Gravity-dependent stick-slip dynamics in penetration experiments on simulated regolith.

The surfaces of many planetary bodies, including asteroids and small moons, are covered with dust to pebble-sized regolith held weakly to the surface by gravity and contact forces. Understanding the reaction of regolith to an external perturbation will allow for instruments including sensors and anchoring mechanisms for use on such surfaces to implement optimized design principles. We analyze the behavior of a flexible probe inserted into loose regolith simulant as a function of probe speed and ambient gravitational acceleration to explore the relevant dynamics. The EMPANADA experiment (Ejecta-Minimizing Protocols for Applications Needing Anchoring or Digging on Asteroids) flew on several parabolic flights. It employs a classic granular physics technique, photoelasticity, to quantify the dynamics of a flexible probe during its insertion into a system of bi-disperse, cm-sized model grains. We identify the grain-scale forces throughout the system for probe insertion at a variety of speeds and for four different levels of gravity: terrestrial, martian, lunar, and microgravity. We identify discrete, stick-slip failure events that increase in both magnitude and frequency as a function of the gravitational acceleration. In microgravity environments, stick-slip behaviors are negligible, and we identify that faster probe insertion can suppress stick-slip behaviors where they are present. We conclude that the mechanical response of regolith on rubble pile asteroids is likely quite distinct from that found on larger planetary objects. Techniques borrowed from Earth-based granular physics experiments provide a promising set of methods for future work analyzing both local and global particle rearrangement events in microgravity conditions.