ENERGY-LOSS AND STICKING MECHANISMS IN PARTICLE AGGREGATION IN PLANETESIMAL FORMATION

A crucial step in the development of planetary systems is the aggregat ion of small solid particles to form planetesimals in gaseous protopla netary disks such as the primordial solar nebula. Among small (centime ter-sized) aggregates for which self-gravity is negligible, a sticking mechanism is needed to hold the aggregate together, even when the rel ative velocities are very low. A similar cohesive process may also det ermine 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 ty pes of frosts, deposited at various (low) temperatures and pressures r elevant to solar nebula conditions. Our preliminary measurements show that several types of frost-coated surfaces stick together when brough t into contact at very low temperatures (similar to 100 K), but the st icking forces depend on the deposition conditions. For ice particles c overed with H2O and CO2 frost: (1) the energy loss in collisions depen ds strongly on the impact speed and surface structure, and (2) particl e ''sticking'' can occur if the impact speed is sufficiently low. Stat ic sticking experiments using methanol (CH(3)0H) frost demonstrate tha t methanol is also an effective ''sticky'' frost. We apply these resul ts to planetesimal formation and suggest that a layer of surface frost provides both the energy loss and the contact sticking required for t he formation of large aggregates. (C) 1996 Academic Press, Inc.