The cohesion bottleneck and the activity of distant comets

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In this German-Ukrainian cooperation project we intend to get to the bottom of the so-called "cohesion bottleneck" of the dust activity of comets. When cometary nuclei approach the sun on their mostly highly elliptical orbits, the sunlight heats up the surface more and more. Through heat transporting to deeper layers, the heat finally reaches the volatile substances (mostly ices of CO, CO2 and H2O), which evaporate as a result. The common paradigm of dust activity says that the outflowing gases carry the dust particles away from the surface, thus explaining the observed dust activity. In recent years, however, more and more doubts have been raised about this idea, because a closer look at the physical processes shows that the gas pressure is far from being able to overcome the cohesion of the dust particles. In this project, we now intend to investigate physical mechanisms that dynamically describe the interactions of gas flow, solar irradiation and particle cohesion. In particular, we plan to find out whether the upper dust layers can be loosened to such an extent by excited vibrations or thermal fatigue processes that the gas flow only has to overcome the weight of the dust particles in order to transport them away from the surface. Our approach is three-pronged and is based on the combination of theoretical modelling, laboratory investigations and observations of comets. While the observational side is contributed solely by the Ukrainian colleagues and the laboratory investigations solely by the German team members, the theoretical sub-project will combine the results of the sub-projects in close Ukrainian-German cooperation to create a new and physically correct model of the dust activity of comets.