On Thin Ice: Experimental Spectroscopy and Ice Mechanics in Support of Ocean World Exploration

Water ice is a fundamental planetary building block and ubiquitous in the outer solar system. On Ocean worlds like Europa, convecting ice may transport material from a subsurface ocean (a potential habitat) to the surface, depositing ices and salts. Evaluating the habitability of Ocean Worlds, requires either unraveling the history of ice on the surface to contextualize biosignatures, or probing the ocean for direct access. There are, however, challenges to both exploration strategies. How can recent exposures of subsurface ice be identified? How can a probe penetrate beneath an ice shell and still communicate with the surface? I have developed techniques to address these questions, and pose new ones, using a two-part approach to exploration of Ocean Worlds, viewed as both remote sensing targets, and sites for in-situ analysis. First, I combined investigations using laboratory spectroscopy and Hapke modeling to identify the diagnostic limits of existing datasets, collected optical and spectral measurements of candidate ices at relevant conditions, and identified the effects of grain size, sample thickness, and thermal cycling on water ice absorption features. I designed this dataset to enable better interpretation of Galileo and upcoming Europa Clipper mission spectra, with a focus on characterization of surface properties. To demonstrate its efficacy, I determined the bulk crystallinity of Europa’s leading hemisphere, the environmental conditions required to meet current age estimates, and developed a criterion for selection of regions of recent exposure. Second, I simulated conditions in Europa’s interior and ice shell faults using cryogenic shear experiments, to evaluate the mechanical behavior of ice and explore the limitations of communication tethers for deployment by a melt probe transiting the ice shell. Surprisingly, I find that these tethers are robust across the range of temperature and velocity conditions expected on Europa and offer capabilities as potential science instruments to detect ice-quakes and characterize the thermal profile of the ice shell. Together, these studies improve the ability to probe the thermomechanical and compositional properties of dynamic ice shells, characterize the environments likely to be encountered by landed missions, and guide future technology development for assessing the habitability of Ocean Worlds.