Water permeability of sunlit lunar highlands regolith using LHS-1 simulant

The regolith on the Earth's Moon 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 the lunar regolith, which should pose one of the most significant physical properties for understanding the lunar surface processes and evaluating regolith utilizations. In this paper, we study the water permeability of the surficial regolith layer on sunlit lunar highlands using the lunar highlands regolith simulant LHS-1. We measure the water permeability of LHS-1 at packing fractions of 0.49-0.60 by simulating lunar gravitational environments and evaluating the influences of freeze-thaw, which is expected to occur on the sunlit lunar highland surfaces. Our results indicate that the water permeability of lunar highland regolith is one to two orders of magnitudes lower than that of fine silica sands and monodisperse spherical glass beads at the given packing fraction. The low water permeability of lunar highland regolith attributes to the regolith particles having irregular and angular shapes with sharp edges and micron-sized chunks due to the least weathering, unlike soil particles on the Earth. The horizontal water permeability of lunar highland regolith simulant before freeze-thaw is 1.6 x 10-13 m2 at a packing fraction of 0.6. From the porosity-depth profile available from previous studies, we also obtained the water permeability-depth curve, representing the vertical water permeabilities of 6 x 10- 13-2 x 10-12 m2 and 7 x 10- 14-3 x 10-13 m2 at the lunar surface and the depth of 5 m within the surficial lunar regolith layer, respectively. We also found that the water permeability of lunar highland regolith increased after one freeze-thaw cycle, achieving 6.4 times higher than the permeability before the freeze, and the increase was more significant with lower packing fractions. We suggest that the pore water in the solid phase due to the freeze encourages the rearrangement of the regolith particles changing the total contact area between the particles. This study will help us better understand the abundance and distribution of molecular water on the lunar surface, lunar surface processes, and regolith utilizations.