Experimental Results from the CoPhyLab - Investigating Filament Formation on Granular Water-Ice Samples Mixed with Carbon Dioxide, Alcohol, or Salt in Comet-Simulation Experiments

The effects of solar illumination on comets are difficult to understand. In order to investigate them, cometary materials can be simulated in vacuum chambers, where light from a halogen lamp is directed onto samples composed of substances known to occur on comets.
The basis of these samples is granular water ice, which constitutes a major fraction of cometary material. When pure granular water-ice samples are observed in vacuum under halogen-lamp illumination, filament formation can be seen. These filaments form on the sample surface, oscillate, rotate, and grow until they eventually detach from the surface. To document these filaments in greater detail, a dedicated experimental setup was developed, consisting of a small vacuum chamber at its center (figure [1]). Using this setup, filaments can be observed and recorded with the aid of a microscope (figure [2]).
This phenomenon has already been investigated for pure granular water ice in the bachelor thesis of Victoria Muthig. To gain further insight into these processes and into how they may occur on actual comets, mixtures of water ice with other substances commonly detected on comets are now being studied. In particular, this work compares filament formation in granular water ice mixed with salt, alcohol, and carbon dioxide, respectively. All of these substances have been identified on comets. The samples consist predominantly of water ice, with approximately ten percent of one of the additional substances. The samples are placed in the vacuum chamber and exposed to illumination. The results provide insight into how the added substances influence filament formation on the sample surface.
For the measurements, a specially designed sample holder is filled with approximately 8 g of sample material. The surface is leveled by removing any material protruding above the surface, ensuring that the microscope is focused precisely at the surface, where the filaments become visible. The lamp is then switched on and focused onto the sample surface. After approximately 20 s, the first filaments become visible, and at a lamp voltage of 12 V, filament formation continues for about 30 min.
Initial measurements with carbon dioxide have already been performed and indicate a larger number of filaments as well as more frequent motion compared to samples with pure granular water ice. Additional measurements will be carried out to verify these preliminary observations.
Measurements with salt and alcohol will be performed in the upcoming months. The expected behavior in these experiments is difficult to predict, since filament formation is not yet fully understood even for pure granular water ice. It may be hypothesized that samples containing salt will show reduced activity, because salt does not sublimate under halogen-lamp illumination. This would lower the gas pressure at the sample surface, which normally builds up due to water sublimation and may influence both filament formation and filament dynamics. Alcohol, in contrast, is also expected to sublimate, possibly even more readily than water ice. This could enhance sample activity and promote both filament formation and filament motion in ice mixtures containing alcohol.

Figure [1]: Experimental setup used for the measurements: A: vacuum chamber, B: vacuum pump, C: microscope, D: camera, E: window for the light, F: voltage source for the lamp.

Figure [2]: Example of a close-up image of the observed filaments in a sample of carbon dioxide and granular water ice. The frame shown has a real height of 1.76 mm.