  Our knowledge about the physical processes determining the activity of comets
were mainly influenced by several extremely successful space missions, the
predictions of theoretical models and the results of laboratory experiments.
However, novel computer models should not be treated in isolation but should be
based on experimental results. Therefore, a new experimental setup was
constructed to investigate the temperature dependent sublimation properties of
hexagonal water ice and the gas diffusion through a dry dust layer covering the
ice surface. We show that this experimental setup is capable to reproduce known
gas production rates of pure hexagonal water ice. The reduction of the gas
production rate due to an additional dust layer on top of the ice surface was
measured and compared to the results of another experimental setup in which the
gas diffusion through dust layers at room temperature was investigated. We
found that the relative permeability of the dust layer is inversely
proportional to its thickness, which is also predicted by theoretical models.
However, the measured absolute weakening of the gas flow was smaller than
predicted by models. This lack of correspondence between model and experiment
may be caused by an ill-determination of the boundary condition in the
theoretical models, which further demonstrates the necessity of laboratory
investigations. Furthermore, the impedance of the dust layer to the ice
evaporation was found to be similar to the impedance at room temperature, which
means that the temperature profile of the dust layer is not influencing the
reduction of the gas production. Finally, we present the results of an extended
investigation of the sublimation coefficient, which is an important factor for
the description of the sublimation rate of water ice and, thus, an important
value for thermophysical modeling of icy bodies in the solar system.
