Gas diffusion and transport in cometary surface material

Introduction

The molecular diffusion of gas is an essential process in our understanding of cometary activity. Gas that is produced through sublimation in active sub-surface layers builds up a pressure, while at the same time diffusing towards the comet’s surface and escaping into space. The governing diffusion constant is strongly dependent on the microscopic structure of the material, i.e., its grain size and porosity (Güttler et al., 2023) but also grain shape and arrangement (Zivithal et al., accepted). Its detailed understanding is therefore bridging between the observed activity of comets with space probes and also analogue material (Kreuzig et al., 2021 and others) on one side and the microscopic material structure on the other side. Through the knowledge of diffusion and gas transport in the near surface layers, we aim to learn about the microscopic structure, thus the nature and formation of comets.

Experiment and Simulation

We have performed laboratory experiments on gas diffusion in the molecular-flow regime through very clean and controlled samples consisting of 0.5 mm diameter steel beads. These simulations were successfully reproduced using a Direct Simulation Monte Carlo (DSMC) approach with the PI-DSMC code (Rose, 2014). The simulations then allowed a broad variation of gas velocity (temperature and molar mass), particle diameter and porosity, confirming the analytic descriptions of Derjaguin (1946) and Asaeda et al. (1974). Residual deviations from absolute values were found on a level comparable to those by Asaeda et al. (1974; they introduced a scaling factor q=1.41) and moreover to be linear dependent on porosity. On a microscopic level, by tracing individual particles, the description of mean path lengths and their distribution by Derjaguin (1946) could also be confirmed.

Further Application of the DSMC Model

After successful application of the DSMC model, as confirmed by our experiments, literature experiments and analytical models, we are now applying it now to more complex geometries.

One aspect is the introduction of macroscopic voids of different geometries to study their effect on the gas-flow field. In particular, we look at the force applied on individual particles (which we consider as pebbles on a comet’s surface; Blum et al., 2017). We look at their lifting force, with and without voids, counteracting their cohesion.

Second, we simulate a gas source (sublimation front) as a plane below the comet surface but with granular material below. This allows the gas to diffuse into space as well as into the interior. We then let the gas that diffuses into the interior adsorb as a function of depth. We find that a substantial fraction of gas that is sublimated in the described plane does not escape into space but rather re-adsorbs in layers below the sublimation plane. This material transport builds up vertical layering, which will be studied in further depth for its eventual application to thermophysical models (Gundlach et al., 2020).

References

Asdaeda, M. et al., 1974. J. Chem. Eng. Jpn. , 7, 93.

Blum, J. et al., 2017. MNRAS 469, S755-S773.

Gundlach, B. et al., 2020. MNRAS 493, p4690-3715.

Güttler, C. et al., 2023. MNRAS 524, pp6114-3123.

Derjaguin, B.Y., 1946. Dokl. Akad. Nauk SSSR, 4, 687.

Kreuzig, C. et al., 2021. RSI 92:115102.

Rose, M., 2014. AIP Conf. Proc. “29th Int. Symp. on Rerefied Gas Dynamics”. Vol 1628.

Zivithal, S. et al., submitted. MNRAS