Experimental Validation of Melt Probe Models for the Exploration of Ocean Worlds

Abstract

Ocean Worlds, where large volumes of water may exist under a layer of ice, are strong candidates for the first discovery of extraterrestrial life. Jupiter’s moon Europa is currently believed to have at least twice as much water as Earth. A key remaining challenge is developing a probe that can traverse tens of kilometers of ice with temperatures ranging from ~100 K to 273 K to reach the oceans of Europa. Initial steps have been taken to develop analytical and numerical models of the thermal and physical dynamics of ice penetrators in cryogenic environments, but experimental validation of these models has been limited.

This thesis presents the design and the experimental results of probes that use melting as the descent mechanism in cryogenic (79 K) vacuum ice. The probes are designed to monitor power, temperature, and descent depth and speed. The melt probes are initially operated in vacuum with internal tether spools, allowing experimental confirmation that, under these conditions, water vapor refreezes on contact with ice to close the hole behind the probe and create a pressurized pocket with liquid water around the probe. These sub-scale melt probes are tested with power input levels ranging from 496 W to 1135 W, and achieve descent speeds between 5.3 cm/h and 59 cm/h, with total travel in ice ranging from 87 cm to 202 cm.

The relationship between the empirical total power and descent speed is then analyzed using both analytic and high-fidelity numerical models. The numerical models enable the separation of inefficiencies in probe performance from inaccuracies in the analytical models, resulting in a better understanding of the range of applicability of the classical models and also clearly demonstrating the types and importance of thermal waste in melt probe designs. Fundamentally, this set of validated models shows that the performance of cryobots that are typical of flight concepts (high-aspect-ratio and fast-moving) can be predicted by analytical models to within 5% error. This error is significantly smaller than other environmental uncertainties of Ocean Worlds, such as the thickness and composition of their icy shells.

In order to determine the feasibility of a mission to Europa’s ocean, it is critical to be able to calculate the duration of the cryobot’s journey through the ice shell. Several example cryobot designs are considered and the time-to-ocean is calculated for each. Using a Monte Carlo simulation, the fastest cryobot design considered presents a median time-to-ocean of 3.4 years and a 90th percentile time of 6.8 years, while the slowest of the designs considered completes the journey with a median time of 6.4 years and a 90th percentile time of 13.8 years.

Finally, these tools are applied to evaluate a selection of different cryobot system architectures focusing on two key figures of merit: time to ocean and payload volume. A roadmap for the development of ice descent technologies is then recommended that can help make a mission to Europa’s ocean viable.