Comets are born of fire as well as ice, the first results from the US space agency's (Nasa) Stardust mission show. In January, Stardust's sample-return capsule landed in Utah, carrying over a million tiny comet grains inside. Some of these grains contain material that formed at extremely high temperatures, scientists have found. This is a surprise. Comets formed in the cold, outer-reaches of the early Solar System, and were never exposed to such extreme heating. The Sun and the planets began forming out of a gaseous cloud called the solar nebula about 4.6 billion years ago. This "accretion disc" consisted of a hot inner region and a cold outer region where ice was able to survive. The high-temperature minerals found in the Stardust samples may have formed in the inner part, where temperatures exceeded 1,000C. But something must then have transported them out to the cold, comet-forming region known as the Kuiper Belt. "These are the hottest minerals found in the coldest place, in the 'Siberia of the Solar System'," said Donald Brownlee, chief scientist on the Stardust mission. "When these grains formed, they were incandescent - they were red or white hot." Details of the analysis were presented here at the Lunar and Planetary Science Conference in Houston, Texas. The Stardust spacecraft encountered Comet Wild-2 in January 2004. It swept up particles from the frozen body of ice and dust, passing some 240km (149 miles) from the comet's core, or nucleus. It then released its sample-return capsule as it flew back to Earth at the beginning of this year. The US-built capsule touched down in the Utah desert on 15 January. They are the first cometary dust samples ever returned to Earth. The high-temperature minerals discovered in the Stardust samples are not oddities.
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