Making a comet nucleus

The chemical composition of a comet nucleus can be very strictly constrained by combining the latest results on: the core-mantle interstellar dust model, the solar system abundances of the elements, the space observed composition of the dust of comet Halley, and the latest data on the volatile molecules of comet comae. The distribution of the components in the comet nucleus fall naturally into two basic categories - refractories and volatiles. The refractory components are tightly constrained to consist of about 26% of the mass of a comet as silicates (a generic term for combinations of the elements Si, Mg, Fe), 23% complex organic refractory material (dominated by carbon), and about 9% in the form of extremely small (attogram) carbonaceous/large molecule (PAH) particles. The remaining atoms are in an H_2O dominated mixture containing of the order of 2-3% each of CO, CO_2, CH_3OH plus other simple molecules. The H_2O abundance itself is very strictly limited to ~ 30% of the total mass of a comet - not much more nor much less. The refractory to volatile (dust to gas) ratio is about 1:1, while the dust to H_2O ratio is ~ 2:1. The maximum mean density of a fully packed nucleus would be ~ 1.65 gcm(-3) . The morphological structure of the component materials, following the interstellar dust into the final stage of the presolar cloud contraction, is as tenth micron silicate cores with organic refractory inner mantles and outer mantles of ``ices'' with each grain containing many thousands of the attogram carbonaceous/large molecule particles embedded in the icy and outer organic fraction.