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G.E. Hassel (RPI, Siena College), W.G. Roberge (RPI)
The water ice mantles on interstellar grains trap volatile molecules, such as CO and CH3OH, with an efficiency that depends on the amorphous or crystalline structure of the ice. The ice structure therefore affects the composition of comets formed from the icy grains. We present a detailed study of the processing of mantled grains by shock waves in protoplanetary disks. The grains suffer a sudden increase in temperature that can evaporate the mantles, followed by an extended cooling time during which the mantles recondense. We find that H2O ice most effectively transforms to the cubic crystalline phase by removal and recondensation, particularly for models at the higher ends of the density and shock speed ranges considered. We evaluate different scenarios for re-deposition of the mantle, and determine the degree of retention of guest molecules for different water:guest ice mixtures. The results show distinct changes in the solid abundances of the guest species over a range of conditions representative of the giant planets region of the solar nebula. We associate hydrodynamic parameters with radial position by means of a viscous accretion disk model (Aikawa et al. 1998) in order to make this connection. In particular, the crystallization of ice and the exclusion of volatiles from the matrix may explain the volatile-depleted composition observed in Comet C/1999 S4 (LINEAR), an Oort-cloud comet originating from the Jupiter-Saturn region (Mumma et al. 2001). This work is supported by the New York Center for Studies on the Origins of Life (NSCORT) at RPI, under NASA grant NAG5-7598.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.