Enceladus and the Icy Moons of Saturn (2016)
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TOWARDS SIMULATION-BASED MISSION SUPPORT FOR SUBSURFACE EXPLORATION OF THE ICY MOONS J. Kowalski1, K. Schüller1, A. Zimmermann1, 1Aachen Institute for Advanced Study in Computational Engineering Science, RWTH Aachen University, Schinkelstr. 2, 52062 Aachen, Germany,
[email protected] Introduction: Direct exploration of the Icy Moons of our Solar System implies landing on its icy surface. Advanced mission scenarios suggest to deploy a robotic exploration probe in order to migrate into the subsurface aiming for data collection and sampling of subglacial liquid [1]. Since water ice melts at a moderate temperature many of the robotic concepts are inspired by the idea of melting their way through the ice. This approach means that while migrating into the ground the probe is surrounded by convecting melt water and experiences a complex thermal state. In order to (autonomously) control the probe while making optimal use of the collected sensor data, it is crucial to have a solid understanding of the probes’ ambient environment during a melting scenario. In our contribution, we will address this objective and present an integrated computational model for an exploration probe migrating through ice and its ambient cryospheric state. When we first started this project, we had a particular application in mind, namely modeling the dynamic behavior of the robotic exploration probe IceMole [2,3]. Though the IceMole still inspires much of our work, we now reached a level of generality that allows us also to study related processes such as self-burrying meteorites. Instead of the robotic probe one can hence also think of any generic heat source. Observation scale: Existing computational models for studying the Icy Moons often act on very large spatial and time scales and address objectives such as the evolution of our Solar System. These are hence macroscale models. Other studies investigate micro-scale processes, e.g. when studying details about the ice structure. In our project, we are most interested in time scales in the order of hours to months and spatial scale that range from the submilimeter scale (melt film in the vicinity of the probe) to tens or hundreds of meters (length of the melting channel). It is important to us to consider both transient heat and mass transport, phasechange processes, as well as natural or forced convection in the melt. We distuinguish two modules: Modeling the ambient cryo-state: Currently our model is capable of computing phase-change processes that are fully coupled to the convecting liquid melt. We compute the temperature distribution in the ice, convection and temperature distribution in the melt, as well as the evolving location of the solid-liquid phase
interface. Our algorithm relies on a fixed-grid, weak interface approach and implements the enthalphyporosity method. First results realize simplified 2d and 3d geometries and have been computed with an OpenFoam implementation. The implementation can be used both on a desktop PC or parallelized on a compute cluster. It has been tested with analytical reference solutions and with experimental data, both of which showed very nice agreements. Modeling the heat source migration: The objective of the second module is to determine the trajectory of a rigid heat source in a migrating state. Here, our current approach updates position and attitude through minimizing an appropriately chosen energy functional. The exact form of the energy functional depends on the design of the probe. In this presentation, we will describe the current state of both modules, discuss scope and validity of their underlying assumptions and current implementation, and show various test cases. We will introduce two collaborations in the context of the Enceladus Explorer Initiative, during which our model will be applied in the near future, namely 1. re-analysis of laboratory and field experiments of the IceMole team at FH Aachen University of Applied Sciences, Germany, 2. optimal control of melting probes together with the CAUSE (Cognitive Autonomous Subsurface Exploration) team at University of Bremen. Finally, we will sketch next steps such as quantifying the error of simplified models [4], and future development areas on our way towards a fully capable tool for simulation-based mission support. Acknowledgements: As part of the Enceladus Explorer Initiative, the project is supported by the Federal Ministry of Economics and Technology, Germany, on the basis of a decision by the German Bundestag (FKZ: 50NA 1502). References: [1] Konstantinidis K. et al. (2015), Acta Astronaut., 106, 63–89. [2] Kowalski J. et al. (2016), Cold Reg. Sci. Technol., 123, 53-70. [3] Dachwald B. et al. (2014), Ann. Glaciol., 55(65), 14-22. [4] Schüller K. et al. (2016) Int. J. Heat Mass, 92, 884892.
