Dust emission phenomena of cometary analogues

Experiments with cometary analogues are reported, which were performed at DLR - Köln in a small vacuum chamber and in the Space Simulator. The results apply to the emission processes of ice-dust grains from the surface of mineral-ice mixtures during insolation. The emission phenomena were observed with CCD-video-cameras. The camera used at the small chamber was equipped with a magnifying (10x) macro-lens. The magnified pictures show that the ice-dust agglomerates carry tiny ice particles on their surfaces. These may play the role of interlinking bonds between one grain and its neighbors. In the enlarged view it can be seen that the ice-dust grains do not erupt with high velocity out of the surface. The upstreaming gas jet originating from the sublimation of the volatile components first erodes the interlinking bonds between the particles. When the drag force exerted on the particles exceeds the force of gravity, the particles lift off. In most of the observed cases this initial lift off velocity is very small e.g. a few cm/s.

During a comet-simulation experiment in the Space-Simulator in November 1989 the trajectories of emitted dust particles were recorded for the first time with a high speed shutter video system 60 cm from the model comet. The video tapes were evaluated with a computerized image processing system. Unique insights in the gas-dust-interaction immediately at and above the insolated icy body were obtained. The trajectories differ markedly in all cases from the the ballistic parabolas calculated from the measured starting conditions: elevation angle and speed. The trajectory is mainly dominated by the momentum transfer from the gas jet to the dust particles. Many of the particles have millimeter sizes. The observed velocities at the end of the visible trajectory are in the order of a few meters per second.