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We find spectacular silicate crystals in the comet.The presence of crystals in comets was suggested by astronomical observations but the Stardust mission results provide important new insight into their origin and history. When the presence of crystals in comets and in disks of dust orbiting other stars was first observed astronomically it was a mystery because crystals are not detected in interstellar dust (the dust between stars), the building materials that are used to form solar systems and comets. Interstellar dust is composed of glassy materials with no crystalline order. The widely accepted hypothesis for the origin of cometary crystals is that they are interstellar grains that were transformed to crystals by mild warming in the vicinity of the star that they orbit.
The comet samples collected by Stardust contain abundant crystalline minerals and in most cases it is clear that they did not form by the predicted mild heating of interstellar dust. Many are too large, and have complex mineralogical and chemical compositions that could not have formed by this process. Instead of the mild heating that astronomers envisioned the comet samples were heated during their formation to severe temperatures, temperatures high enough to melt or vaporize them. The temperatures above 1300 ºC and the samples were white hot.This is quite remarkable because the some of the ice components of comets appear to have formed only 30 degrees above absolute zero. The comet is very odd mix of materials that formed at the highest and lowest temperatures that existed in the early solar system. Comets have been cold for billions of years but their ingredients are remarkable products of both fire and ice. Because the rocky materials in comet Wild 2 formed at such high temperatures, we believe that they formed in the hot inner regions of the young solar system and were then transported all the way to beyond the orbit of Neptune. There had been several theories that suggested that such large scale mixing might have occurred and the Stardust results appear to be proof that this large scale mixing did occur and that many of the large rocky particles formed close to the Sun. This implies that while the comets contain ices that formed at the edge of the solar system, the rocky materials that actually make up the bulk of a comet’s mass actually formed in the hottest possible conditions. The inner solar system can be thought of as a factory producing rocky materials that were distributed outwards to all the bodies and regions of the solar system.
One of the most remarkable particles found in the Stardust collection is a particle named after the Inca Sun God Inti. Inti is collection of rock fragments that are all related in mineralogical, isotopic and chemical composition to rare components in meteorites called “Calcium Aluminum Inclusions” or CAI’s for short. CAI’s are the oldest materials that formed in the solar system and they contain a remarkable set of minerals that form at extremely high temperature. In addition to these same minerals, Inti also has tiny inclusions that may have been the first generation of solids to condense from hot gas in the early solar system. These include compounds of titanium, vanadium and nitrogen (TiN and VN) as well as tiny nuggets of platinum, osmium, ruthenium, tungsten and molybdenum. In certain chemical environments and at high enough temperature in the early solar system these exotic materials were the only solid materials that could survive without being vaporized.
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Track the CAI particle was found in from Aerogel cell #54. Courtesy: NASA |
The comet particles returned by the Stardust mission have been a real bonanza.They do contain some stardust grains from other stars but the majority of solids are solar system materials that appear to have formed over a very broad range of solar distances and perhaps over an extended time range. Comet Wild 2 is a collection of materials that probably came from all regions of the young solar system and thus it has turned out to be wonderful “time capsule”.Hundreds of scientists around the world have worked on these samples and the first results from these studies were presented in the December 15, 2006 issue of Science magazine. Having samples from the edge of the solar system has provided a fabulous way to explore the early solar system and test ideas for its origin. The samples will be explored for decades to come and we believe that they provide a wonderful new way to better understand our origins.
