H. Kohl

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.

The effect of non‐volatile porous layers on temperature and vapor pressure of underlying ice

The space missions to comet Halley have confirmed the view that on large parts of the surface of a cometary nucleus the ice is buried below a non-volatile dust layer or even a sintered cohesive crust. In order to obtain a better understanding of such ice/crust systems, the thermal behavior of ice samples covered by non-volatile porous material under illumination by an artificial ‘sun’ was studied by means of laboratory experiments. Both loose dust mantles and cohesive porous crusts were used as covers. For comparison an experiment with uncovered ice was also performed. We recorded temperature and gas pressure close to the surface of the porous ice and it was found that due to the suppression of gas outflow the ice temperatures in the covered ice became always higher than in the ‘free sublimation’ experiment. Furthermore, a pressure build-up of several Pa was observed in the covered ice close to the surface. In some experiments the measured pressures remained significantly below the saturation vapor pressure which corresponds to the measured temperature at the same depth, indicating strong deviations from equilibrium conditions.

Dust emission of mineral/ice mixtures: residue structure and dynamical parameters

In a series of experiments the particle emission from ice/dust mixtures under insolation at space conditions was investigated using an array of particle collectors and Piezo impact detectors. The dust data provide an estimate of the ratio of ice containing to ice free particles for different size ranges, indicating a decrease in the frequency of ice free particles with increasing particle size. The measurement of the main dynamical parameters of the particle emission reveals a significant interrelation of particle size, starting angle and velocity of the emission process. The near surface particle stream exhibits an internal structure of a low-speed central core of large grains superimposed by a high-speed background of small grains. Possible implications for the dynamics of a dust jet emanating from a comet nucleus are discussed.

A new method for analyzing low velocity particle impacts

Abstract Particle detection and characterization by piezoceramic acoustic detectors was tested for lowvelocity impacts of ice/dust particles with respect to comet simulation (KOSI) experiments. In these experiments ice/dust samples consisting of water and carbon dioxide ice, as well as mineral dust grains are irradiated with simulated solar light. As a consequence the sample emits particles in a wide size range up to a few millimetres. To detect these particles and to get information on their physical parameters like size, momentum, mass and velocity, acoustic detectors are used. It has been shown that piezoceramic detectors are suitable for characterizing low-velocity particle impacts at speeds of about 1m s − and at particle diameters > 200 μ m. Agglomerates with high porosities (filamentary sublimate residues) show only weak and complex piezoelectric signals because of their inhomogeneous momentum transfer to the acoustic detector during impact processes. It has been shown that at least some fraction of emitted particles in the KOSI experiments still contains a large amount of icy constituents.

Particle injection into cometary and planetary atmospheres

Abstract Elevation processes of particles by steam injection may play an important role for aerosol production in cometary and planetary atmospheres. The most relevant processes are reviewed. Particles may be lifted both by sublimating or desorbing gases in the upper surface layers. Results from simulation experiments for interplanetary space conditions are reported.