Viscous and Knudsen gas flow through dry porous cometary analogue material

Abstract

According to current theories of the formation of stellar systems, comets belong to the oldest and most pristine class of bodies to be found around a star. When approaching the Sun, the nucleus shows increasing activity and a pressure increase inside the material causes sublimated and trapped gas molecules to stream away from their regions of origin towards the surface. The present work studies two essential mechanisms of gas transport through a porous layer, namely the Darcy and the Knudsen flow. Gas flow measurements are performed in the laboratory with several analogue materials, which are mimicking dry cometary surface properties. In this first series of measurements, the aim was to separate gas transport properties from internal sources like local sublimation or release of trapped gases. Therefore, only dry granular materials were used and maintaining a low temperature environment was unnecessary. The gas permeability and the Knudsen diffusion coefficient of the sample materials are obtained, thereby representing the relative importance of the respective flow mechanism. The experiments performed with air at a stable room temperature show that the grain size distribution and the packing density of the sample play a major role for the permeability of the sample. The larger the grains, the bigger the permeability and the Knudsen diffusion coefficient. From the latter, we estimated effective pore diameters. Finally, we explain how these parameters can be adapted to obtain the gas flow properties of the investigated analogue materials under the conditions to be expected on the comet.

Cometary dust analogues for physics experiments

“…This is hard to achieve with the current set-up, therefore, the number of samples whose measured properties can be used for data interpretation is limited. In the same way as the experiments done with glass beads by Schweighart et al 2021, one can see a dependency of the gas flow on the size of the single agglomerates. Although the average size-frequency distribution of samples is known (see Section 3.1.1), small variations can lead to non negligible changes in the gas permeability measurements.…”

“…The gas flow is controlled by a thermal mass flow sensor, allowing for gas flow in the range of 0.15 mg s −1 to 20 mg s −1 . This setup was previously used to measure the gas permeability for cometary analogues (Schweighart et al 2021).…”

“…As the system adapted to the change, the pressure approached a steady state. Whenever a steady pressure level was reached, the mass flow was doubled (more details can be found in Schweighart et al 2021).…”

“…Gas permeability and Knudsen diffusion coefficient were calculated from the measured pressure values (see Schweighart et al (2021) for further details).…”

Are there any pristine comets? Constraints from pebble structure

“…Indeed, recent experiments that focus on the relation between the tortuosity and permeability, measure a similar range in tortuosity (see e.g. figure 19 in Schweighart et al (2021)).…”