Drop tower experiments on sticking, restructuring, and fragmentation of preplanetary dust aggregates

Abstract

For the determination of the aggregation efficiency of preplanetary dust, we performed impact experiments with fractal dust aggregates in the drop tower Bremen. We found that for the lowest impact velocities, the dust aggregates, which consisted of micron-sized, monodisperse SiO₂ spheres, hit and stuck with no measurable impact restructuring. For intermediate collision velocities, compact aggregate structures formed, and at the highest impact velocities, aggregates were fragmented. Our experimental results are in quantitative agreement with a numerical dust aggregate collision model (Dominik, C., Tielens, A. G. G. M. 1997, Astrophysical Journal vol. 480, p. 647), when the latest experimental values for the rolling-friction and break-up energies are used. However, the presence of a rarefied gas flow, in which the incoming aggregates were embedded, increased the threshold velocity for sticking. Although the impinging aggregates were disintegrated at high impact velocities, the resulting fragments were dragged back to the target on which they could stick due to a then considerably lower collision velocity. This aerodynamically-supported aggregation process might be responsible for the rapid growth of preplanetary bodies in the size range from ∼0.1 m to ∼10 m. Such a rapid growth is necessary to prevent a loss of most of the solid bodies of these sizes due to gas-drag-induced fast orbital decay.