Ejection of dust by elastic waves in collisions between millimeter- and centimeter-sized dust aggregates at 16.5 to 37.5 m/s impact velocities.

We report on experiments in which millimeter-sized Si O2 dust aggregates consisting of (sub)-micrometer-sized grains impact into centimeter-sized targets that consist of the same kind of dust particles. The porosity of the granular targets is between 74% and 88%. Impact speeds are between 16.5 and 37.5 m/s with most impacts around 25 m/s . Compaction of the target by the impacting dust aggregate creates a crater which is several millimeters deep and 2-3 cm in diameter. We do not detect a significant amount of ejecta originating at the crater. We do observe a large amount of ejecta though. These are dust granules that are ejected from the whole target surface up to significant distances away from the impact site. This implies that elastic waves induced by the impact are an efficient mechanism to eject material. The estimated mass of these ejecta can be larger than 10 times the projectile mass. The ejecta velocity is uniform across the surface. It is typically 0.5% of the impact velocity. We apply these results to the problem of planetesimal formation. Under microgravity ablation of a dusty body or mass gain in a dust-dust collision might result. This depends on the parameters of the impact. Due to the low ejecta velocities, net growth is also possible in secondary collisions after an eroding primary collision if the body is placed in a gas flow. Thus, for a large number of typical conditions for dust-dust collisions in protoplanetary disks, formation of a larger body results from an impact.