10:45 AM - 12:15 PM
[PPS07-P10] Evaluating permeability of lunar highlands regolith and its change by freeze-thaw using LHS-1
Keywords:lunar, regolith, permeability, freeze-thawing
The regolith on the Lunar and Mars has been attracting attention from the perspective of understanding the planetary surface processes and procuring in-situ resources. However, very few studies have investigated the hydraulic properties of regolith, which are one of the significant physical properties in understanding the surface processes and evaluating regolith utilization. We aimed to study the permeability and particle packing structure of the regolith on lunar highlands, where expected to deposit with the highest thickness, by comparing different experimental gravity environments and the influences of freeze-thaw cycles.
In this study, we used the regolith simulant LHS-1, and prepared the specimens filled with LHS-1 granular particles at different packing fractions that assumed the uppermost several meters of the lunar subsurface. We flowed water into the specimen at a constant flow rate in this experiment. W then evaluated the permeability (m2) by applying Darcy’s law to the water flow rate and the differential pressure between the inlet and outlet of the specimen. We simulated the experimental gravity environment by changing the tilt angle at which we placed the specimens. We also evaluated the permeability and changes in the particle packing structure before and after one freeze-thaw cycle (down to -20°C). Silica sand and glass beads with similar median particle diameters were also used to prepare the reference packing media.
Our results indicated that the permeability of LHS-1 was at least one order of magnitude lower than that of the reference materials. In the lunar gravity environment (1/6G), the permeability was 1.57×10-13 m2 with a packing fraction of 59%, and the permeabilities with all packing fractions showed their minimum at 1/6G. In contrast, the permeability at 1/6G – 1G increased as the gravity increased. This is because the higher gravity increases the contact area between regolith particles and decreases tortuosity. We also found that the changing rate in permeability before and after one freeze-thaw cycle was greater with lower packing fractions. We attributed this finding to the fact that the amount of water that could be present within pore spaces of the given specimen was greater when the packing fraction was smaller, and that the pore water in the solid phase due to freezing allowed the rearrangement of the LHS-1 particles and changed the contact ratio between the particles. The tortuosity of the specimen with a packing fraction of 49% packing fraction decreased by about 60% with freezing and thawing, suggesting a significant increase in the total flow path. No significant difference was observed for glass beads before and after one freeze-thaw cycle due to differences in particle strength and shape.
In addition to the very small permeability on the order of 10-13 m2 measured from the highland regolith at the presumed depths of the uppermost few meters below the lunar surface, the simulated gravity environment showed the smallest permeability in the lunar gravity environment. The permeability was also found to increase with freezing and thawing. Therefore, we suggest that the regolith on the lunar surface may have higher permeability than that acquired in this study, given that the lunar surface should have been subject to multiple freeze-thaw cycles.
In this study, we used the regolith simulant LHS-1, and prepared the specimens filled with LHS-1 granular particles at different packing fractions that assumed the uppermost several meters of the lunar subsurface. We flowed water into the specimen at a constant flow rate in this experiment. W then evaluated the permeability (m2) by applying Darcy’s law to the water flow rate and the differential pressure between the inlet and outlet of the specimen. We simulated the experimental gravity environment by changing the tilt angle at which we placed the specimens. We also evaluated the permeability and changes in the particle packing structure before and after one freeze-thaw cycle (down to -20°C). Silica sand and glass beads with similar median particle diameters were also used to prepare the reference packing media.
Our results indicated that the permeability of LHS-1 was at least one order of magnitude lower than that of the reference materials. In the lunar gravity environment (1/6G), the permeability was 1.57×10-13 m2 with a packing fraction of 59%, and the permeabilities with all packing fractions showed their minimum at 1/6G. In contrast, the permeability at 1/6G – 1G increased as the gravity increased. This is because the higher gravity increases the contact area between regolith particles and decreases tortuosity. We also found that the changing rate in permeability before and after one freeze-thaw cycle was greater with lower packing fractions. We attributed this finding to the fact that the amount of water that could be present within pore spaces of the given specimen was greater when the packing fraction was smaller, and that the pore water in the solid phase due to freezing allowed the rearrangement of the LHS-1 particles and changed the contact ratio between the particles. The tortuosity of the specimen with a packing fraction of 49% packing fraction decreased by about 60% with freezing and thawing, suggesting a significant increase in the total flow path. No significant difference was observed for glass beads before and after one freeze-thaw cycle due to differences in particle strength and shape.
In addition to the very small permeability on the order of 10-13 m2 measured from the highland regolith at the presumed depths of the uppermost few meters below the lunar surface, the simulated gravity environment showed the smallest permeability in the lunar gravity environment. The permeability was also found to increase with freezing and thawing. Therefore, we suggest that the regolith on the lunar surface may have higher permeability than that acquired in this study, given that the lunar surface should have been subject to multiple freeze-thaw cycles.