Primordial Pebbles

Most scientists believe that the solar system--sun, planets and all--condensed out of a vast nebula of gas and dust. Graduate Student Craig M. Merrihue of the University of California at Berkeley is even convinced that some of the first objects that condensed are still around and can be identified. They are "chondrules"--round, pea-sized or even smaller globules of stony material. When they happened to be embedded in meteorites, the tiny pebbles were preserved by the cold and vacuum of interplanetary space and lasted for billions of years.

Merrihue started his research by cadging a 4-lb. chunk of the stony Bruderheim meteorite that fell in Canada in 1960. He crushed it carefully and separated 14 chondrules from the debris. Then he ground the remainder and purified a sample of meteor material until it was free of chondrule fragments. He heated both samples separately and measured the amount of xenon gas that was driven out of them. The chondrules, he found, contained considerably more xenon 129 than the rest of the meteor.

Xenon 129 is a rare xenon isotope that is the descendant of iodine 129, a radioactive form of iodine that was created with the rest of the elements that formed the solar nebula and became extinct not many million years later. Since chondrules contain xenon 129, Merrihue argues that they must have acquired it from the decay of iodine 129. This means that they condensed as droplets during the infancy of the solar system, when everything else in the nebula was dust or gas--and they must be older than the earth or the sun.

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