Measuring the mechanical properties of small body regolith layers using a granular penetrometer

Small bodies in the solar system are known to be covered by a layer of loose unconsolidated soil composed of grains ranging from dusty sands to rugged boulders. Various geophysical processes have modified these regolith layers since their origin. Therefore, the landforms on regolith-blanketed surfaces hold vital clues for reconstructing the geological processes occurring on small bodies. However, the mechanical strength of small body regolith remains unclear, which is an important parameter for understanding its dynamic evolution. Furthermore, regolith mechanical properties are key factors for the design and operation of space missions that interact with small body surfaces. The granular penetrometer, which is an instrument that facilitates in situ mechanical characterization of surface/subsurface materials, has attracted significant attention. However, we still do not fully understand the penetration dynamics related to granular regolith, partially because of the experimental difficulties in measuring grain-scale responses under microgravity, particularly on the longer timescales of small body dynamics. In this study, we analyzed the slow intrusion of a locomotor into granular matter through large-scale numerical simulations based on a soft sphere discrete element model. We demonstrated that the resistance force of cohesionless regolith increases abruptly with penetration depth after contact and then transitions to a linear regime. The scale factor of the steady-state component is roughly proportional to the internal friction of the granular materials, which allows us to deduce the shear strength of planetary soils by measuring their force-depth relationships. When cohesion is included, due to the brittle behavior of cohesive materials, the resistance profile is characterized by a stationary state at a large penetration depth. The saturation resistance, which represents the failure threshold of granular materials, increases with the cohesion strength of the regolith. This positive correlation provides a reliable tool for measuring the tensile strength of granular regolith in small body touchdown missions.