Penetration and relaxation behavior of dry lunar regolith simulants

A return to the lunar surface has been identified as an important near-term goal for NASA and other international space agencies. Along with recent and upcoming missions to asteroid bodies and other extraterrestrial surfaces, this demands a comprehensive understanding of the mechanical properties of the regolith we are likely to encounter. Historically, penetrometers have been used to garner information about the near-surface, but in situ measurements give correlational data at best. To extend the usefulness of penetrometer measurements and provide more information about the regolith properties, we conducted laboratory penetration and subsequent relaxation measurements of two regolith simulants, one angular and cohesive and the other rounded and noncohesive, using a mechanical conical probe specially-designed to allow bulk sample relaxation. We measured penetration resistance and load-relaxation behavior for both 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, and certain elastic parameters of a best-fit mechanical relaxation model can be used to differentiate between simulants, sample density, and test pressure. A combination of penetration and relaxation data may therefore be used to explore these properties in a field setting, and the relaxation behavior can also be tentatively correlated to more fundamental soil mechanics properties such as shear moduli. Furthermore, penetration/relaxation tests into sieved samples (coarse- and fine-grained versions of the simulants) show that penetration resistance can be used to determine relative particle size with respect to conical probe diameter and relaxation behavior indicates that a combination of fine- and coarse-grain particles produces a more cohesive behavior (in a mildly cohesive simulant) than either the fine- or coarse-grained counterparts. Additional testing in varying simulants and at more extreme ranges of temperature, pressure, density, and water ice content is suggested to refine these measurements, along with field testing to determine the viability of the use of this technology terrestrially as well as on future planetary missions.