COMET HALLEY - SPATIAL AND TEMPORAL VARIABILITY OF THE SILICATE EMISSION FEATURE

Narrow- and broadband photometry of Comet Halley covering the wavelength range 2-20-mu-m was obtained at the NASA Infrared Telescope Facility (IRTF) both preperihelion (UT 1986 January 17 and 18) and postperihelion (UT 1986 March 3-5). Strong features at 10 and 20-um-m, indicative of silicate emission, were evident in our data. In addition to day-to-day variations in brightness at all wavelengths, we observed changes in the strength of the silicate emission feature on timescales as short as 3 hr. While the strength of the 10-mu-m feature varied with location in the coma, no such variability was observed in the shape of this emission feature, in agreement with higher spectral resolution observations (Campins & Ryan 1989). Calculated color temperatures indicate that brightness and temperature do not correlate directly, i.e., brighter did not necessarily imply hotter dust grains. Our observations are used to refine a relation between the color temperature of the dust and the heliocentric distance derived by Tokunaga et al. (1988).