The effect of non-volatile porous layers on temperature and vapor pressure of underlying ice

The space missions to comet Halley have confirmed the view that on large parts of the surface of a cometary nucleus the ice is buried below a non-volatile dust layer or even a sintered cohesive crust. In order to obtain a better understanding of such ice/crust systems, the thermal behavior of ice samples covered by non-volatile porous material under illumination by an artificial sun was studied by means of laboratory experiments. Both loose dust mantles and cohesive porous crusts were used as covers. For comparison an experiment with uncovered ice was also performed. The authors recorded temperature and gas pressure close to the surface of the porous ice and it was found that due to the suppression of gas outflow the ice temperatures in the covered ice became always higher than in the free sublimation experiment. Furthermore, a pressure build-up of several Pa was observed in the covered ice close to the surface. In some experiments the measured pressures remained significantly below the saturation vapor pressure which corresponds to the measured temperature at the same depth, indicating strong deviations from equilibrium conditions