Most experts agree that the solar system's most ancient rocks from asteroids and comets should be sprinkled with microscopic diamond dust, a remnant of ancient stars. But a group of researchers has found that at least some of the most primitive, unaltered rock in the solar system contains no diamond star dust at all. The finding raises questions about just how star stuff came to form the solar system.
Researchers have found interstellar dust grains in some less-altered meteorites by a drastic procedure: dissolving a specimen until all that remains are the hardy mineral bits condensed in the atmospheres of stars long ago--silicon carbide, graphite, and diamond. The diamond flecks are so small that a single grain may contain just a few thousand atoms. Three years ago, microscopist Zurong Dai of Georgia Institute of Technology in Atlanta and his colleagues decided to extend the diamond hunt to microscopic interplanetary dust particles (IDPs) that flaked off asteroids and comets and now sift down through the stratosphere.
In an analytical tour de force, Dai and his colleagues exposed the nanodiamonds a gentler way. They partially dissolved samples and identified diamonds by measuring their distinctive atom-to-atom distance under high-resolution transmission electron microscopy. As the team reports in the 11 July issue of Nature, there were plenty of nanodiamonds in four large, “cluster-type” IDPs from the stratosphere. But none turned up in five smaller IDPs, although they were just as primitive as the cluster IDPs--and therefore also presumably came from the outer parts of the solar system, where stellar nanodiamonds are most likely to have survived.
"It really was an unexpected result," comments microscopist Lindsay Keller of NASA's Johnson Space Center in Houston. "Why nanodiamonds are not there is uncertain." The simplest answer, Dai's group writes, would be that most nanodiamonds were not formed around ancient stars at all, but in the inner parts of the disk-shaped solar nebula as the solar system formed. But if popular theories about chemical conditions in the early solar system are correct, diamonds shouldn't have been able to form there.