The shape of a sublimating ice surface at the bottom of a non-volatile cylinder: Monte Carlo simulations and 2-D analysis

Abstract

Gas flow rate at a cometary surface depends on the microscopic structure of the porous dust-ice material that makes up the nucleus. The standard model for the structure of such material is a bundle of cylindrical, non-crossing tubes. Even within this idealized model the emergent flux depends on the shape of the sublimating ice surface at the bottom of each tube. The present work simulates the evolution of this surface with the use of a kinetic model based on the Test Particle Monte Carlo Method. The results of these simulations are compared with 2-D analytical integration. The surface is found to evolve to an asymptotic concave shape in three distinct phases. During the initial phase, when the surface is at the top of the tube, the sublimation is similar to that of a free surface as long as the depth of the tube is small compared to its diameter. In the second phase, the curvature of the surface develops due to the non-isotropic recondensation of the molecules back scattered from the walls of the tube. The strength of the curvature is self regulating by the direct flux of the molecules from one section of the sublimating surface to another. A final asymptotic stage is reached when the two competing recondensation flows are in equilibrium.