[PPS02-P07] Thermal conductivity of lunar regolith simulant and implication to grain size estimate using thermal inertia
Many air-less planetary bodies are covered by regolith. The thermal conductivity of the regolith is an essential parameter controlling the surface temperature variation and depends on physical condition such as grain size and density. In order to estimate physical condition of the planetary surface materials using infrared remote sensing data, a thermal conductivity model applicable to natural soils as well as planetary surface regolith is required. In this study, we investigated the temperature and compressional stress dependence of the thermal conductivity of the lunar regolith simulant JSC-1A under vacuum conditions. Moreover, we prepared some sieved JSC-1A samples and their thermal conductivities were also measured. The comparison with the thermal conductivity of original JSC-1A has an important role for evaluating the effect of the grain size distribution, and comparison with the thermal conductivity of spherical beads aids in determining the effect of the grain shape on the thermal conductivity.
As a result, we found that JSC-1A has similar solid and radiative conductivities to Apollo regolith samples, and it can be used to simulate the thermal conductivity of the lunar top surface. We also confirmed that a series of the experimental data for JSC-1A can be calibrated by an analytical thermal conductivity model we developed (Sakatani et al., 2017, AIP Adv.). Moreover, the thermal conductivity of sieved samples with grain size about 100 μm had a similar conductivity to the original JSC-1A. The original sample has a volumetric median grain size about 100 μm, so that it is inferred that the thermal property of soils with a broad grain size distribution can be modeled as mono-sized grains with a volumetric median. In other words, a grain size estimated from the thermal observation data is a volumetric median size. Comparison with the experimental data for size-sorted spherical glass beads indicated that the irregular shapes of the natural samples reduce the conductivities by a few ten percent. We are planning to apply the thermal conductivity model calibrated in this study to the Hayabusa2 Thermal Infrared Imager data of asteroid Ryugu so as to determine the spatial distribution of the grain size of surface materials on the asteroid.
As a result, we found that JSC-1A has similar solid and radiative conductivities to Apollo regolith samples, and it can be used to simulate the thermal conductivity of the lunar top surface. We also confirmed that a series of the experimental data for JSC-1A can be calibrated by an analytical thermal conductivity model we developed (Sakatani et al., 2017, AIP Adv.). Moreover, the thermal conductivity of sieved samples with grain size about 100 μm had a similar conductivity to the original JSC-1A. The original sample has a volumetric median grain size about 100 μm, so that it is inferred that the thermal property of soils with a broad grain size distribution can be modeled as mono-sized grains with a volumetric median. In other words, a grain size estimated from the thermal observation data is a volumetric median size. Comparison with the experimental data for size-sorted spherical glass beads indicated that the irregular shapes of the natural samples reduce the conductivities by a few ten percent. We are planning to apply the thermal conductivity model calibrated in this study to the Hayabusa2 Thermal Infrared Imager data of asteroid Ryugu so as to determine the spatial distribution of the grain size of surface materials on the asteroid.