Ice sublimation below artificial crusts: Results from comet simulation experiments

Dust mantles or non-volatile mineral crusts most probably exist on large parts of the surface of many cometary nuclei. Even when such a layer is only a few millimetres thick and porous, its existence reduces substantially the gas emission rate of the underlying ice. In the present paper we report the results of systematic laboratory experiments, where a sample of porous, granular ice covered by a dark steel plate with holes was irradiated under vacuum conditions by an artificial light source simulating the Sun. The dark steel plate containing holes of defined size was intended to simulate the effect of a porous non-volatile cometary crust or dust mantle. We observed the build-up of vapour pressure below the artificial crust and measured the temperature profile developing in the ice sample for different hole sizes. In all experiments a drastic reduction of the gas emission rate (compared with the case of free sublimation from a dark icy surface of the same albedo and emissivity) was observed. The main effect of the porous crust is a much faster heating of the underlying ice due to suppression of gas outflow. The experimental results are interpreted in terms of a new heat conduction model that allows for all conduction modes that may act in such a structured ice, in particular Knudsen gas diffusion, infrared radiation, and solid-state heat conduction via intergranular connection points.