Close-contact melting - process insights and relevance for ice exploration

The Earth is partially covered by ice, which provides answers to important scientific questions in its interior. Additionally, ice can preserve scientifically interesting regions. For example, sub-glacial lakes that have been isolated from the outside world for hundreds of thousands of years can provide important insights to the evolution of life on Earth. Since it is known that ice not only exists on Earth, but is also found elsewhere in our solar system, exploring these distant icy worlds is of great scientific interest. Good conditions for extraterrestrial life are especially assumed at Saturn's moon Enceladus and Jupiter's moon Europa, which are known to harbor liquid water below their ice sheet. A proven approach for terrestrial ice exploration are melting probes. In addition, due to their small size, mass, and the lack of complicated mechanics, melting probes are a promising approach for extraterrestrial ice exploration in future space missions. Melting probes can move through ice using heating elements in the melting head. For this purpose, the phase change from solid to liquid is forced in the direction of the probe's trajectory. The melt water flows along the melting probe to the rear where it freezes again after some time, depending on the ice temperature. The objective of the present work is to develop models that help to describe this multiphysics process. In order to describe the heat transport along the melting probe and into the surrounding ice, a macro-scale model for the solution of natural convection coupled melting processes was developed. It is based on the enthalpy porosity method. This is a fixed-grid approach, which allows the unknowns (phase interface position, velocity, pressure and temperature) to be determined in just one domain using a phase field. To validate the model, an experiment was developed in which controlled melting of water ice can be studied at high spatial and temporal resolution. The experimental results, i.e. image recordings of the phase interface propagation, were processed in a collaborative work for model validation. The optical images have been segmented in order to determine the position of the phase interface. A good agreement was observed with the simulation results. In addition, a model for describing the melting probe trajectory based on close-contact melting theory has been developed. As it can be assumed that the melting velocity is determined essentially by the heat transfer in the region of the melt film below the melting head, the model contains only this micro-scale domain. Furthermore, a tailored model for the IceMole melting head design has been developed, which allows for curved trajectories within the ice through a hybrid melting head. The boundary condition at the melting head, i.e. the temperature or heat flux can thus be inhomogeneous and asymmetric. The curvilinear melting was validated by a suitable experiment. The model can be applied to a variety of important issues. For example, the model has been used to identify a good candidate for the optimal heat flux distribution of a planar circular melting head. It is a Gaussian distribution, i.e. most of the power has to be applied in the center of the melting head for efficient melting. For straight melting of a cylindrical melting probe with a planar melting head, a simplified model based on the contact melting theory has been derived. Compared to the simpler established model, which is used by default for melting probe design and can be solved analytically, the simplified close-contact melting model takes into account the influence of force on the melting velocity. This makes it possible, for example, to investigate the influence of gravitational acceleration on the melting velocity. A corresponding study for different icy worlds of our Solar System showed that significant differences in the melting velocity can be observed with the same melting probe configuration. Furthermore, the simplified close-contact melting model has been used in another study to determine if a melting probe requires additional heating elements along its hull. These additional heating elements may be necessary to ensure that liquefied ice does not refreeze on the probe, which could prevent the probe from further movement through the ice. Both the macro-scale and micro-scale models provide important tools for the simulation-based analysis of advanced melting probes for terrestrial and extraterrestrial ice exploration. For example, using a one-way coupling, the melting velocity of a melting probe was first determined in the micro-scale model and then further used in the macro-scale model to investigate the global heat transfer in the phase change material. This showed that convective heat transport is smaller for celestial bodies with less mass than on Earth, which is due to the lower gravitational acceleration. Consequently, it is not enough to qualify melting probe designs for future space missions under terrestrial gravitational acceleration. Instead, the target gravitational acceleration needs to be simulated, either in the experiment or numerically