The cohesion bottleneck and the activity of distant comets

在这个德国-乌克兰合作项目中，我们打算弄清彗星尘埃活动的所谓“凝聚瓶颈”。当彗星核以其高度椭圆的轨道接近太阳时，太阳光会使彗星表面越来越热。通过向更深层的热传输，热量最终到达挥发性物质（主要是CO、CO2和H2O的冰），其结果是蒸发。尘埃活动的常见范例是，流出的气体将尘埃颗粒带离表面，从而解释了观测到的尘埃活动。然而，近年来，越来越多的人对这一想法提出了质疑，因为仔细研究物理过程表明，气体压力远远不能克服尘埃颗粒的凝聚力。在这个项目中，我们现在打算研究动态描述气体流动，太阳辐射和粒子凝聚力相互作用的物理机制。特别是，我们计划找出上层灰尘层是否可以通过激发振动或热疲劳过程松散到这样的程度，即气流只需克服灰尘颗粒的重量，以便将它们从表面运走。我们的方法是三管齐下的，是基于理论建模，实验室调查和彗星观测相结合。虽然观测方面完全由乌克兰同事作出贡献，实验室调查完全由德国小组成员作出贡献，但理论分项目将通过乌克兰和德国的密切合作，将分项目的成果联合收割机结合起来，以建立一个新的、物理上正确的彗星尘埃活动模型。

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.