Structure and density of cometary nuclei

We are still at a very primitive stage in our understanding of the structure and density of cometary nuclei. Much of the evidence at our disposal is fragmentary and often indirect. Never- theless, a compelling picture is beginning to emerge of cometary nuclei as collisionally processed fractal aggregates, i.e., rubble piles. The evidence comes from observations of split and disrupted comets, in particular Shoemaker-Levy 9, from theories of planetesimal formation in the early solar nebula, from a recognition of the role of collisions in the evolution of cometary nuclei, and from theoretical and experimental studies of the fragmentation and reassembly of asteroids. This paradigm-shift away from nuclei as monolithic bodies parallels that which has occurred for aster- oids in the past decade. A related factor that strongly suggests that nuclei contain substantial macroscopic voids is estimates of the nuclear density, which, like asteroids, show comets to be “under-dense” compared with their constituent materials. We find that the bulk density of com- etary nuclei lies in the range 0.5–1.2 g cm–3, with a perhaps “best” current value of 0.6 g cm–3.