Heat Flow Probe for Europa Deployable from a Small Lander.

We have developed a design of a heat flow probe deployable to the surface of Europa by a small robotic lander. It adopts the sensor deployment mechanism originally developed for the heat flow probes slated to be deployed on Earths Moon in 2023 and 2025. The probe is a compact, modular system and can be accommodated to the underbelly of the landers platform or one of its legs. The deployment mechanism spools out a metal tube of 6.4-mm diameter and inserts it into the ground. While the lunar probe uses a pneumatic drill in excavating a hole into the regolith, the Europa probe uses a hot tip in subliming the ice ahead of it. On the way deeper into Europas surface ice, the probe makes multiple stops, and at each stop, it measures temperature and thermal conductivity of the ice. Heat flow is obtained as the product of the thermal gradient and the thermal conductivity of the depth interval penetrated by the probe. The thermal sensor is integrated into the penetrating tube and occupies its leading end, immediately above the hot tip. The sensor consists of a ~30-cm-long electric heater and a resistance temperature detector (RTD) placed at its mid-point. The heater for the sensor subsystem is separate from the hot tip for subliming ice. When the probe reaches a depth targeted for measurement, the hot tip is turned off, and the RTD monitors temperature of the surrounding ice as it cools off and reaches an equilibrium. Then, the heater is turned on. Thermal conductivity of the ice is determined from the rate of temperature rise in response to the heater. This methodology for thermal conductivity measurement (the needle probe or hot wire method) is used widely for terrestrial rocks and soils, except that the probe for Europa is longer and of a greater diameter for the mechanical strength necessary for penetrating into ice. We have tested the thermal conductivity measurement capability of the Europa probe using simulants of Europa ice in a vacuum chamber. Three types of simulants were used: solid ice, granular ice, and ice powder. The probe used for these tests consists only of the thermal sensor section of the penetrating tube. The test results are used to construct a numerical thermal model for the shallow subsurface of Europa. Based on the model, we determine the penetration depth needed to measure the internal heat flow of Europa, unaffected by insolation.