Phenomenology and dynamic behavior of the dust component in the KOSI experiments

The KOSI (Kometensimulation) project (1987–1993) was intended as a series of multi-discipline experiments to investigate porous ice-dust mixtures under space conditions in view of a better understanding of comets. The present paper gives a synoptic summary of results obtained in the simulation experiments that are related particularly to the phenomenology and dynamic behavior of the dust component. Sample preparation was achieved by spraying aqueous suspensions of mineral powders (olivine, montmorillonite) into liquid nitrogen, which implies contact to liquid water. After sublimation of the ice both montmorillonite and olivine containing residues show a size dependence in porosity and mass density that is typical for fractal-like particles. The montmorillonite containing dust residues after artificial insolation were found to form coherent “tactoids” of high electrical conductivity. The decrease of the dust emission activity of fresh ice-dust mixtures with increasing time of insolation is explained by the formation of a volatile-depleted dust mantle that quenches further activity. The surface temperature was found to be directly related to the thickness of the ice-free dust cover and to the elevation angle of the light source above the local horizon. The surface topography of the sample after irradiation indicates the occurrence of local mantle displacements (“dust avalanches”) on inclined surfaces due to gas drag induced lifting and slipping down of parts of the dust cover. The local dust removal and deposition leads to the formation of valleys and ridges parallel to the gradient of inclimation. Similar features are expected to occur on cometary nuclei. Test particles of defined size and density were used to simulate meteoroid impact events on a developed dust mantle during insolation. The mean local surface temperature was found to drop immediately after impact by 1–7 K, depending on the total cross-section of the particles. A simultaneous enhancement of the gas emission was observed, the increase of the local gas flux density being anticorrelated to the surface temperature. Particle acceleration due to the enhanced gas drag was found to vary from <10 to 17 m s−2 depending on the particle size. Implications for impact induced phenomena on comets are discussed.