Characterizing the microstructure of icy lunar regolith simulants using microcomputed tomography

To date, the physical texture and polymorphic structure of frozen deposits of water ice in lunar polar cold traps has not yet been properly defined, and neither has its geotechnical nor thermophysical properties. In an attempt to simulate these icy materials, various sample preparation techniques have been used, resulting in materials with strengths ranging from hard concrete to weak snow. This has direct implications for the in-situ resource utilization (ISRU) of ice on the Moon, as validation of flight hardware prior to launch requires high fidelity materials for accurate testing. To address this gap, this article investigates how preparation methods for different icy simulants influences the soil skeleton and ice microstructure. Microcomputed tomography (Micro-CT) is used to characterize the three-dimensional internal structure of icy simulants prepared as four distinct morphologies: cemented (wet mix), unsintered and pressure-sintered granular, and vapor-deposited. Samples were prepared using Lunar Highlands Simulant (LHS-1) and imaged at voxel resolutions of 15 µm 3 under cooled conditions, and 1.5 µm 3 under ambient conditions for wet mix. A 2.5D U-Net convolutional neural network was trained then used for the trinary segmentation into ice, lithic, and void phases, and image reconstruction and quantitative analyses were performed using Dragonfly ORS Pro. The change in soil skeleton and morphology of ice-cemented samples is characteristic of the transition through pendular, funicular and capillary saturation regimes. Increasing water content corresponded to pore coarsening from 0.57 to 0.76 mm at 1 and 12 wt% respectively, due to formation of continuous spideweb-like networks of reticulated structures that bound lithic particles together with water. Unsintered and pressure-sintered samples consisted of discrete grains suspended in lithic groundmass and underwent no changes with increasing ice content, apart from particle agglomeration due to solid-state sintering. The vapor-deposited sample consisted of diffuse, highly porous surface frost and rinds across grain surfaces, with evidence of pore cavity and throat constriction, interpreted as ice growth. These observations provide clear evidence that the classical and widely adopted preparation of ice-cemented mixtures by freezing mud is due to morphological changes mediated by the matric suction and capillary forces of liquid water. Given that liquid water is not stable on the Moon, ice-cemented morphologies may not occur, and lunar ice deposits are therefore expected to exhibit geotechnical strengths lower than the prevailing consensus in the experimental literature-which uses concrete and rock analogues. These findings advance our current understandings of grain-scale microstructure of icy lunar regolith simulants, and highlight the importance of sample preparation technique, its implications on material properties, and large-scale simulation testing. • Simulants made with liquid water, granular ice or vapor deposition (desublimation). • Scanned at 15 and 1.5 µm 3 with temperatures of 75(5) °C and 24 °C respectively. • Matric suction of liquid water causes pore coarsening and particle agglomeration. • Discrete grains in lithic groundmass undergo no change in soil skeleton with more ice. • Vapor deposited ice occurs as a diffuse frost that fills pore cavities and doesnt alter soil skeleton.