Geometry-Dependent Penetration in Microgravity Reveals a Transition From Compaction-Dominated to Shear-Dominated Failure

Abstract

Penetration into granular media plays a central role in regolith–structure interactions on planetary bodies, where gravitational acceleration is far lower than on Earth. Although penetration resistance and bearing capacity are commonly inferred from terrestrial experiments, the extent to which reduced gravity alters penetration mechanics and failure behavior remains insufficiently understood. Here, we present a controlled experimental study of quasi-static penetration into dry granular media under terrestrial gravity and microgravity using a drop-tower facility. Penetration resistance, surface deformation, and grain-scale kinematics were measured simultaneously to resolve both macroscopic response and underlying particle motion. The results show that penetration induces compaction of the granular packing under both gravity conditions. However, microgravity leads to markedly different deformation patterns, including enhanced surface rebound, steeper transient slopes, and substantially reduced penetration resistance. Grain-scale displacement fields reveal that these macroscopic differences arise from a gravity-dependent transition in failure behavior: penetration-dominated deformation under terrestrial gravity gives way to shear dilation failure accompanied by vortex-like particle motion under microgravity. These findings demonstrate that penetration behavior observed on Earth cannot be directly extrapolated to low-gravity environments and provide new physical insights into gravity-controlled granular mechanics relevant to planetary surface processes.