Planar cone penetration resistance in granular matter under low gravity

In situ penetration exploration of the Moon and asteroids requires understanding the mechanical properties of their surface granular matter, yet cone penetration tests (CPTs) under low-gravity conditions are scarce. We performed velocity-controlled planar CPTs in the Bremen (∼10−6 g) and Beijing (∼10−3–10−2 g) Drop Towers, intruding penetrometers at 15–140 mm/s into glass beads and comparing results with terrestrial (1 g) tests. Unlike the strong depth-dependent resistance observed on Earth, the penetration resistance under low gravity shows a dramatically reduced depth dependence. Remarkably, the decrease in resistance is far less than the reduction in gravity level. Under low gravity, penetration resistance shows complex rate dependence: positive between 40 and 100 mm/s, turning negative above 100 mm/s, contrasting with rate-independent 1 g behavior. A transition of the granular material into a fluid-like state, dilation and cavity formation were also enhanced. Ground experiments align with Mohr–Coulomb limit equilibrium solution, whereas reduced-gravity penetration demands μ(I) rheology for faithful reproduction. This study provides fundamental insights into the transition from rate-independent quasi-static flow to rate-dependent dense flow and further to rate-dependent collisional flow.