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For the investigation of collisions among protoplanetesimal dust aggregates,we performed microgravity experiments in which the impacts of high-porositymm-sized dust aggregates into 2.5 cm-sized high-porosity dust aggregates can bestudied. The dust aggregates consisted of micrometer-sized dust grains and wereproduced by random ballistic deposition with porosities between 85% and 93%.Impact velocities ranged from ~0.1 m/s to ~3 m/s and impact angles were almostrandomly distributed. We also used "molded" target aggregates such that theradii of the local surface curvatures corresponded to the projectile radii. Theexperiments showed that impacts into the highest-porosity targets almost alwaysled to sticking, whereas for the less porous dust aggregates, the collisionswith intermediate velocities and high impact angles resulted in the bouncing ofthe projectile with a mass transfer from the target to the projectileaggregate. Sticking probabilities for the impacts into the "molded" targetaggregates were considerably decreased. For the impacts into smooth targets, wemeasured the depth of intrusion and the crater volume and could derive someinteresting dynamical properties which can help to derive a collision model forprotoplanetesimal dust aggregates. Future models of the aggregate growth inprotoplanetary disks should take into account non-central impacts, impactcompression, the influence of the local radius of curvature on the collisionaloutcome and the possible mass transfer between target and projectileagglomerates in non-sticking collisions.

The Physics of Protoplanetesimal Dust Agglomerates II. Low Velocity Collision Properties