PHENOMENOLOGY AND DYNAMIC BEHAVIOR OF THE DUST COMPONENT IN THE KOSI EXPERIMENTS

The KOSI (Kometensimulation) project (1987-1993) was intended as a ser ies of multi-discipline experiments to investigate porous ice-dust mix tures under space conditions in view of a better understanding of come ts. The present paper gives a synoptic summary of results obtained in the simulation experiments that are related particularly to the phenom enology and dynamic behavior of the dust component. Sample preparation was achieved by spraying aqueous suspensions of mineral powders (oliv ine, montmorillonite) into liquid nitrogen, which implies contact to l iquid water. After sublimation of the ice both montmorillonite and oli vine containing show a size dependence in porosity and mass density th at is typical for fractal-like particles. The montmorillonite containi ng dust residues after artificial isolation were found to form coheren t ''tactoids'' of electrical conductivity. The decrease of the dust em ission activity of fresh ice-dust mixtures with increasing time of ins olation is explained by the formation of a volatile-depleted dust mant le that quenches further activity. The surface temperature was found t o 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. Th e surface topography of the sample after irradiation indicates the occ urrence of local mantle displacements (''dust avalanches'') on incline d surfaces due to gas drag induced and slipping down of parts of the d ust cover. The local dust removal and deposition leads to the formatio n of valleys and ridges parallel to the gradient of inclination. Simil ar features are expected to occur on cometary nuclei. Test particles o f defined size and density were used to simulate meteoroid impact even ts on a developed dust mantle during insolation. The mean local surfac e temperature was found to drop immediately after impact by 1-7 K, dep ending on the total cross-section of the particles. A simultaneous enh ancement of the gas emission was observed, the increase of the local g as flux density being anticorrelated to the surface temperature. Parti cle 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 impa ct induced phenomena on comets are discussed.