Multidimensional physicochemical models of the near-nucleus coma: Present achievements and requested future developments

The published multidimensional physicochemical simulations of the near-nucleus coma (hereafter “NNC”) are reviewed. The goals of these simulations are to understand the physical origin of the observed NNC structure(s), and to relate the latter to properties of the nucleus - an inference which is often done on the basis of subjective impressions only. The NNC simulations are mostly based on ad hoc benchmark model nuclei. However, a systematic program of simulation of the NNC of comet Halley exists, which uses the observed shape of its nucleus with or without filtering of topographic details. The main conclusions to be derived from the simulations of the NNC around idealized benchmark nuclei are that: (1) only fully comprehensive quantitative modelling of the whole NNC can reveal the origin of NNC structures: there does not exist any universal relation between one nucleus feature and the NNC structure adjacent to it, independent from its surrounding; (2) in particular, it is impossible to separate the effect of the nucleus heterogeneity on the NNC, from that of the surface topography, except by global quantitative modelling; (3) these two conclusions follow from the physical laws of gas flow and dust motion, hence are independent from the nucleus gas and dust production mechanism. Regarding comet P/Halley, it was possible, using the nucleus shape derived from the cameras, to show that the observed dust “filament” structures are the signatures of the nucleus shape, and are insensitive to whether the nucleus is assumed compositionally homogeneous or inhomogeneous. Application of similar models to the high quality nucleus and NNC observations made during the recent flybys of comets Borelly, Wild-2, and Tempel-I is badly needed. In the future, the nucleus-orbiting mission Rosetta should provide the ultimate benchmark to achieve our understanding of the NNC.