HOUSTON, TEXAS--Scientists analyzing the first samples returned from a comet announced startling news yesterday. They are finding not the unprocessed "stardust" thought to have glommed together in the frigid fringes of the early solar system, but bits of rock forged in white-hot heat. The discovery may mean that the disk of dust and gas from which all planetary bodies formed was far more violently mixed than previously thought.
At the Lunar and Planetary Science Conference here, leaders of the 150-strong Stardust science team described the slicing, dicing, and analyzing of 10-micrometer particles collected by the Stardust spacecraft as it swept past comet Wild 2 in 2004. Since the return to Earth on 15 January, analysts have isolated mineral crystals such as olivine, pyroxene, anorthosite, spinel, and titanium nitride. All these minerals formed at moderately high to extremely high temperatures, Stardust principal investigator Donald Brownlee of the University of Washington, Seattle, told a press conference at NASA's nearby Johnson Space Center. Temperatures greater than 1100 degrees Celsius would be needed to fuse the particles, especially those resembling calcium-aluminum inclusions found in meteorites. "These are hot minerals from the coldest place in the solar system," Brownlee said, referring to the comet-forming region beyond Neptune.
Brownlee offered two possible solutions to the hot-and-cold conundrum. First, the crystals could have come from the innermost region of the still-forming solar system, he said. Astrophysicist Frank Shu of National Tsing Hua University in Taiwan has advanced that idea to explain these inclusions and other minerals in meteorites (Science, 20 June 1997, p. 1789). Shu argues that the young, violently active sun would have blasted nearby solids to their melting points and magnetically flung them out over the disk as far as the comet-forming region. The second option, Brownlee said, is that the Stardust minerals crystallized from melts near other stars and somehow reached our early solar system.
"If this were astronomy, we'd stop there," Brownlee told his colleagues. "But we have samples; that will solve this mystery." The key will be isotopes, he said. The mix of isotopes in solar system material is wildly different from that of other stars, he noted, as evidenced in rare bits of interstellar material long known from meteorites. The Stardust team plans to measure isotope ratios in their samples to determine if the comet crystals formed in our neighborhood or in another part of the galaxy. "We'll know in weeks or months," Brownlee said.