The energy for the sublimation of ices happens near the surface (in the top metre or so) based on temperature readings. Thus the volatiles travel from where they are heated under the black surface to the jets, picking up solid particles as they go. There is considerable mass loss in each orbit, while the position of the jets remains relatively stable. Thus the black "skin" needs to shrink with the comet, in the places where the volatiles are sublimated.
The jets appear not to collapse in on themselves, and thus have a more solid structure around them.
The combination of observations would contradict the models of "fissures" ie cracks. Cracks causing the out gassing cannot be reconciled with the observations of discrete jets and temperature profile.
Models of regularly collapsing and new jets cannot be reconciled with non gravitational accelerations for Halley class comets that are consistent from apparition to apparition despite substantial mass loss. Jets need to be in the same configuration from apparition to apparition for the adjustments to work the way they do for Halley.