Measuring Thermal Properties on Mars: Relaxation of Requirements for Transient Heated Needle Measurements of Material Thermal Properties

Thermal properties of geological materials are commonly measured using transient heated needle techniques. The application of this theory depends on the heater needle approximating an infinitely long line heat source, a criteria that cannot be met for certain applications. We Decagon Devices developed the Thermal and Electrical Conductivity Probe (TECP) which was used to measure thermal properties of the Martian surface in situ as part of NASA's Phoenix Scout Lander. Because the probe is inserted by a robot arm into the Martian surface with undetermined hardness it is likely that long, thin needles could be damaged or destroyed during insertion. Hence, this application requires violation of the assumptions outlined above. We evaluated the effects of using a shortened, conical heater instead of the more common long, cylindrical heater. First, we used a finite element analysis (FEA) software package to simulate the heat flow from conical needle geometries in several materials. To verify the model analysis, the shortened, conical probes were also evaluated in the laboratory with media of known thermal properties. The results of the FEA and laboratory studies indicate that the use of simple calibrations can account for errors associated with the violation of the heated needle theory, and accurate determination of material thermal properties are still possible.