Studying gas flow on cometary surfaces with diffusivity variations using flow simulations and experiments

Comet surfaces have a complex morphology on large scales (such as pits, depressions, scarps and faults) as well as on small scales (such as particle size, porosity distribution and roughness of the comet surface). Little is known about the influence especially of small-scale structures on the gas-flow and the ability to lift off dust particles. Focusing on small-scale structures, we simulate diffusion processes applying Fick's law. For the upper and lower boundary of the simulated box, a flat sublimation front with constant pressure below the inactive surface layer and a perfect vacuum on the surface is assumed. By applying periodic boundary conditions in the planar direction, we mimic an infinite surface with periodic inhomogeneity. We performed the simulation with different variations of diffusivity. In one example, the simulation shows that a region of high porosity within a region of low porosity experiences an increase in the flow rate, as would be expected according to Fick's Law. Nevertheless, it also significantly changes the flow rate in the surrounding region due to lateral flows in the vicinity of the high diffusivity region. We analyze the results qualitatively and compare them with 3D Monte Carlo simulations in a similar setting, which shows a general agreement.

In addition, we conduct experiments with a dedicated vacuum-chamber to measure the viscous permeability and Knudsen diffusion of granular materials by applying the binary friction model. The design allows measurements in different gas-flow regimes, with most samples mainly covering the free molecular flow and the transition region. In the last measurement campaign, bi-disperse samples of spherical particles were measured and the results show a good agreement with generalized models depending on the specific surface area of the sample. The current measurement campaign focuses on angular materials and the influence of shape properties on diffusivity. The results show that packings of highly porous hollow cylinders have a lower diffusivity than expected compared to more compact packings of spherical particles. A comparison with Monte Carlo simulations (from [1,2]) of packings of highly porous spherical particles also shows a higher diffusivity compared to the same measurements. Further measurements will show whether a dependence on a particular shape property could explain this discrepancy.

[1] Macher, Wolfgang, et al. "Transmission probability of gas molecules through porous layers at Knudsen diffusion." Journal of Engineering Mathematics 144.1 (2024): 1-26.

[2] Güttler, Carsten, et al. "Simulation and experiment of gas diffusion in a granular bed." Monthly Notices of the Royal Astronomical Society 524.4 (2023): 6114-6123.