Abstract

We consider the hypothesis that the layering observed on the surface of Comet 9P/Tempel 1 from the Deep Impact spacecraft and identified on other comet nuclei imaged by spacecraft (i.e., 19P/Borrelly and 81P/ Wild 2) is ubiquitous on Jupiter Family cometary nuclei and is an essential element of their internal structure. The observational characteristics of the layers on 9P/Tempel 1 are detailed and considered in the context of current theories of the accumulation and dynamical evolution of cometary nuclei. The works of Donn (1990), Sirono and Greenberg (2000) and the experiments of Wurm et al. (2005) on the collision physics of porous aggregate bodies are used as basis for a conceptual model of the formation of layers. Our hypothesis is found to have implications for the place of origin of the JFCs and their subsequent dynamical history. Models of fragmentation and rubble pile building in the Kuiper Belt in a period of collisional activity (e.g., Kenyon and Luu, 1998, 1999a, 1999b; Farinella et al, 2000; Durda and Stern, 2000) following the formation of Neptune appear to be in conflict with the observed properties of the layers and irreconcilable with the hypothesis. Long term residence in the scattered disk (Duncan and Levison 1997; Duncan et al. 2004) and/or a change in fragmentation outcome modeling may explain the long term persistence of primordial layers. In any event, the existence of layers places constraints on the environment seen by the population of objects from which the Jupiter family comets originated. If correct, our hypothesis implies that the nuclei of Jupiter family comets are primordial remnants of the early agglomeration phase and that the physical structure of their interiors, except for the possible effects of compositional phase changes, is largely as it was when they were formed. We propose a new model for the interiors of Jupiter Family cometary nuclei, called the Talps or “layered pile” model, in which the interior consists of a core overlain by a pile of randomly stacked layers. We discuss how several cometary characteristics – layers, surface texture, indications of flow, compositional inhomgeneity, low bulk density low strength, propensity to split, etc, might be explained in terms of this model. Finally, we make some observational predictions and suggest goals for future space observations of these objects.