Experimental Validation of Cryobot Thermal Models for the Exploration of Ocean Worlds

Accessing the potentially habitable subsurface waters of Ocean Worlds requires a cryobot to traverse tens of kilometers of ice with temperatures ranging from ∼100 K to 273 K. Designing and planning such a mission requires excellent understanding of ice probe behavior as a function of its local environment and design parameters. We present experimental results of two different laboratory probes using heat as the penetration mechanism in cryogenic (79 K) and warm (253 K) ice. The melt probe tested in warm ice had multiple adjustable heaters, enabling optimization of the system efficiency. The melt probes tested in cryogenic ice initially operated in vacuum and had internal tether spools, allowing experimental confirmation of hole reclosure and creation of a pressurized pocket with liquid water around the probe, while allowing for continued descent in the completely-closed melt cavity. These melt probes were tested with power levels ranging from 114 W to 1135 W, and achieved descent speeds between 5.3 cm/h and 59 cm/h. By analyzing the relationship between power and speed using both analytical and high-fidelity numerical models, we demonstrate progress in understanding important aspects of melt probe performance. We distinguish between the previously-confounding terms of probe operational inefficiency and analytical model inaccuracy. This allows us to both understand the range of applicability of the analytical models and clearly demonstrate the importance of controlling heat distribution in cryobot design. The validated models show that the performance of an efficient cryobot designed for operation on an Ocean World can be predicted by analytical models to within 5% error.