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Gas-flow through the dry layer on a cometary surface

Presentation #218.08 in the session Comets: Coma, Nucleus, Dynamics (Oral Presentation)

Published onOct 23, 2023
Gas-flow through the dry layer on a cometary surface

The findings from space missions to comets have indicated that the coverage of pure water ice on the surface of a comet is insufficient to account for the observed high rates of gas production leading to the formation of the coma [1,2]. One plausible explanation is the sublimation of ices in a subsurface layer, with a dry dust layer on top of it. However, the depth of such a layer remains an unanswered question, not least because of the complex and non-uniform structure of the comet’s surface.

To investigate the physics of the uppermost layer of a dry cometary surface, we conduct experiments, using a dedicated setup designed to measure gas-flow properties within a granular medium under vacuum conditions. The vacuum chamber consists of two separate compartments, with the sample container positioned between them. Precise pressure measurements can be obtained in both compartments, while an adjustable gas-flow is introduced from the top of the chamber. By employing the binary friction model (which allows for both Knudsen Diffusion and viscous flow), we can measure gas-flow properties in different regimes by examining different gas-flow rates and the corresponding pressures.

Our research specifically focuses on two parameters of interest: the Knudsen diffusion coefficient and the viscous permeability. Existing analytical models for these parameters are currently limited to explain gas-flow in homogeneous mono-disperse samples. However, when studying complex structures such as the surface of a comet, these models prove insufficient. Therefore, we developed more general models based on the specific surface area and particle size distribution within the existing models. To validate these models, we conducted experiments using a bi-disperse sample consisting of spherical glass beads. The results show good agreement with the generalized model, allowing us to investigate also inhomogeneous samples with more complex void structures. Additionally, we developed a simulation setup to generate polydisperse samples, which we plan to study by means of DSMC simulations. First results show good agreement between the model for mono- disperse spherical packings and the simulation.

1. J. M. Sunshine, et al., Exposed Water Ice Deposits on the Surface of Comet 9P/Tempel 1.Science311,1453-1455(2006).DOI:10.1126/science.1123632 2. F. Capaccioni, et al., The organic-rich surface of comet 67P/Churyumov-Gerasimenko as seen by VIRTIS/Rosetta.Science347,aaa0628(2015).DOI:10.1126/science.aaa0628

https://doi.org/10.1126/science.1123632

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