Experimental and Numerical Studies of Planetsimal Formation via Collisional Accretion

Small particles (μm to cm-size) in planetary ring systems and protoplanetary disks collide at low velocities (less than 1 m/s) and tend to aggregate through non-gravitational interactions. Additionally, exploration and sample return missions to asteroids and other small, airless bodies involve low energy interactions with weakly-bound regolith particles. Therefore, to characterize the processes that lead to planetesimal formation and planetary ring collisional evolution, and to develop appropriate procedures for asteroid sample return missions, it is necessary to understand how regolith responds to low-energy collisions in a microgravity environment. The COLLIDE (Collisions Into Dust Experiment) and PRIME (Physics of Regolith Impacts in Microgravity Experiment) programs produced observations of mass transfer of regolith onto cm-scale projectiles at impact velocities < 40 cm/s in microgravity conditions. These experiments were carried out on orbit (COLLIDE, COLLIDE-2), in suborbital space (COLLIDE-3), and on parabolic airplane flights (PRIME) under vacuum. To study this phenomenon with significantly reduced cost and time constraints we have developed an experimental apparatus that makes use of a laboratory drop tower (free-fall time 0.75 s). The experiment consists of a tube pumped to vacuum conditions containing a cm-diameter marble suspended from a spring in contact with a bed of regolith. The spring contracts during free-fall allowing us to simulate the rebound portion of a low-velocity collision in a laboratory microgravity environment. We are also carrying out concurrent numerical simulations with the use of a discrete element method particle simulation program, LIGGGHTS (Kloss et al. 2012, Prog. Comp. Fluid Dyn., 12, 140), to replicate these environmental conditions and study the role of cohesion in mass transfer events. We will present the results of our laboratory and numerical studies where we vary rebound velocity, target material type and grain size distribution, projectile size and material, and regolith packing density designed to bolster our understanding of collisional evolution in the protoplanetary disk and planetary rings.