Navier–Stokes and direct Monte-Carlo simulations of the circumnuclear gas coma III. Spherical, inhomogeneous sources

We pursue our program of comparative simulations of the cometary gas coma by the two most advanced techniques available: (1) numerical solution of Navier–Stokes equations coupled to the Boltzman equation in the surface boundary layer, and (2) direct Monte-Carlo simulation. Here, we consider two different spherical but compositionally inhomogeneous nuclei, at three very different levels of gas production. The results show the same excellent agreement between the two methods in a domain adjacent to the surface as found precedingly, practically down to free-molecular conditions. A wealth of coma density patterns with non-intuitive structure is obtained. Some of these structures appear even under free-molecular effusion from the surface. The physical origin of all structures is discussed, and their evolution with changing gas production is studied. The computed comae are compared to those computed by various authors precedingly. Intercomparison of the present results demonstrates that differing inhomogeneity patterns may lead to similar structures in the gas coma. Comparison between these structures and those created by homogeneous, aspherical surfaces shows that it is not possible to guess from empirical rules which one of the two processes is responsible for the creation of a given structure. The implications for the interpretation of future high resolution images, or of future in situ mass spectrometric samplings of the near-nucleus gas coma are discussed.