Astromaterials Research of Comet Wild 2: Terameters to Nanometers
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Research of Comet Wild 2: Terameters to Nanometers Sean Brennan
The following article is based on a Symposium X: Frontiers of Materials Research presentation given by Sean Brennan of Stanford University on April 16, 2009, at the Materials Research Society Spring Meeting in San Francisco.
Abstract Stardust, a NASA sample return mission, safely landed in the Utah desert in January 2006 after a seven-year mission, bringing with it the first cometary material from a known parent source, Comet 81P/Wild 2. One of the mission goals is to determine the starting material of the solar system. By sampling a comet, which has spent most of the past 4.6 Gyr beyond the orbit of Neptune, we expect to measure material presumed to be unaffected by the ignition of the sun. The Stardust spacecraft swept through the tail of the comet, collecting hundreds of micron-sized particles from that stream into aerogel, a low-density silica foam. An international team of materials scientists have studied the mineralogy, petrology, and elemental and isotopic abundance of these materials. Our group has studied elemental abundance using an x-ray microprobe; the morphology of the particles was examined using an x-ray microscope, which enables nanotomography of the particles while encased in aerogel. The unexpected conclusions are that much of the material from this comet was formed near the sun, after its ignition, and soon thereafter transported to the outer reaches of the solar system. These results have changed the way astrophysicists think about solar system formation.
rial from the Kuiper Belt, the region beyond the orbit of Neptune that includes the former planet, Pluto. The region, approximately 50 AU (astronomical units, the distance from the sun to Earth), is far enough from the sun that there has been little or no heating from that source, and equilibrium temperatures rarely exceed 30 K. Thus, we are interested in material some 5 billion miles or roughly 1 trillion meters from the sun. One solution is to use telescopes to perform spectroscopy on distant solar systems with stars similar to our own, looking for evidence of amorphous and crystalline materials (seen as absorption lines) in the dust surrounding those suns. Alternatively, we can take advantage of the occasional visitor from the Kuiper Belt, such as comets Tempel 1 or Wild 2. If one is focused on the leastaltered material from the pre-solar nebula, then new additions to the family of shortperiod comets are most interesting. Another source of extraterrestrial material is the asteroid belt, the region between Earth and Mars thought to be a “failed planet.” From this region we get meteorites, both those that are large enough to survive entry through the atmosphere and are found on Earth, as well as those that are collected in the upper regions of the stratosphere. The latter, because of their low mass, are thought to be least-altered by the heating that occurs during deceleration and thus most representative of the original meteoritic material. However, due to the asteroid belt’s
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