Gas release from ice/dust mixtures: results from eleven KOSI experiments

Porous mixtures of ice from water and other volatile components and of mineral grains were irradiated by artificial sunlight in a space simulator allowing sample sizes of tens of centimeters. The flow of gases released was measured by means of pressure gauges and a mass spectrometer, and gas flux densities at the insolated surface were inferred at 11 KOSI experiments. Gas release rate is closely related to the developments of an outer dust layer depleted of ice. At varied composition and radiative power the decline of sublimation rate during insolation was found to be similar for all experiments when the dust/ice ratio was at least 0.09 by weight. At a lower dust content of 0.02 no dust mantle was formed allowing sublimation at a nearly constant rate. Phenomena observed by video, IR and ejected particle sensing techniques lead us to conclude that gas flux may be quite inhomogeneously distributed over the surface where areas covered with and uncovered from dust fluctuate on a scale of centimeters or less. Gas flux rate and its response to radiative power is shown to be indicative of the evolutionary status of the near-surface part of a porous ice/dust body. Estimates of the cohesive force between grains and of heat conductivity are derived from our results. The composition of the released gas is different from that of the ice which has implications for the interpretation of cometary observations.