Monolithic Nucleus	Multi-component Nucleus	Differentiated Nucleus

We have many ideas but not many constraints on the interior structure of cometary nuclei. What little we know is inferred from observations of comets that split, and from our limited knowledge of the way in which comets accrete in the protoplanetary nebula.

Monolithic Nucleus
Perhaps the simplest idea is that the nucleus is a single, coherent body of uniform internal composition. There is a mantle (red in the figure) composed of rubble exposed by sublimation. The monolithic nucleus would be strong, so it is not clear that his model can explain observations of comets splitting in response to tiny tensile forces.

Multi-component Nucleus
The multi-component nucleus (also known as Rubble Pile nucleus) consists of many structurally independent bodies loosely held together by mutual gravitational attraction. There would be many more pieces than in the sketch above - I couldn't draw them, though. The bodies are sometimes identified with individual precursor planetesimals from the protoplanetary disk. Some people like to think that the sub-units within the nucleus might be planetesimals from very different original locations in the solar nebula. If so, they might have different compositions. A mantle envelopes the whole assemblage as with the Monolithic nucleus model. Because the building blocks are not tightly bound, a multi-component nucleus could be very weak. This fits with what we think we know about the strengths of comets.

Differentiated Nucleus
Some people have speculated that nuclei might be partially differentiated, just as the Earth has differentiated into an iron core + rock mantle structure. I don't expect such dramatic differentiation in comet nuclei because these objects are mostly too small to generate very high internal temperatures. However, radioactive heating from long-lived nuclei of Potassium, Thorium, Uranium might be sufficient to drive internal migration of the most volatile ices (e.g. CO, N2) away from the "core" (shaded pink in the 3rd panel, above). In smaller comets, the short-lived Al26 nucleus could supply the heat.

Real Nucleus
Very probably, real comet nuclei incorporate features of all the models shown here. Small nuclei may be monolithic, while larger ones consist of agglomerated pieces. Cometary material is such a poor conductor of heat that it is entirely possible that large nuclei possess a core depleted of the most volatile ices. The challenge is to take observations that constrain the many models.

Nucleus	Comet	Jewitt