Why some meteorites contain microscopic diamonds

Celestial diamonds, falling to Earth in interplanetary space. Diamond is a form carbon acquires only when subjected to extremely high pressure and possibly high temperatures as well. Thus, when microscopically sized diamonds were found in the Canyon Diablo meteorite which formed Meteor Crater in Arizona, scientists concluded that the shock of the impact had transformed some of the meteor's carbon.

But this can't be said of similar diamonds which Daniel E. Appleman, Roy S. Clark, and Daphne Ross of the Smithsonian Institution have reported finding in a meteorite from the Allen Hills in Antarctica. That meteorite is much smaller than the Canyon Diablo specimen. It would have been slowed too much by the atmosphere and would have landed too softly for the impact to have formed diamonds. Thus the diamonds are probably the result of two asteroids colliding in space -- an event more than violent enough for a shock wave to produce the pressures and temperatures needed.

This is the only iron meteorite besides Canyon Diablo known to contain diamonds. These two bodies are similar chemically adn structurally and are quite distinct from a horde of other iron meteorites found in the Allen Hills in recent years. This, the Smithsonian scientists say, suggests that the Canyon Diablo diamonds may also have formed in a collision in space rather than when the meteorite hit the ground.

Thus, diamonds in iron meteorites join a variety of mineral crystals that line cavities in some of the stony meteorities to provide clues to inerplanetary smashups. Edward Olsen of Chicago's Field Museum of Natural History has recently pointed out in the journal Meteoritics that the cavities and their minerals seem to have formed by heating such as that caused by shock waves.

Scientists such as Olsen and the Smithsonian researchers don't think meteoritic crystals or diamonds formed under gravitational pressure on an ancestral body which later broke up into asteroids. However, the planets Uranus and Neptune may have diamond-coated cores.

These planets are thought to have inner rocky cores surrounded by liquid water, methane, adn ammonia. Marvin Ross of the Lawrence Livermore National Laboratory has combined laboratory experiments on these liquids with theory to suggest that methane breaks down into hydrogen and carbon, which then converts at least partly to diamond.

Ross's report, which like that of the Smithsonian scientists appeared in Nature, shows that carbon and hydrogen might also exist as metals. This would give the planets an electrically conducting inner layer which, in turn, might generate magnetic fields. For planetary scientists, that is a more intriguing prospect than the presence of a material that happens to be considered precious on Earth.