Poster Presentation
DSMC Simulations of Global and Localized Outgassing and Dust Jets of Comet
The application of a parallel 3D Direct Simulation Monte Carlo (DSMC) code, named PDSC++, using an unstructured grid, allows us to study the expansion of gas from surface or subsurface sublimation of volatile ices from the nucleus of comet 67P/C-G. Our calculations can automatically link the hydrodynamic flow region close to the nucleus surface to the Knudsen flow regime with infrequent collisions. In addition, because of the unstructured grid technique, it becomes much easier to conduct investigation of the gas emission and dust jet formation at different length-scales caused by the irregular topography and inhomogeneous composition distribution of the surface materials of the comet. Another computational advantage is that the geometry model of comet 67P can be used to produce the surface temperature distribution and its diurnal variation. In this work, we present our preliminary results on two types of gas/dust emission when comet 67P reaches a heliocentric distance of 3 AU at the point where the Rosetta spacecraft is about to perform its rendezvous mission. The first one has to do with the global structure of the gas and dust coma if most of the outgassing is concentrated at an active region of 0.056 km2 (about 0.14 % of the total surface area). It is found that the (1-10 μm sized) dust distribution is characterized by a sunward jet with radial speed of 1-9 m/s plus a disc of slow-moving dust particles which are nearly perpendicular to the sun-comet direction. The second category of model calculations is about the gas and dust emission from a small-scaled active area in the form of a concave-shaped crater or a convex-shaped dome. The size of these topographic structures is assumed to be 100 m. By taking into account the thermal conductivity of the surface materials (from water ice to dust mantle), we show that the collimated dust jet could have different temporal behaviors according to the solar insolation condition, thus providing a valuable probe to the physical properties of the subsurface material.
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