11:00 AM - 11:15 AM
[PPS08-08] Experimental results on thermal conductivity of regolith simulants for lunar exploration
★Invited Papers
Keywords:LUPEX, Lunar regolith, Lunar exploration
Introduction Lunar Polar Exploration (LUPEX) is a joint mission with the Indian Space Research Organisation (ISRO), the National Aeronautics and Space Administration (NASA), and European Space Agency (ESA) to explore water resources in the lunar polar region. Lunar thermogravimetric analyzer (LTGA), which is one of the instruments on board the LUPEX rover, collects data on the quantity of water resources. LTGA can heat a container with lunar regolith and measures the weight of volatile substances contained in the regolith. In LTGA, the temperature of the regolith is calculated from the temperature of the container. The thermal properties of regolith simulants are necessary to obtain these temperature correlations, but there is a scarcity of useful information. In this study, thermal conductivity measurements of lunar regolith simulants were performed under various temperature and pressure conditions.
Experimental method The thermal conductivity was investigated according to the line heat source method reported by Carslaw and Jaeger.1 The experimental configuration is similar to that reported by Sakatani et al.2,3 Three line heat source sensors were installed equally spaced in a copper sample container (60 mm x 100 mm x 24 mm). They include nichrome wires as line heat sources and alumel-chromel thermocouples as temperature sensors. The container with FJS-1 at a density of 1688 kg/m3 was put into the vacuum chamber and evacuated (2 x 103 - 10-4 Pa, this experiment was conducted in air.). After the temperature in the chamber was set (100, 200, 300, 400, 450 K) and kept in a steady state, constant current (10 - 180 mA) was input to the nichrome wires. The correlation between heating time and regolith temperature was observed by monitoring temperature sensors.
Experimental results The thermal conductivity at each temperature and pressure condition was calculated according to the method of Sakatani et al. 2,3 The calculated values of thermal conductivity were about 6.0 x 10-2 Wm-1K-1 under the condition of 2 x 103 Pa and about 1.5 x 10-3 Wm-1K-1 under 1 x 10-2 Pa, respectively, when the temperature of the system was 273 K. The values were also comparable under pressure conditions below 1 x 10-2 Pa. Furthermore, these values showed temperature-dependent property, e.g., under pressure conditions of 1 x 10-2 Pa, they increased from 1.0 x 10-3 to 5.1 x 10-3 Wm-1K-1 as the temperature of the system increased from 150 K to 450 K. The trends in these results were consistent with previous studies.4,5
Summary In this study, the thermal conductivities of the regolith simulant were measured as essential information for weight measurement of volatile components by LTGA. Establishing the physical properties and evaluation methods of the simulants under various conditions is expected to be beneficial for future exploration missions, including lunar and Martian exploration. In this presentation, we first show in detail the correlations of their thermal conductivities with temperature and pressure.
References 1. H. S. Carslaw and J. C. Jaeger, Conduction of Heat in Solid, second ed. Oxford University Press, London (1959).
2. N. Sakatani et. al., Experimental study for thermal conductivity structure of lunar surface regolith: Effect of compressional stress, Icarus, 221 (2012) 1180-1182.
3. N. Sakatani et. al., Compressional stress effect on thermal conductivity of powdered materials: Measurements and their implication to lunar regolith, Icarus, 267 (2016) 1-11.
4. P. O. Hayne et. al. Global regolith thermophysical properties of the Moon from the Diviner Lunar Radiometer Experiment, Journal of Geophysical Research: Planets, 122 (2017) 2371-2400.
5. R. Woods-Robinson et. al., A model for the thermophysical properties of lunar regolith at low temperatures, Journal of Geophysical Research: Planets, 124 (2019).
Experimental method The thermal conductivity was investigated according to the line heat source method reported by Carslaw and Jaeger.1 The experimental configuration is similar to that reported by Sakatani et al.2,3 Three line heat source sensors were installed equally spaced in a copper sample container (60 mm x 100 mm x 24 mm). They include nichrome wires as line heat sources and alumel-chromel thermocouples as temperature sensors. The container with FJS-1 at a density of 1688 kg/m3 was put into the vacuum chamber and evacuated (2 x 103 - 10-4 Pa, this experiment was conducted in air.). After the temperature in the chamber was set (100, 200, 300, 400, 450 K) and kept in a steady state, constant current (10 - 180 mA) was input to the nichrome wires. The correlation between heating time and regolith temperature was observed by monitoring temperature sensors.
Experimental results The thermal conductivity at each temperature and pressure condition was calculated according to the method of Sakatani et al. 2,3 The calculated values of thermal conductivity were about 6.0 x 10-2 Wm-1K-1 under the condition of 2 x 103 Pa and about 1.5 x 10-3 Wm-1K-1 under 1 x 10-2 Pa, respectively, when the temperature of the system was 273 K. The values were also comparable under pressure conditions below 1 x 10-2 Pa. Furthermore, these values showed temperature-dependent property, e.g., under pressure conditions of 1 x 10-2 Pa, they increased from 1.0 x 10-3 to 5.1 x 10-3 Wm-1K-1 as the temperature of the system increased from 150 K to 450 K. The trends in these results were consistent with previous studies.4,5
Summary In this study, the thermal conductivities of the regolith simulant were measured as essential information for weight measurement of volatile components by LTGA. Establishing the physical properties and evaluation methods of the simulants under various conditions is expected to be beneficial for future exploration missions, including lunar and Martian exploration. In this presentation, we first show in detail the correlations of their thermal conductivities with temperature and pressure.
References 1. H. S. Carslaw and J. C. Jaeger, Conduction of Heat in Solid, second ed. Oxford University Press, London (1959).
2. N. Sakatani et. al., Experimental study for thermal conductivity structure of lunar surface regolith: Effect of compressional stress, Icarus, 221 (2012) 1180-1182.
3. N. Sakatani et. al., Compressional stress effect on thermal conductivity of powdered materials: Measurements and their implication to lunar regolith, Icarus, 267 (2016) 1-11.
4. P. O. Hayne et. al. Global regolith thermophysical properties of the Moon from the Diviner Lunar Radiometer Experiment, Journal of Geophysical Research: Planets, 122 (2017) 2371-2400.
5. R. Woods-Robinson et. al., A model for the thermophysical properties of lunar regolith at low temperatures, Journal of Geophysical Research: Planets, 124 (2019).