Infrared emission from comets

IR observations of seven comets from 4 to 20 microns are discussed. It is found that the IR emission depends primarily on the comet's heliocentric distance. A model is applied to the observations based on grain populations composed of a mixture of silicate and amorphous carbon particles in the mass ratio of about 40 to 1, with a power-law size distribution similar to that inferred for Comet Halley. The results suggest that the 3.4-micron feature observed in Comet Halley is primarily due to thermal emission from the comet dust when the comet is close to the sun. It is found that the emission curve from about 4 to 8 microns provides the best spectral region for estimating the cometary grain temperature distribution.