Extreme low-temperature freezing process and characteristic curve of icy lunar regolith simulant

Water distribution uniformity is an important control criterion for icy lunar regolith samples during ground simulations and is easily influenced by the sample freezing process. However, the freezing process and char-acteristics of icy lunar regolith are rarely studied. Therefore, to address this issue, simulated samples of icy lunar regolith with two raw material ratios, various water contents and dry densities were prepared in this study, and the unfrozen water contents of samples at different freezing temperatures were tested by nuclear magnetic resonance instrument. The freezing process of icy lunar regolith from 20 degrees C to-192.8 degrees C was analyzed based on the variation of the T2 spectrum, and the influence of initial water content and dry density of the samples on the variation rules of unfrozen water content with temperature was discussed. The results show that the freezing process includes the subcooling stage, the fast freezing stage, the slow freezing stage and the stable freezing stage. Below-80 degrees C, the unfrozen water content in both unsaturated and saturated samples reaches a very small order of magnitude. However, at-190 degrees C, unfrozen water is still present in the pores (<1 nm) of the samples. Unfrozen water can be divided into bound water, capillary water and gravity water. Unfrozen water in samples with the water content of 5 wt% and 10 wt% is mainly bound water, and water migration occurs during freezing. The solid ice formed during the freezing process of saturated samples has similar adsorption or capillary effect on unfrozen water to rock particles. A lower dry density of the sample means a higher initial freezing temperature and can more significantly promote the formation of capillary and gravity water. Finally, a new freezing char-acteristic model was constructed based on the linear relationship between particle and pore radii. The model well predicts the four variation stages of unfrozen water content with temperature from room temperature (20 degrees C) to extreme low temperature (-190 degrees C). And the applicability of the model was verified by quantifying the available surface permafrost data.