MAKING A COMET NUCLEUS

The chemical composition of a comet nucleus can be very strictly const rained by combining the latest results on: the core-mantle interstella r dust model, the solar system abundances of the elements, the space o bserved composition of the dust of comet Halley, and the latest data o n the volatile molecules of comet comae. The distribution of the compo nents in the comet nucleus fall naturally into two basic categories - refractories and volatiles. The refractory components are tightly cons trained 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% comple x organic refractory material (dominated by carbon), and about 9% in t he form of extremely small (attogram) carbonaceous/large molecule (PAH ) particles. The remaining atoms are in an H2O dominated mixture conta ining of the order of 2-3% each of CO, CO2, CH3OH plus other simple mo lecules. The H2O abundance itself is very strictly limited to similar 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 du st to H2O ratio is approximate to 2 : 1. The maximum mean density of a fully packed nucleus would be approximate to 1.65g cm(-3). The morpho logical structure of the component materials, following the interstell ar 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 ic y and outer organic fraction.