Minerals Point to a Hot Origin for Icy Comets

Abstract

HOUSTON, TEXAS— Scientists analyzing the first samples returned from a comet announced startling news this week. 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 told how team members on four continents have been slicing, dicing, and analyzing 10-micrometer particles collected by the Stardust spacecraft. It swept by comet Wild 2 two years ago and returned its samples to Earth on 15 January. Working first on the larger particles snared in the Stardust collectors, analysts are finding mineral crystals such as forsterite, pyroxene, anorthosite, spinel, and titanium nitride. These “are all minerals that formed at moderately high to extremely high temperatures,” Stardust principal investigator Donald Brownlee of the University of Washington, Seattle, later told a press conference at NASA's nearby Johnson Space Center. “These are hot minerals from the coldest place in the solar system,” the comet-forming region beyond Neptune. ![Figure][1] A hot one. This 2-micrometer bit of comet Wild 2—a magnesium-rich olivine called forsterite—formed at a high temperature, perhaps near the young sun. CREDIT: NASA/JPL-CALTECH/UNIVERSITY OF WASHINGTON The minerals must have formed at 1400 K or hotter, Brownlee said, especially a couple of particles resembling the so-called calcium-aluminum inclusions (CAIs) known from meteorites. In contrast, the dust the analysts expected to find in comets would be submicrometer in size and lacking in any crystalline structure. That's the form they would have taken as they condensed from vapor in deep space after being blown off other stars. Brownlee offered two possible solutions to the hot-and-cold conundrum. 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 CAIs and once-molten droplets called chondrules that dominate the most common type of meteorite coming from the asteroid belt ( Science , 20 June 1997, p. [1789][2]). Shu argues that the young, violently active sun would have blasted nearby solids to their melting points and magnetically flung them—including CAI and chondrule particles—out over the disk as far as the comet-forming region. Alternatively, says Brownlee, the Stardust minerals may have crystallized from melts near other stars and reached the forming solar system by some unspecified means. “If this were astronomy, we'd stop there,” Brownlee told his colleagues. Astronomers have nothing to go on but the electromagnetic spectrum, which would yield no further information in this case. “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. “We'll know in weeks or months,” says Brownlee. [1]: pending:yes [2]: /lookup/volpage/276/1789