Experimental results from the CoPhyLab - determination of particle size and acceleration in comet-simulation experiments

The formation of a cometary dust tail is phenomenon that has been known for a very long time and can be seen from earth with the naked eye if the comet is close enough. The mechanism for the formation of the cometary dust tail is believed to be caused by the sublimation of volatile components, which drag away the dust particles. In the lab, we aim to reproduce this effect by placing mixtures of ice and dust in a cooled vacuum chamber and illuminating them with an artificial sun. In preparation of these experiments, we placed samples consisting entirely of µm-sized spherical water-ice grains in the chamber. As soon as the illumination is turned on, solid particles are being ejected from the surface, hence the water-ice sample ejects a fraction of its own mass in the form of solid grains, which are accelerated by the surrounding gas. We analysed the particle motion, using multiple cameras in high-speed and low-speed mode, as well as a scale, a mass spectrometer, an infrared camera, several distributed temperature sensors and a scanning line-laser system. From the high-speed camera images, we determined that most of the ejected particles are flat disks with a height of ∼ 20 µm and a diameter of ∼  80 µm. Furthermore, we performed 3D particle tracking to gain information about the trajectories and accelerations of the particles. We discovered that the speed of the particles at the first image visible in the cameras is much higher than expected. From that point on, the particle trajectories can be fit with the expected value for a gas-driven acceleration with and a gas-flow rate determined by the surface temperature using the Hertz-Knudsen equation. However, if we extrapolate the trajectory backwards in time using the same acceleration, the zero-velocity starting point of the particles would be several centimeters below the sample surface. Since this is impossible, the particles must possess an unresolved starting acceleration about tenfold higher than what could be explained by the vapour pressure of the water ice. This finding means that the initial acceleration must be driven by another process, which we aim to explain by thermal modelling and by combining data from all instruments used during the experiments.