Comets Halley, crystalline silicates

Also, observations of crystalline silicates in the comets Halley (Swamy etal. 1988) and Tempel 1 (Harker et al. 2005) suggest that a large fraction of cometary dust was processed or formed in the hot inner regions of the protoplanetary disk and transported to the region where the comets formed. 

Among the goals for the Stardust mission was identifying the origin of the crystalline silicates in comets, whose presence in comets had been known from observations of comets Halley and Hale-Bopp (as reviewed in Bockelee-Morvan... 

Both the analysis of Comet Wild 2 dust particles (Brownlee et al. 2006) and infrared spectroscopy of dust particles from comets Halley, Hale-Bopp, and Tempel 1 (Lisse et al. 2006) have shown that crystalline silicates are common constituents of comets. The presence of crystalline silicates in comets indicates that the crystallization of amorphous interstellar silicates occurred in the comet-forming region... 

Fine structure on the silicate feamre was first seen in observations of comet Halley. It consisted of a small 11.2 pm bump on the silicate feature detected in high spectral resolution and good signal-to-noise ratio observations. This small bump has now been seen on the silicate feature of several comets and it is widely interpreted as evidence for the presence of olivine because IR studies of powdered olivine samples show fine stmcmre at 11.2 pm. In the astronomical literature this feature is considered to be crystalline olivine as compared to amorphous silicates that cannot produce the pronounced 11.2 pm bump on the overall 10 pm silicate feamre. 

Keller H. U. and Thomas N. (1997) Comet Halley s nucleus a physical interpretation. Adv. Space Res. 19, 187-194. Kemper F. and Tielens A. G. (2003) The crystallinity of intersteller silicates. Astrophysics of Dust, Ester Park Co., May 26-30 (in press). 

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