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Cometary activity is not yet fully understood. One goal is to determine thelocations on a comet more likely to be active. This work focuses on how surfacestructures influence cometary activity. Therefore, laboratory experiments wereconducted to examine different surface structures. To simulate the most simpleversion of a comet, the samples were made out of granular water ice. Differentstructures were embedded into the samples. Three structures were chosen forthe experiment to simulate some possible structures to find on a surface. A nar-row but deep hole, a cliff with a 45 angle and an even square imitating a crackare realized.The prepared samples then were inserted into an actively cooled vacuum cham-ber. At a pressure of about 1 105mbar the samples were illuminated witha halogen lamp which simulates the Sun. Exposed to a radiation of around 2solar constants, the samples show the first signs of activity. The whole processis filmed with a camera. Every video is about 20 minutes long. In the end, allthe recorded images are stacked and color coded to visualize the most activeregions of the sample. In Fig. 1, the results of an experiment with a cliff arepresented. The upper plot shows the accumulated brightness of the particlesover the position on the sample. The peaks mark the most active areas. Thecolor coded trajectories of the particles are visualized in the middle plot. Thelower plot indicates the location and shape of the realized structure.The required intensity of radiation to show activity is lowered by structuringthe samples as observed in the experiments. A reason for that might be lessbound material on the edges of the structures. Another reason could be thatstructured surfaces can produce up to 61 % higher sublimation rates than flatsurfaces as results from simulations ( Hfner, 2021).Further, structured samples feature areas with directed jets. This might be dueto directed gas flow carrying the particles in a certain direction. Since the struc-tures were embedded into the sample and the camera is observing the sampleparallel to the surface, there is no information about the activity inside thestructures. This setup only observes particles leaving the surface of the sample.Improving the setup to investigate the structures inside will be part of futurework.Figure 1: (center) Color coded trajectories of ejected particles from a sampleafter 20 minutes of illumination with an artificial sun. (bottom) Schematics ofthe position of the structure inside of the sample. (top) Accumulated brightnessof each column of the particle trajectories. The two peaks at 3.5 mm and 5 mmare not related to the measurements but result from pixel errors.Figure 2: Sample holder filled with micro-granular water ice. Inserted intothe sample holder is a bridge with a stamp used to create the embedded struc-ture. The left side of the stamp is located in the middle of the sample holder tocompare the flat and structured parts of the sample.
Knoop et al. (Wed,) studied this question.