SAN FRANCISCO, CALIFORNIA--Fragments of a comet retrieved from space by NASA's Stardust mission have yielded a big surprise. The comet was thought have been formed exclusively in the icy backwaters of the solar system. Instead, scientists have found that some of the particles from Comet Wild 2 (pronounced "Vilt 2") had been cooked at near-record high temperatures. The discovery is forcing a new look at solar system formation and creating a tantalizing hint at the basic processes of biology.
According to the prevailing hypothesis, the solar system condensed out of an immense disk of gas and dust about 4.6 billion years ago, with the sun in the middle. Trillions of comets circled the outer edges, in a region called the Oort Cloud. Any hot material formed entirely near the sun, while comets consisted solely of icy material. That's what the Stardust mission was supposed to confirm, anyway. In January 2004, the spacecraft flew through Wild 2's tail and captured tiny particles in a special collector made of a glasslike substance called aerogel that was designed to trap comet grains without altering them.
Last January, Stardust's collector parachuted to Earth in the Utah desert and, ever since, scientists have been carefully examining its contents (ScienceNOW, 14 March). The results have thrown the conventional solar system formation hypothesis on its head. Instead of an orderly condensation from center to edges, it appears that the sun was a nasty baby, throwing frequent tantrums in the form of mass ejections. The activity cooked nearby minerals and then heaved them out for billions of miles, where they eventually were captured and preserved in comets.