Collisional coma models: An unorthodox overview

An overview of recent inner coma models is attempted from the somewhat unconventional point of view of the physics of rarefied inhomogeneous jets : in doing this we try to bridge the gap which separates cometary modelling from the treatments of similar problems which appear in other contexts. We introduce the Navier Stokes Equations as the relevant method by which this class of problem should be dealt with. The characteristics of 19 recent inner coma models are given. The question of their mutual consistency is raised the discussed in detail in one particular but very important case : that of the water temperature field. It is shown that none of the reviewed works can claim to produce a fully satisfactory temperature field. As a consequence, while all works demonstrate remarkable achievements, their relevance for the interpretation of specific experimental results may be smaller than implied by the authors. The main conclusion of this overview is that standardisation of the modelling of the inner coma (parameter values and algorithms by which elementary processes are accounted for) is in urgent need.