The role of the hot porous layer in the gas flow in the inner coma

¹ Physikalisches Institut, University of Bern, Sidlerstrasse 5, Bern 3011, Switzerland
² Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
³ Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, 38106 Braunschweig, Germany
⁴ CNRS, Observatoire de la Côte d’Azur, Laboratoire J.-L. Lagrange, Nice, France
⁵ European Space Agency (ESA), ESAC, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain

^★ Corresponding author; omar.mokhtari@unibe.ch

Received: 24 July 2024
Accepted: 3 December 2024

Abstract

Aims. The objective of this work is to study the influence of a highly non-isothermal porous dust layer on the formation of a comet’s inner coma. We studied the water gas activity of comet 67P/Churyumov-Gerasimenko to find a link between the gas properties around the comet and the properties of the dust surface crust. The effects on the radiative transfer spectral lines were studied and compared with MIRO remote sensing observations.

Methods. For cases of spherical and complex nucleus shapes, we validated surface boundary conditions for gas flow obtained from the two-layer consistent thermophysical model. This model accurately estimates the properties of the sublimation products as the gas diffuses through the layer. The gas expansion was then modeled using a 3D parallel implementation of a direct simulation Monte Carlo algorithm. A multi-beam linear interpolation was used to extract the gas density, velocity, and temperature profiles along a given line of sight. Finally, the radiative transfer equation was used to calculate the brightness temperature of the water vapor.

Results. The presence of a porous layer results in an increase in gas temperature and a decrease in gas density at the surface. The gas has a greater acceleration due to the higher initial temperature and increased conversion of translational energy to kinetic energy. This reduces the difference in density between the different models, with the densest gas being the coolest, and increases the terminal expansion velocity of the hotter gas. While the gas density differences are small at large distances, the observable water absorption lines are significantly affected.

Conclusions. The presence of a porous layer has a large effect on the properties of the gas in the coma, which can be seen by comparing the spectral lines. This demonstrates the potential interest of the approach in improving surface activity models and placing physical constraints on the dust layer.