Experimental Simulation for the Analysis of Geomorphological Properties of Cometary Surfaces with Volatile Outgassing

Abstract

Despite a wealth of information from observations and experiments, the chemical and physical properties of cometary nuclei are not yet fully understood. The work presented here is intended to contribute to a better understanding of the processes on the surface of cometary nuclei by means of experiments in the laboratory. Special attention is given to the influence of sublimating volatiles in variable amounts. For this purpose, a number of consecutive laboratory experiments were performed with the aim to reproduce morphological features as observed on the surface of comets. The obtained results were used to draw conclusions about how representative the different material combinations in the laboratory are with respect to chemical and physical characteristics of a comet. This work combines three experimental studies, each published in peer-reviewed journals. The first series of experiments tested which mixtures of mineralic and volatile components were suitable as analogs in the laboratory. SiO2 dust, fly ash and water ice were tested. Of particular interest were the tensile strength and the cohesion of mixtures of these materials as functions of grain shape, mixing ratio, and at temperatures below 150 K. For this purpose, cylindrical samples of various mixtures of these components were prepared and their average tensile strength was determined by using the Brazilian disk test method. It was found that the tensile strength of two-component mixtures is dominated by that component with the higher tensile strength. The experiments also showed that it is possible to prepare cometary analogues of spherical fly ash and ice particles with an average tensile strength of a few hundred pascals. For the second study, the previously characterized fly ash-ice mixtures were placed in a vacuum sublimation chamber and insolated with a light source for several hours. Different morphologies evolved and were depended on the insolation flux, insolation angle, and sample composition. Large amounts of ice caused rapid sublimation of the samples and the development of exotic morphologies. With decreasing ice content, the occurrence of episodic collapse events or outbursts increased, resulting in a rough surface texture of the samples. In addition, a dust layer formed on the surface that reduced or prevented further sublimation processes. The third study extended the experiments of the second study by enriching the water ice particles with organic components. The amino acid glycine and the organic salt sodium acetate were used. Both are components that have been detected on comets and represented cometary organic components in the laboratory. The experiments showed that organic components have a distinct adhesive effect on the samples when the ice sublimates. Dominant morphological alterations of the samples surfaces are no longer due to collapse events or outbursts, but by a loss of volume of the ice-depleted remains and due to the formation of shrinkage fractures on the sample surface. The morphologies produced in the experiment indicate that an ice content of >25 mass% (>40 vol%) is likely at the surface and near-surface layers of a comet. The amount of only a few percent of organic material is sufficient to adhesively solidify the ice-depleted comet surface and cause the formation of fracture patterns.