Angularity and Polydispersity effects on gas-flow and possible implications for cometary activity

Introduction:

Cometary acitivity models (such as used in: Skorov et al. 2023, Gundlach et al. 2020, Bischoff et al. 2023 and Fulle et al 2020) depend on an accurate permeability prediction of surface near materials. The refractory to ice ratio of this material is studied extensively, but little is known about the structure of the porous layer or the shape of the icy regolith and how it might affect surface activity. To support cometary erosion models with an improved gas-flow prediction, we study various models which are used to describe the flow of rarefied gases through porous materials. These diffusion models are usually expressed for mono-disperse packed beds of spherical and simulations and experiments have confirmed the dependency on grain size and porosity (Güttler et al., 2023). But observations using Rosetta have shown, that cometary particles might be of elongated, non-spherical shape. Therefore, we generalised the current models to be applicable to polydisperse and non-spherical particles (Zivithal et al., in revision) and performed experiments which identified the gas flow parameters, Knudsen diffusion coefficient and viscous permeability, of different granular packings.

Measurements and results:

We use a dedicated measurement setup that allows to measure the gas flow parameters in the transition regime. To study the effects of angularity and polydispersity we conducted measurements with mono- and bi-disperse spherical packings, packings of elongated metal pins in different orientation, and highly porous packings. Special attention had to be paid to biases in measuring the porosity and the pressure drop in the sample. The conducted measurements confirm that the Knudsen diffusion coefficient is inversely proportional to the specific surface area of the grains and that the viscous permeability is inversely proportional to the specific surface area squared. This was not only valid for the mono-disperse but also for the bi-disperse packings measured. Further, we were able to identify a relation between the gas flow parameters, represented by a parameter 𝛽, which seems to be an indicator for the mean orientation of the grains. The findings give further evidence of the importance of the grain size distribution, the grain shape and the orientation for rarefied gas flow and how sensitive it is to changes within those parameters.

Discussion:

Cometary surfaces as seen by Rosetta and other space mission have a diverse morphology. Our results suggest that different morphological structures (consisting of packings of various grain size distributions and grain shapes) will have vastly different outflow behaviours and therefore, the findings could help to explain different erosion patterns. Most importantly the findings of this work emphasise the importance of further research about cometary surface composition and the interacting, sensitive gas flow.

References

Skorov, Y. et al., 2021. MNRAS 501.2, pp. 2635-2646.

Gundlach, B. et al., 2020. MNRAS 493, pp. 4690-3715.

Bischoff, D. et al., 2023 MNRAS, pp. 5171-5186.

Fulle, M. et al., 2020 MNRAS 493, pp. 4039-4044.

Güttler, C. et al., 2023. MNRAS 524, pp. 6114-3123.

Zivithal, S. et al., in revision. MNRAS