A review on the preparation techniques and geotechnical behaviour of icy lunar regolith simulants

This review provides an in-depth analysis of the methods used to produce icy regolith simulants with a focus on the Moon. Current knowledge of the form and morphology of ice on the Moon is limited, yet a wide range of studies have used ice and regolith analogues to simulate the icy material in lunar cold traps. The physical properties of ice on the Moon may vary significantly depending on its form as either an amorphous or crystalline solid, as well as its physical texture with lunar regolith. As a result, the choice of a particular preparation technique used to create an icy regolith simulant may directly impact the results of a study. It is found that sample preparation techniques can be broadly classified based on the mechanism of ice formation, proposed as either the freezing of liquid to solid, direct deposition (desublimation) of vapor to solid, or the comminution of large blocks to smaller fragments. Secondary growth structures including re-cementation, vapor re-deposition or ductile deformation of ice can also be achieved by post-processing techniques like thermal or pressure-induced metamorphism of the ice. Differences in the form and morphology of icy regolith is also found to significantly influence geotechnical behaviour to a greater extent than changes in ice content, bulk density or temperature. Specifically, vapor-deposited and amorphous ices which best simulate the formation conditions of ice on the Moon are the weakest, whilst ice-cemented ’mud-pies’ which are commonly produced by gradually freezing simulant mixtures containing liquid water, which is not stable on the Moon, can be orders of magnitude stronger. The limitations of current simulants are highlighted, and several opportunities to combine or improve existing techniques are provided. In conclusion, further study using techniques that involve comminution and deposition of ice in the absence of the liquid phase of water are recommended to better emulate the formation and weathering pathways of icy regolith on the Moon, and ultimately its physical properties.