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We discuss the possibility of the erosion of dusty bodies in protoplanetary disks by a subsonic laminar gas flow. Our analysis is based on wind tunnel experiments on centimeter-size dust targets in an air gas flow of 63 m s À1 at static gas pressures between 0.1 and 4.5 mbar. We compare the results to numerical calculations of gas flow through porous bodies and the resulting drag force on dust aggregates at the surface. Our studies imply that a dusty body can be efficiently eroded if the dynamic gas pressure of the surface flow exceeds gravity and/or cohesion. In protoplanetary disks, we find that objects on circular orbits might be relatively safe against erosion in a laminar gas flow even in a dense disk. However, if a body is stirred up to an eccentric orbit, its motion relative to the gas increases. Such objects can be significantly eroded if they consist of dust. As an extreme, a 100 m body with the rather low eccentricity of an Earth orbit might be eroded in a single orbit. This effect leads to a bias for planetesimals in low-eccentricity orbits, as objects with large eccentricities are destroyed more easily. Erosion of bodies in high-eccentricity orbits, and reaccretion of the dust aggregates by low-eccentricity planetesimals, might provide a special growth mode of planetesimals and protoplanets.

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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.

Planetesimals and their precursors in protoplanetary disks are very porous. Thus, a gas flow around such bodies will be accompanied by gas flow through them. We calculate how this gas flow will influence the impact of a small body on a body larger than 1 m in size. On the front side of a large body (target) with high porosity there is a boundary layer that is characterized by a gas flow toward the surface. We find that under typical conditions with respect to collisions in protoplanetary disks, fragments of a collision will stay inside this boundary layer. These fragments will return to the target by gas drag. Net growth of the larger body in these secondary collisions will occur. The mechanism works for all sizes up to planetesimal size. This supports the idea that planetesimals (kilometer-sized bodies) build up from collisions of smaller bodies.