Probing Granular Materials in Extraterrestrial Gravities*

The Hayabusa2 and OSIRIS-REx missions have identified the surfaces of asteroids Ryugu and Bennu as being composed of weakly attracted grains primarily bound by cohesive forces, raising questions about granular dynamics in these exotic conditions. Operating instruments on such surfaces will require efficient and intentional design principles; we identify the insertion of a flexible probe as a function of speed and gravity as a prototypical example to explore these dynamics. We employ a classic granular physics technique, photoelasticity, to quantify the dynamics of a flexible intruder during its insertion into a laboratory system of bi-disperse, cm-sized model grains at various speeds and under four different levels of gravity: terrestrial, martian, lunar, and microgravity. This technique is used to identify the grain-scale forces throughout the system, and shows good agreement with direct load cell measurements. In most cases, the interparticle forces are characterized by discrete, stick-slip failure events that increase in both magnitude and frequency as a function of the gravitational acceleration. For microgravity experiments, stick-slip behaviors are negligible. We additionally find that varying the probe speed can suppress stick-slip behavior.