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Ancient Meteorites Give Clues to Planet Formation

Meteoritesthat are among the oldest rocks ever found have provided new clues about theconditions that existed at the beginning of the solar system, solving alongstanding mystery and overturning some accepted ideas about the way planetsform.

The ancientmeteorites, called angrites, still contain magnetic records about the veryearly history of planets, like disk drives salvaged from an ancient computer, newresearch by MIT planetary scientist Benjamin P. Weiss indicates.

The resultsof the study, which was by a grant from the National Science Foundation?sInstrumentation and Facilities Program, are detailed in the Oct. 31 issue ofthe journal Science.

Theanalysis showed that surprisingly, during the formation of the solarsystem, when dust andrubble in a disk around the sun collided and stuck together to formever-larger rocks and eventually the planets we know today, even objects muchsmaller than planets ? just 100 miles (160 kilometers) across or so ? werelarge enough to melt almost completely.

This totalmelting of the planet-forming chunks of rock, called planetesimals, causedtheir constituents to separate out, with lighter materials including silicatesfloating to the surface and eventually forming a crust, while heavier iron-richmaterial sank down to the core, where it began swirling around to produce amagnetic dynamo. The researchers were able to study traces of the magneticfields produced by that dynamo, now recorded in the meteorites that fell toEarth.

"Themagnetism in meteorites has been a longstanding mystery," Weiss said, andthe realization that such small bodies could have melted and formed magneticdynamos is a major step toward solving that riddle.

Untilrelatively recently, it was commonly thought that the planetesimals ? similarto the asteroids seen in the solar system today ? that came together to buildplanets were "just homogenous, unmelted rocky material, with nolarge-scale structure," Weiss said. "Now we?re realizing that many ofthe things that were forming planets were mini-planets themselves, with crustsand mantles and cores."

That couldchange theorists? picture of how the planets themselves tookshape.

If thesmaller bodies were already molten as they slammed together to build up largerplanet-sized bodies, that could "significantly change ourunderstanding" of the processes that took place in the early years of thenascent planets, as their internal structures were forming, Weiss said. Thiscould have implications for how different minerals are distributed in theEarth?s crust, mantle and core today, for example.

"Eventshappened surprisingly fast at the beginning of the solar system," Weisssaid. Some of the angrite meteorites in this study formed just 3 million yearsafter the birth of the solar system itself, 4,568 million years ago, and showsigns that their parent body had a magnetic field that was 20 to 40 percent asstrong as Earth?s today.

"Weare used to thinking of dynamo magnetic fields in rocky bodies as uncommonphenomena today," Weiss said. "But it may be that short-livedplanetesimal dynamos were widespread in the early solar system."

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