Energy Loss and Sticking Mechanisms in Particle Aggregation in Planetesimal Formation

Abstract

A crucial step in the development of planetary systems is the aggregation of small solid particles to form planetesimals in gaseous protoplanetary disks such as the primordial solar nebula. Among small (centimeter-sized) aggregates for which self-gravity is negligible, a sticking mechanism is needed to hold the aggregate together, even when the relative velocities are very low. A similar cohesive process may also determine the size distribution of particles in planetary rings. In order to provide the crucial data, we carry out experiments to investigate the contact sticking that occurs for surfaces coated with different types of frosts, deposited at various (low) temperatures and pressures relevant to solar nebula conditions. Our preliminary measurements show that several types of frost-coated surfaces stick together when brought into contact at very low temperatures (∼100 K), but the sticking forces depend on the deposition conditions. For ice particles covered with H2O and CO2frost: (1) the energy loss in collisions depends strongly on the impact speed and surface structure, and (2) particle “sticking” can occur if the impact speed is sufficiently low. Static sticking experiments using methanol (CH3OH) frost demonstrate that methanol is also an effective “sticky” frost. We apply these results to planetesimal formation and suggest that a layer of surface frost provides both the energy loss and the contact sticking required for the formation of large aggregates.