Rethinking Comet Origins

A tiny comet particle recently brought to Earth by NASA’s Stardust mission was likely forged in the furnace of the inner solar system and then carried outward to the system’s icy edge. The finding, reported by a team led by Jennifer E. P. Matzel of Lawrence Livermore National Laboratory, is forcing scientists to rethink the idea that comets formed in the solar system’s outer reaches from primordial material. Rather, the researchers say, dust returned from comet Wild 2 more closely resembles meteorites. Recent studies show that much of the material from the comet Wild 2 returned by Stardust was formed at high temperatures. Now, Matzel’s radioisotope study of “Coki,” a 5-μm Wild 2 particle rich in calcium and aluminum, solidifies this new idea. The evidence for Coki’s origins comes from its lack of the radioactive isotope ²⁶Al. The solar system’s oldest solids, known as calcium aluminum inclusions, contain ²⁶Mg formed from the decay of ²⁶Al—an isotope with a 730,000-year half-life that was present during the solar system’s early history. This implies that Coki formed or was last melted after the solar system’s ²⁶Al had already decayed—at least 1.7 million years after the formation of Ca-Al inclusions.