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Genesis of Planets: Meteorites--Emissaries from the Past




Don Dixon

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Editor's Note: This story is a supplement to the Feature "The Chaotic Genesis of Planets" from the May 2008 issue of Scientific American.

Meteorites are not just space rocks but space fossils—planetary scientists’ only tangible record of the origin of the solar system. Planetary scientists think that they come from asteroids, which are fragments of planetesimals that never went on to form planets and have remained in deep freeze ever since. The composition of meteorites reflects what must have happened on their parent bodies. Intriguingly, they bear the scars of Jupiter’s early gravitational effects.

Iron and stony meteorites evidently originated in planetesimals that had melted, thereby allowing their iron and rocky silicate material to separate from each other, the heavy iron sinking to the core and the lighter silicates becoming concentrated in the outer layers. Researchers believe that this heating was brought about by the radioactive isotope aluminum 26, which has a half-life of 700,000 years. A supernova explosion or nearby star probably seeded the protosolar cloud with this isotope, in which case the first generation of planetesimals in our solar system contained plenty of it.

Yet iron and stony meteorites are very rare. Most meteorites consist instead of chondrules, which are millimeter-size pebbles that predate the formation of planetesimals and cannot survive melting. It there­fore seems that most asteroids are not left over from the first generation of planetesimals. That generation must have been cleared out, presumably by Jupiter. Planetary scientists estimate that the region now occupied by the main asteroid belt used to have 1,000 times as much material as it does now. The few grains that eluded Jupiter’s clutches, or later drifted into the region of the belt, collected into new planetesimals, but little radioactive aluminum 26 was left by then, so these bodies never fully melted. The isotopic composition of chondrules in meteorites dates them to about two million years after the solar system started forming.

The glassy texture of the chondrules suggests that before being incorporated into planetesimals, they were abruptly heated, turned to molten rock and allowed to cool. The waves that drove Jupiter’s early orbital migration should have evolved into shock fronts, which could account for this flash heating.­

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