Formation of icy planetesimals

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The first phase of planet formation is the growth of micron-size particles to km-size planetesimals by coagulation. So far, experimental studies deal with coagulation of dust particles (mostly silicates), as silicates are dominating in the inner parts of protoplanetary disks. In the outer parts of protoplanetary disks the temperature in the disk is low enough that water ice is the dominant solid. In different growth stages (mm-size and meter size), particle growth is difficult to explain by sticking forces only, as collision velocities get large and bouncing and fragmentation are typical collision outcomes. However, cohesion forces for ice aggregates are supposed to be larger than for silicates and therefore also the coagulation efficiency is supposed to be greater. We propose experimental studies on collisions between porous ice agglomerates as they are expected in the outer parts (beyond the snow line) of protoplanetary disks. An experimental setup to generate porous ice agglomerates will be developed, as well as a setup for multiple collisions under cryogenic conditions. Multiple collisions in the velocity range between 0.5 m/s and 10 m/s will be studied as well as the properties of the resulting aggregates. The results will be important for the understanding of the formation of the planets in the Solar System and will provide crucial input for theoretical coagulation models.