Surviving in Ocean Worlds: An Experimental Characterization of Mechanical and Optical Transmission Performance of Fiber Optic Tethers Across Ice Faults

A successful mission to explore the subsurface of an Ocean World entails navigating through an ice shell with unknown thermophysical and mechanical properties, surviving tidal stressing and potential faulting, and/or exposure to potentially caustic chemistries. A robust communication strategy and hardware to achieve science and exploration objectives are critical. To prepare for this eventuality, the Europa STI team is studying the interior of Icy Ocean Worlds, along with developing, and reducing the technical risk of tethered communication techniques to enable communication through many kilometers of ice. Environmental conditions in our lab shear tests (95<T(K)<260) simulate active ice faults (with creeping faults to ice quake velocities) and were used to evaluate the mechanical behavior of ice and explore the limitations of optical tethers. Surprisingly, we find that these tethers are robust across a range of conditions expected on Europa and offer capabilities as potential science instruments to detect ice-quakes and characterize the thermal profile of the ice shell. Mechanical properties of the tether-ice assemblage, such as peak shear stresses of ~70 to 1500 kPa achieved to activate faulting, a phenomenological brittle to ductile transition at ~230 K, and variations in sliding with temperature and velocity (figure below) demonstrate the striking range of conditions that the tether experienced during testing. The stability of friction is so closely tied to temperature, it suggests that a seismogenic zone may be present at depth on Europa, as has been identified on Earth, such that the uppermost and lowest portions of the ice shell would be sliding smoothly (and slowly), whereas at a mid-range in temperature and depth, icy faults could initiate stick-slip, rapid Europa-quake events. Optical transmission loss of tethers was also measured, with negligible drops during testing. We will also discuss results from our continued SESAME efforts, which will include tension robustness and long tether soaks in mixed chemistry ice (e.g., ammonia, salts) for data transmission evaluations. Our ongoing work improves the ability to unravel the behavior of the ice, identify the challenges arising due to environmental conditions, and develop the technology required for assessing the habitability of Ocean Worlds.