A major goal of comet research is to determine the properties of comet nuclei. They provide clues about the chemical and thermodynamic conditions of the nebula in which comets formed. In situ spacecraft investigations, supplemented by long-term ground-based observations, provide data for these investigations. We combine our knowledge from in situ investigations of Comet Halley with ground-based observations of Comets Shoemaker-Levy 9, Hyakutake, and Hale-Bopp. From Comet Halley we learned that the nucleus is very black, irregular in shape, and of low density, i.e., porous. Comet Shoemaker-Levy 9 revealed a friable structure, rich on dust. The close approach of Comet Hyakutake provided exceptional opportunities to determine the composition of the coma close to the nucleus. Taking advantage of new technology and the early detection of the very active Comet Hale-Bopp permitted determination of mixing ratios of chemical species in the coma as a function of heliocentric distance that are useful tools to probe the interior of the nucleus.

To get the composition of the coma at its interface with the nucleus, we must consider photolytic reactions, kinetics of the coma gas, coupling of the gas to the dust, solar wind interaction with coma gas, and energetic reactions of pick-up ions and their charge exchange to become energetic neutrals. Energetic ions and neutrals can sputter dust and the surface of the nucleus.

The chemical species at the coma -- nucleus interface are the same as in the comet nucleus, but the abundances are not the same. This is immediately clear from the change in the mixing ratio (e.g., CO/H₂O) of coma gases which heliocentric distance r.

For our analysis we consider three sources for the coma gas:

1. The surface of a comet nucleus which provides mostly H₂O and dust entrained by the escaping coma gases.
2. The distributed source in the coma, which gives rise to additional water that was trapped in grain aggregates and to organic species released from hydrocarbon polycondensates that are part of the dust.
3. The interior of the nucleus, which generates gases that diffuse through the porous nucleus after being liberated by heat conducted to ices more volatile than water (e.g., CO, CO₂, CH₃OH, NH₃, etc.).

The mixing ratio as function of r is an important part of this investigation. Power balance between the rate of solar energy input on the surface and the rate of transport and dissipation of energy are very important aspects for such an analysis. The question about amorphous water ice with trapped gases versus intimate mixtures of ices with crystalline water ice plays an important role in the investigation. Comet Hale-Bopp, which was observed from large heliocentric distances before perihelion to large heliocentric distances after perihelion, is ideally suited for this purpose. We investigate power balance and mixing ratios as evidence for composition and structure of the nucleus. Among structural changes of the nucleus we consider porosity and chemical differentiation.