, he notes, it could help tell us how varying amounts of water ended up on Venus, Earth, and Mars.

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