Localised sublimation under cometary dust mantles

Observations of cometary surfaces have indicated that the amount of exposed surficial water ice is insufficient to supply the measured water production rates within the inner solar system. The measured water production rates are also a small fraction of that which would arise from freely subliming pure water ice bodies of similar surface area. A plausible mechanism, given the now well-established, high porosity of comets is that water ice is subliming below the visible surface with the produced gas escaping through the pores in a non-subliming hot dust matrix. If this process leads to an additional heating of the evolving gas, this may provide an elegant explanation for the temperature of the gas required to match measured terminal velocities. We investigate this process using a series of numerical models based on the Direct Simulation Monte Carlo (DSMC) method for rarified gas dynamics. We show that inhomogeneities in the source are not evident in the flow field once the thickness of the porous layer reaches a specific size and that lateral flow through the porous layer (parallel to the surface) can broaden the apparent source size when the source is small and isolated. The temperatures of the gas flow at the visible surface can be significantly increased and will respond to the temperature distribution within the non-volatile porous layer. The influence of the structural properties of the porous layer (how the porous layer is mechanically constructed) is shown to have only limited influence on the final gas parameters at the surface.