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.
"This comet may have formed far from the sun," says astromaterials researcher and Stardust team member Michael Zolensky of NASA's Johnson Space Center in Houston, Texas. "But things in it did not." Some of the grains found in the collector are the product of extremely high temperatures, well over 1500 degrees C, he noted here yesterday at the annual meeting of the American Geophysical Union.
Reporting in seven papers in today's Science, Zolensky and an international team of researchers also describe how the comet grains represent a surprising variety of minerals--including organic matter--which proves that the early solar system was a maelstrom of activity, with material spread far and wide within the protoplanetary disk. That could have huge implications for astrobiology, says astrophysicist and Stardust team member Scott Sandford of NASA's Ames Research Center at Moffett Field, California. If this is how organic materials were distributed in our solar system, he says, it likely happened the same way in other parts of the universe. "So if this is how life got started, it's a model for all solar systems."