Klaus Thiel

DEVELOPMENT OF A DUST MANTLE ON THE SURFACE OF AN INSOLATED ICE-DUST MIXTURE : RESULTS FROM THE KOSI-9 EXPERIMENT

Astronomical observations indicate that formation and destruction of dust mantles on cometary nuclei may be the cause for erratic and systematic variations of cometary activity, i.e. emission of dust. A laboratory experiment (KOSI-9) has been performed to study the evolution of a dust mantle on top of a sublimating ice-dust mixture in vacuum. A sample consisting of water ice with a 10% (by weight) admixture of olivine grains has been insolated in three periods at variable intensities from 200 to 1900 W/m2. Both increasing surface temperature of the sample and decreasing gas and particle emissions indicated the formation of a dust mantle during the first period. During the second insolation period after the gas flux had reached a critical value of a few 1021 water molecules m−2 s−1, avalanches of mantle material occurred on the inclined sample surface, broke up the mantle locally, and opened up a fresh icy surface. Enhanced ice and dust particle emission resumed for some time from these spots. A large number of the emitted dust particles were of a fluffy aggregate structure, i.e., they had large cross section to mass ratios compared to compact particles. During the third period the critical gas flux was not reached and no enhanced dust and ice emission was observed. A dry dust mantle of a few millimeters thickness developed during the course of the experiment. Consequences of these findings for cometary scenarios are discussed.

Thermal properties of cometary ices and sublimation residua including organics

The simulation of cometary surface conditions by laboratory experiments has proven to be an efficient way towards a better understanding of physical phenomena at and below the surface of a comet nucleus (Grun et al., Geophys. Res. Lett. 18, 245–248, 1991). A question which has not yet been answered by the comet simulation (KOSI) experiments performed so far is the influence of organic matter on the physical properties of the sublimation residua. Therefore, a number of experiments performed in a small vacuum chamber cooled by liquid nitrogen are reported on, which were dedicated to study the influence of organics on the thermal properties of a cometary analogue sample. Using aliphatic hydrocarbons of low volatility (paraffins) as model substances for the organic compounds, the formation of a several centimetres thick cohesive residuum in response to heating of the sample was observed. In one of the experiments the evolution of an originally homogeneous multi-component sample (containing water ice, organics, and minerals) to a residuum containing (finally) only minerals and organics was followed. During this evolution the thermal properties changed dramatically. The heat conductivity of the cohesive residuum was found to be at least an order of magnitude larger than the typical value for a loose dust mantle containing no organic material. Thus the evolution of comets with the same thermal history containing a considerable amount of organics might be quite different from that of a comet consisting only of ices and minerals.

Temperature evolution and vapour pressure build-up in porous ices

Abstract 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.

The Rosetta Lander Experiment SESAME and the new Target Comet 67P/Churyumov-Gerasimenko

SESAME is an international instrument complex carried by the Rosetta Lander. Most of the instrument sensors are mounted within the soles of the landing gear feet in order to provide good contact with or proximity to the cometary surface. The main aim of these instruments is to measure physical properties of the cometary surface layer and of emitted particles by acoustic and electrical methods. The scientific goals, the measuring principles and performance parameters are described. In addition, we discuss the impact by selecting 67P/Churyumov-Gerasimenko as the new target comet.