Abstract
Abstract. The thermal properties of the surface and subsurface layers of planets and planetary objects yield important information that allows us to better understand the thermal evolution of the body itself and its interactions with the environment. Various planetary bodies of our Solar System are covered by so-called regolith, a granular and porous material. On such planetary bodies the dominant heat transfer mechanism is heat conduction via IR radiation and contact points between particles. In this case the energy balance is mainly controlled by the effective thermal conductivity of the top surface layers, which can be directly measured by thermal conductivity probes. A traditionally used method for measuring the thermal conductivity of solid materials is the needle-probe method. Such probes consist of thin steel needles with an embedded heating wire and temperature sensors. For the evaluation of the thermal conductivity of a specific material the temperature change with time is determined by heating a resistance wire with a well-defined electrical current flowing through it and simultaneously measuring the temperature increase inside the probe over a certain time. For thin needle probes with a large length-to-diameter ratio it is mathematically easy to derive the thermal conductivity, while this is not so straightforward for more rugged probes with a larger diameter and thus a smaller length-to-diameter ratio. Due to the geometry of the standard thin needle probes they are mechanically weak and subject to bending when driven into a soil. Therefore, using them for planetary missions can be problematic. In this paper the thermal conductivity values determined by measurements with two non-ideal, ruggedized thermal conductivity sensors, which only differ in length, are compared to each other. Since the theory describing the temperature response of non-ideal sensors is highly complicated, those sensors were calibrated with an ideal reference sensor in various solid and granular materials. The calibration procedure and the results are described in this work.
Highlights
One way for a better understanding of our Solar System is a better knowledge of the objects in it, the planets and their satellites, as well as asteroids and comet nuclei
In a first step the thermal conductivity of different samples was determined by the LNP03 as well as by the shorter LNP04 thermal conductivity probe
The LNP03 sensor has already been calibrated by Kämle et al (2013), but it was done again with an improved measurement set-up, while the LNP04 was calibrated for the first time
Summary
One way for a better understanding of our Solar System is a better knowledge of the objects in it, the planets and their satellites, as well as asteroids and comet nuclei. There is a need for further geophysical in situ exploration of the surface and subsurface of moons, comets and planets in future lander missions This is necessary to allow us more detailed insights into the composition and evolution of those planetary bodies, especially for prospecting water ice and water deposits or other resources. Such instruments are recommended for the payload of the upcoming Russian Lunar Polar Lander missions, which have the goal to investigate the properties of the regolith in the lunar south polar regions by in situ measurements and by returning samples to Earth (ESA, 2014). One can find the theoretical approach for needle-shaped thermal sensors, a short presentation of the used measurement probes and sample materials, the results of thermal conductivity measurements in various sample materials, as well as the calibration of the LNP sensors and a comparison between LNP03 and LNP04
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