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

Abstract

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 stall 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's 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 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 stall, 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.

The Autonomous Pinger Unit of the Acoustic Navigation Network in EnEx-RANGE: an autonomous in-ice melting probe with acoustic instrumentation

“…The melting system needs to be adapted to Enceladus ice properties and environmental conditions. In preparation of this adaption simulations (Schüller and Kowalski, 2019) of the melting behavior in low gravity, low pressure and at low temperatures are currently being carried out. Additionally, the system for the EnEx mission needs to be refitted in accordance to the space standards (ECSS or NTSS) for robustness, radiation hardness and an extended temperature range.…”

Modeling of hot-point drilling in ice

“…To validate our model, our calculating results are compared with the analytical solutions given by Aamot (1967) and Schüller and Kowalski (2019). The initial conditions are taken from Schüller and Kowalski (2019) where a cylindrical thermal head with a radius of 6 cm is used to drill ice in the vicinity of Enceladus' tiger stripes. During drilling, the power of the thermal head changes from 1000 to 5000 W while BCF is 0.1, 10 and 1000 N. Aamot (1967) ignored the power loss, so the ROP increases linearly with input power and the BCF had no influence on it (Fig.…”

“…This indicates a good agreement of our model with the analytical solution. Schüller and Kowalski (2019) assumed that the heat transported by convective flow affects temperature distribution and a quadratic polynomial Ansatz for the temperature field in s direction was taken. This is the main reason which leads to the difference between our results and the analytical solution.…”

“…In these models, more attention was paid to the shape of the thermal head, but the effects of other variables were neglected. More recently, Schüller and Kowalski (2019) investigated the effect of power, BCF and ice temperature on the drilling performance by analyzing the CCM process with respect to extraterrestrial ice. In their study, the shape effect of the thermal head was not considered, and the developed model could only be applied to PDCs.…”

Radioisotope Heat Sources and Power Systems Enabling Ocean Worlds Subsurface and Ocean Access Missions

“…International scientific modeling efforts [9] clearly show that dimensions of a melt probe have an enormous impact on the ice penetration duration per unit length of ice. Probe speed is a function of Q1 [10] and probe diameter, while probe length dictates Q2 to prevent the probe being frozen into the ice.…”