Temperature evolution and vapour pressure build-up in porous ices

We report on a new series of comet simulation experiments performed in the "Small Chamber" at the DLR Koln. Different types of non-volatile materials have been used to simulate the effect of dust mantles on heating and vapour pressure of an underlying porous ice sample. In all experiments the flow resistance of the mantle material caused a measurable pressure build-up inside the sample and a temperature rise larger than would be expected from a freely sublimating icy surface exposed to the same energy input. Comparison of experiments with different mantle grain sizes revealed similar vapour pressures and ice temperatures. This result is probably caused by the fact that the smaller (and lighter) particles, even if they are not blown away, are redistributed by the gas stream and form pores much larger than the original grain size. The flow resistance is then determined by the flow resistance of the whole structure, not by the grain size of the original particles. Theoretical estimates and model calculations reveal that deviations from the equilibrium vapour pressure should be small inside porous water ice. Only within a distance of about three pore radii from a sublimating surface can major deviations occur. The experiments utilizing pure (white) ice confirmed this prediction: reasonable agreement between the theoretically expected saturation vapour pressure and the measured gas pressure inside the sample was found. Admixture of a darkening agent (solution of carbon and an organic constituent) in even very small amounts to the water out of which the porous ice is produced, caused, however, a significant reduction of the vapour pressure inside the pores.