Cometary nucleus and active regions

On the basis of the icy-conglomerate model of cometary nuclei, various observations demonstrate the 'spotted' nature of many or most nuclei; i.e., regions of unusual activity, either high or low. Rotation periods, spin axes, and even precession of the axes have been determined. Narrow dust jets near the nuclei of some bright comets require that small sources be embedded in larger active areas. Certain evidence suggests that very dusty areas and very dusty comets may be less active, respectively, than surrounding areas or other comets.