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H. Campins (U. Central Florida), D. S. Lauretta (U. of Arizona, Lunar and Planetary Laboratory)
Based on the observed characteristics of cometary dust and of interplanetary dust particles, Campins and Swindle (1998) favored the preponderance of anhydrous silicates in cometary solids and cometary meteorites (if they exist). However, could there be abundant yet undetected hydrated silicates in comets? The answer is yes, a significant fraction of cometary solids could be in the form of hydrated silicates. For example, in comet Hale-Bopp the observed spectral features attributed to anhydrous crystalline silicates can be produced by as little as 15% of the particles; the rest of the silicates could be a mixture of hydrated and amorphous grains. This is in part because anhydrous crystalline silicates are easier to identify spectrally than hydrated or amorphous silicates. The mineralogy of fine-grained chondrule rims provides constraints relevant to cometary silicates. The prescence of hydrated and anhydrous silicates in direct contact with each other suggest the fine-grained chondrule rims of at least some meteorites (specifically CM chondrites) formed from accretion of multiple reservoirs in the solar nebula (Lauretta et al. 2000, Geochim. et Cosmochim. Acta, v. 64, p. 3263). A model by Ciesla et al. (2003, Science, v.299, p.549) proposes that chondrule-forming shock waves in icy regions of the solar nebula can produce rapid mineral hydration, thus supporting a nebular origin for the hydrated silicates in the chondrule rims studied by Lauretta et al. (2000). Previous arguments had suggested hydration had to occur in parent bodies and not in the nebula. Hence, comets may have also accreted hydrated silicates from the solar nebula. Observationally, cometary hydrated silicates could be detected using near-infrared reflectance spectroscopy of cometary surfaces and mid-infrared spectroscopy of cometary dust. This work was supported by NASA and by the National Science Foundation.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.