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Cryogenic penetration and relaxation behavior of dry and icy lunar regolith simulants
Atkinson, Jared W. G.
Atkinson, Jared W. G.
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2019
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NASA and other international space agencies have identified a return to the lunar surface as a primary objective for the coming decade. Speculation about resources in space – from platinum-group metals to water ice – has permeated space-focused media, culminating with the establishment of a price for water delivered in low-earth orbit. Much of the discussion about space resources focuses on the potential of regolith (extraterrestrial soil) to provide valuable materials, yet regolith properties are poorly characterized at environmental extremes. Extraction concepts require detailed knowledge of the mechanical properties of dry and ice-saturated regolith. This study aims to characterize two lunar regolith simulants using a specially-designed penetrometer at various conditions of temperature, pressure, density, and ice content. We measured penetration resistance and load-relaxation behavior for both dry simulants under constant displacement rate penetration at pressures ranging from 1E-5 Torr to 700 Torr and at relative densities of ~20% and ~80%. Penetration resistance was sensitive to density and insensitive to test pressure and simulant type. Relaxation was sensitive to all three conditions, differentiable using elastic parameters of a best-fit mechanical relaxation model, and may also be tentatively correlated to more fundamental soil mechanics properties such as shear moduli. Furthermore, we measured penetration resistance and relaxation behavior for a basaltic lunar simulant at ~300 mTorr pressure, ~110 K sample temperature, 170 K to 190 K probe temperature, and ice contents of 0% to 12% and 100%. Penetration resistance and relaxation behavior both showed sensitivity to ice content, with parameters of best-fit curve models offering simple empirical predictors of saturation. A critical ice content of 1% to 3% is identified as being fundamentally influenced by the filling of substantial pore space with grouting ice. A decrease in viscoelastic behavior of high ice content samples at cryogenic temperatures is noted, and inhibition of relaxation mechanisms due to activation-energy-based temperature effects is demonstrated in dry, ice-free simulant. Additional research across the spectrum of ice saturations, temperatures, and pressure conditions likely to be found in extraterrestrial environments is suggested to improve upon these results and advance the potential use of this technology in future exploratory missions.
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