Morphological structure and chemical composition of cometary nuclei and dust

The chemical composition of comet nuclei derived from current data on interstellar dust ingredients and comet dust and coma molecules are shown to be substantially consistent with each,other in both refractory and volatile components. When limited by relative cosmic abundances the water in comet nuclei is constrained to be close to 30% by mass and the refractory to volatile ratio is close to 1:1. The morphological structure of comet nuclei, as deduced from comet dust infrared continuum and spectral emission properties, is described by a fluffy (porous) aggregate of tenth micron silicate core-organic refractory mantle particle on which outer mantles of predominantly H2O ices contain embedded carbonaceous and polycyclic aromatic hydrocarbon (PAH) type particles of size in the of 1 - 10 nm range.