FORMATION AND PROPERTIES OF FLUFFY PLANETESIMALS

A mechanism of accumulation of grains in the primordial solar nebula i s described. This process produces porous, low density compressible ag gregates. Compaction of the aggregates in a collision between them dis sipates the kinetic energy of the collision and can result in efficien t growth. A simple analysis of such collisions is developed and applie d over a range of aggregate sizes and relative velocities. The results indicate that large planetesimals could grow through collisions rathe r than fragment if the conditions are favorable. Our modelling suggest s that primordial asteroids and comets on the order of a kilometer in size will have low densities and irregular shapes.