| Popis: |
Enceladus, a moon of Saturn, is one of the most promising candidates for the search for life beyond Earth. The Cassini-Huygens mission revealed that Enceladus has a thick crust composed of water ice. Beneath this crust there is a subsurface liquid water ocean that erupts into space through jets near the south pole, forming a plume of ice and gas. It is suggested that this ocean may be habitable and future missions to Enceladus will likely involve life detection experiments on ejected plume material or of the surface around the plume source. A limitation to habitability on Enceladus is the freezing point of water; however, the presence of dissolved salts extends this freezing point to lower temperatures. On Earth, frozen environments such as sea-ice, snow and glacial surfaces, and subglacial lakes contain microbial ecosystems with complex dynamics. The presence of ice does not mean water is unavailable and liquid brine networks can extend throughout the ice, providing an extensive micro-environment for microbial life to inhabit. As a result, it is suggested that the icy crust of Enceladus, especially around the warmer, thinner southern pole, may contain accessible habitats close to the surface. Furthermore, the surface is likely connected to the ocean across short to geological timescales and relict habitable regions may be detectable on the surface. Many questions still remain about the phase behaviour of Enceladus type brines at low temperatures and the evolution of physiochemical param eters as these solutions freeze. This thesis explores the cryogeochemistry of Enceladus-type Na-Cl-CO3 solutions and how microscale freezing dynamics can reveal information about planetary scale processes, and ultimately, the habitability of Enceladus. We present, for the first time, results that significantly improve our understanding of Enceladus’s geochemistry and that will inform future life detection based missions. We first explore the cryomineralogy of Na-Cl-CO3 solutions using powder x-ray diffraction and show that a mixture of hydrohalite and hydrated sodium carbonate minerals form. Several minor phases exist that we are unable to identify but that will be important to investigate further for future missions. Additionally, we look at the microscale freezing dynamics of these solutions using cryomicroscopy. Based on our results, we suggest that behaviour of carbonate minerals will have important implications for ocean CO2 dynamics which impacts our understanding of the predicted pH of the ocean. Next, we explored how this system behaves in three-dimensions using x-ray computed microtomography. We show that the relative precipitation of these salt phases and the ice will affect where they are found on Enceladus, and ultimately, their presence or absence can be used as an indicator of thermal history analogous to igneous and metamorphic petrology. Areas with precipitated salts on the surface may contain vital information about Enceladus’s interior processes and they may be the best place to find evidence of life. Finally, using fluorescence and polarised light cryomicroscopy coupled with cell staining, we show how microbial cells physically behave in these frozen environments and how they may be trapped within ice or salts and transported to the surface. We explore how controlled boiling of Enceladus’s ocean may lead to sputtering and dispersal of microbial material into the plume and how this may impact plume sampling. Our results suggest that larger plume particles will be more likely to contain cells, and as most large particles fall back to the surface, a lander mission would be best suited to finding life. We show that there are many answers to be found with lab-based empirical studies of simple cryogenic systems using modern techniques. Improving our understanding of cryogeochemistry will provide a solid foundation for future missions to frozen environments beyond Earth and ultimately will provide context to information gained from the exploration of the surface and subsurface of icy moons. |
