My remarks will present some views of the behavior of the nucleus and problems with their explanation.
One thing we definitely know about comets is that there has to be a permanent structure which revolves around the sun in the comets orbit. Permanent means carrying over from apparition to apparition during the lifetime of the comet. I adopt some form of the “icy nucleus” model as proposed by Whipple. This structure reasonably fits most of the known cometary features although no completely consistent model accounting for all known phenomena in a satisfactory manner has yet been described. The icy nucleus does not appear to have any major flaws. It has a real advantageous feature in that detailed models can be constructed and their behavior more or less accurately predicted, e.g. (Donn 1963; Delsemme and Miller, 1971; Sekanina 1972, 1973; Huebner, 1975).