Modeling of Cryobot Melting Rates in Cryogenic Ice

There currently is significant interest within NASA and the scientific community to explore outer planetary ocean worlds, including Jupiter's moon Europa. Since the Galileo spacecraft magnetometer data indicated that an ocean of liquid water/slush might exist 10-15Km below Europa's icy shell, ocean access became of particular interest for future missions in view of the possibility of finding signs of life or life itself. In this study, both passive and active melt probes (cryobots) have been theoretically analyzed to determine heating power requirements and rates of descent. It is based in part on earlier experimental JPL cryobot studies, as well as more recent system engineering studies, and provides an analytical estimation adapted to the unique cryogenic Europan environment. The Cryobot probe will descend though Europa's ice core until it reaches liquid water at about 10km depth. This analysis corresponds to the melting and descent rate starting at a 3 km location where water jets are used instead of a mechanical drill. The Computational Fluid Dynamics (CFD) analysis was performed using Transient Multiphase Static model in StarCCM+, but was validated with experimental empirical data performed by Honeybee Robotics, obtained from small-scale probe passive melt data in both cryogenic and warm ice, as well as modeling data from CFX and FLUENT performed by U Pittsburg. The first step of the analysis was focused on the heating power required to form a melt a layer around the probe. Based on earlier passive thermal melt models developed by Stone Aerospace for the conceptual probe developed for this trade study, it was determined that a minimum of 7KWt would be required to meet a 2 yr transit through cryogenic ice. Analysis showed that the water jets at the tip of the probe will significantly increase the melt rate over purely conductive, passive heating. A purely conductive melting probe will descend at a rate of approximate 9 cm/hr, while using water jetting the probe will increase this rate to approx. 50 cm/hr. The rate of descent also increases as the probe travels through warmer ice, so case studies at 180K, 200K, 230K and 250K were used to calculate an approximate melt rate through the 10kn of ice. The water jet locations and spacing are also a significant factor in the rate of descent, since if they too close together, it could create freeze zones that would slow the probe. The paper will show results of water jet optimization for their location and mass flow rate to improve the probe's descent rate.