K.-H. Thiel

Laboratory Simulation of Cometary Processes: Results From First Kosi Experiments

In situ observations of comet Halley provided the first photographs of a cometary nucleus and yielded information about its environment, including the emitted gas and dust. The relation between these measurements and properties of and processes on the nucleus is established by theoretical modelling, while laboratory experiments may provide some of the physical parameters needed. In addition, laboratory tests can stimulate new ideas for processes that may be relevant to cometary physics. Processes to be studied in detail by large-scale laboratory experiments may include: (1) heat transport phenomena during sublimation of porous ice-dust mixtures, (2) material modification and chemical fractionation caused by the sublimation processes, (3) buildup and destruction of dust mantles, (4) detailed studies of gas release from mixtures of volatile ices, and (5) the investigation of ice and dust particle release mechanisms. The KOSI-team (Kometensimulation) carried out sublimation experiments with ice-mineral mixtures in a large Space Simulator. During initial experiments, cylindrical samples of 30-cm diameter and 15-cm thickness were irradiated with up to 2700-W/m2 light energy. The samples consisted of water-ice or water- and CO2-ice mineral mixtures. The experiments showed the importance of advection for heat transport into the interior. It was found that the sublimation of CO2 advances into the sample at a higher speed than that of water vapor release. Therefore, emission of volatile gases responded to insolation changes with a time lag of several hours. The ratio of the emitted gas species, as well as the dust-to-gas mass ratio, differs significantly from the values within the sample. A partly permeable refractory mantle of minerals and carbonaceous material developed with time. Dust and ice particle emission has been observed to occur from irradiated dirty ices as well as from dust mantles.

Ice sublimation below artificial crusts: results from comet simulation experiments

Abstract Dust mantles or non-volatile mineral crusts most probably exist on large parts of the surface of many cometary nuclei. Even when such a layer is only a few millimetres thick and porous, its existence reduces substantially the gas emission rate of the underlying ice. In the present paper we report the results of systematic laboratory experiments, where a sample of porous, granular ice covered by a dark steel plate with holes was irradiated under vacuum conditions by an artificial light source simulating the Sun. The dark steel plate containing holes of defined size was intended to simulate the effect of a porous non-volatile cometary crust or dust mantle. We observed the build-up of vapour pressure below the artificial crust and measured the temperature profile developing in the ice sample for different hole sizes. In all experiments a drastic reduction of the gas emission rate (compared with the case of free sublimation from a dark icy surface of the same albedo and emissivity) was observed. The main effect of the porous crust is a much faster heating of the underlying ice due to suppression of gas outflow. The experimental results are interpreted in terms of a new heat conduction model that allows for all conduction modes that may act in such a structured ice, in particular Knudsen gas diffusion, infrared radiation, and solid-state heat conduction via intergranular connection points.

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.

Dust particle emission dynamics from insolated ice/dust mixtures: results from the KOSI 5 experiment

Abstract In comet simulation (KOSI) experiments mineral-ice mixtures are studied under space conditions. In this paper the temporal variation of emission events is studied during the irradiation phase in the experiment KOSI 5. Physical properties like mass, size and mean mass density of emitted dust particles are investigated. Dynamical parameters of the particle motion are determined from recorded trajectories close to the sample surface. The drag coefficient governing the momentum transfer from the released gas to the emitted particle is also estimated. From the trajectories, the spatial distribution of starting points of the respective particles on the sample surface is estimated. These results suggest a sample surface development in two phases: formation of dry dust particles on the surface and, subsequently, further increase of the dust mantle.