Melting probe technology for subsurface exploration of extraterrestrial ice – Critical refreezing length and the role of gravity

The ‘Ocean Worlds’ of our Solar System are covered with ice, hence the water is not directly accessible. Using melting probe technology is one of the promising technological approaches to reach those scientifically interesting water reservoirs. Melting probes basically consist of a heated melting head on top of an elongated body that contains the scientific payload. The traditional engineering approach to design such melting probes starts from a global energy balance around the melting head and quantifies the power necessary to sustain a specific melting velocity while preventing the probe from refreezing and stalling in the channel. Though this approach is sufficient to design simple melting probes for terrestrial applications, it is too simplistic to study the probe’s performance for environmental conditions found on some of the Ocean Worlds, e.g. a lower value of the gravitational acceleration. This will be important, however, when designing exploration technologies for extraterrestrial purposes.

We tackle the problem by explicitly modeling the physical processes in the thin melt film between the probe and the underlying ice. Our model allows to study melting regimes on bodies of different gravitational acceleration, and we explicitly compare melting regimes on Europa, Enceladus and Mars. In addition to that, our model allows us to quantify the heat losses due to convective transport around the melting probe. We discuss to which extent these heat losses can be utilized to avoid the necessity of a side wall heating system to prevent the probe from stalling, and introduce the notion of the ‘Critical Refreezing Length’. Our results allow to draw important conclusions towards the design of melting probe technology for future missions to icy bodies in our Solar System.