Planets originate in so-called protoplanetary (accretion-) disks around their central star (e.g. the sun). The disks, consisting of gas and dust, take shape in early phases of star formation, which is initiated by the collapse of molecular cloud cores. This event produces an accretion shock front that gas and dust particles have to pass through as they fall onto the accretion disk. Hereby, the dust and ice particles evaporate and recondense in the accretion disk to 0.1-10 µm-sized particles.
It is generally assumed that the particles in turbulence-free regions of the disk grow by Brownian motion, sediment towards the centre plane and then drift toward the disk’s axis of rotation.
ICAPS stands for “Interactions in Cosmic and Atmospheric Particle Systems” and studies the agglomeration of μm-sized, monomeric SiO2 particles under microgravitational conditions. To achieve stable microgravity, the experiment flew onboard the Texus-56 sounding rocket, continuously monitoring and controlling the movement of the dust cloud. It therefore employed a thermal trap, consisting of four rings. During the experiment, the agglomeration processes inside the dust cloud were observed by two overview cameras (OOS) and a long distance microscope (LDM).
In our laboratory experiment, we simulate the growth of dust and ice particles through sedimentation. Large particles sediment faster and grow by accumulating smaller ones. We obtain cm-sized particles with a filling factor of 10-15 %. This is equivalent to particle sizes that are being reached in protoplanetary disks as they sediment to the centre plane. Here, we simulate the sedimenting particle with a target that a mixture of gas and µm-sized particles is flowing around. It thereby grows through accumulation of the particles contained in the gas, similar to real sedimentation.
Afterwards, we let the resulting agglomerates collide to derive impact properties and compare with impact models, developed by us. For this, we use a 2.5 m high, evacuated glass tube, which we also call the laboratory droptower. In it, two agglomerates are positioned one above the other and dropped with a short delay. Their resulting relative velocity leads to a collision, which is recorded by a high speed camera, falling outside of the tube. The camera is dropped exactly between the agglomerates’ release times and therefore always looks at the center of mass of both particles. With this experiment, the adhesion limit speed can be measured, which is essential for numeric planet growth models.
The associated movies show collisions in the laboratory droptower, one with sticking dust samples and one with ice samples bouncing off each other.
In order to experimentally simulate agglomeration in the initial phase of planet formation, weightless environments are required. However, particularly long experiments in weightlessness are associated with a lot of effort, as the free fall time in drop tower experiments is no longer sufficient. Therefore, long-term experiments have to be carried out on expensive rocket flights, on parabolic flights or on the ISS.
In experiments with the levitation drum, instead of a weightless environment, the friction of the dust particles on a gas flow is used to keep the dust in suspension.
The levitation drum consists of a rotating vacuum cylinder chamber which is filled with a residual gas in the millibar range. The rotation leads to a gas flow which rigidly moves with the chamber. At the so-called stability point, the force of gravity and the gas friction force of the dust in the chamber cancel each other out. A cloud of dust forms, which circles around this stability point. The dust can float for up to an hour and agglomerated. Optical properties and microscope images can be seen in the floating dust
be made during the agglomeration. At the end of an experiment, the dust can be caught and later analyzed with a microscope.
A long-term experiment with the levitation drum is significantly less complex and there are significantly fewer demands on the instruments than in comparable weightlessness experiments. However, specific effects of the levitation drum on the agglomeration must be taken into account.
In summer 2018, a suborbital flight with the DUST experiment was carried out in the Esrange Space Center (Kiruna, Sweden) in cooperation with the University of Hokkaido (Japan), the TU Braunschweig, the German and Japanese space agencies DLR and JAXA.
The aim of the DUST experiment is to investigate the condensation and agglomeration of titanium carbide nanoparticles from the ejection of AGB stars under weightlessness. It is assumed that these particles are part of the starting material of protoplanetary disks and are thus involved in the beginning of planet formation.
The experiment consists of a vacuum chamber with a dilute argon atmosphere in which a titanium-wrapped carbon rod is electrically heated. During the heating phase, a gas mixture of titanium and carbon is created, which condenses to form a cloud of titanium carbide nanoparticles. The nucleation of titanium carbide from the gas phase is disturbed due to the convection in the earth's gravitational field, which is why weightlessness is required. In the particle cloud, the nanoparticles agglomerate to form fractal structures and then fall into a sampler, the contents of which are then examined in a transmission electron microscope after the experiment has "softly" landed.
