The growth of super-large pre-planetary pebbles to an impact erosion limit

The early evolution of dust in protoplanetary disks is dominated by sticking collisions. However, this initial phase of particle growth faces constraints, notably from destructive encounters. To find the maximum particle size achievable, we studied collisional processes during a prolonged microgravity experiment aboard a suborbital flight. Specifically, we describe an impact erosion limit. We observed individual basalt beads, each measuring 0.5 mm in diameter, colliding with and either eroding or adhering to a cluster several centimetres in size. This cluster, formed from tribocharged particles, simulates an electrostatic growth phase that surpasses the classical bouncing barrier. We found a threshold velocity of about 0.5 m s⁻¹, which separates additive and erosive impacts of individual beads. Numerical simulations of grains impacting clusters, for both low and high charge constituents, corroborate the experimental findings of surface erosion within the observed velocity range. This specific velocity threshold supports the potential formation of pebbles several centimetres in size within protoplanetary disks. Such dimensions place these pebbles well into a regime in which hydrodynamic interactions might facilitate the formation of planetesimals.