On one possible mechanism for a cometary nucleus surface destruction and the ejection of solid particles from the surface layer

According to the current understanding, the nucleus of a comet consists of a mixture of diverse ices, minerals and organics. The atmosphere of a comet is formed by the sublimation products of ices and the dust particles ejected from the nucleus and entrained by the gas flow.

In the absence of a direct exploration of the nucleus, the observations of cometary atmosphere allow us to deduce parameters of the nucleus – its composition, structure etc. and, in this way, to get information about the Solar System formation. To this end, it is necessary to have a model that links processes in the nucleus and in the circumnuclear dusty-gas environment.

 

We present a modified model of gas flow through the porous layer of a comet's nucleus. We consider a time-dependent process that includes the rotation of the nucleus, its heating (under solar illumination) and its cooling. The nucleus consists of a mixture of ices (H₂O, CO₂, CO etc.) and refractory materials.

The standard model for surface layer heating and gas outflow through pores involves solving the diffusion equation in conjunction with the energy equation. A distinctive feature of the modified model is that it accounts for the presence of a certain proportion of closed cavities within the layer. The model allows for a complex composition of a surface layer consisting of multiple ices, open and closed cavities, and refractory material. The model accounts for the dependence of the thermophysical parameters on the porosity. To calculate the thermal conductivity coefficient, the Hertz factor (a coefficient characterizing the ratio of the thermal conductivities of porous and non-porous materials) is introduced. When evaluating the thermal conductivity coefficient, the contribution of radiative heat transfer in a porous medium is taken into account.

Based on the modified model, a numerical study was conducted of the heating process in the surface layer of a comet, which consists of H₂O and CO₂ ice, as well as refractory particles. The model predicts significantly higher gas pressure than that predicted by a model that accounts only for the presence of water ice, or by a model that describes a mixture of different types of ice but does not account for enclosed cavities.

High gas pressure in the cavity might leads to the destruction of the surface layer and ejection of its fragments. We present the results of a numerical simulation that accounts for the gas-dynamic detachment of refractory fragments caused by the presence of cavities containing gaseous products of sublimation from the surface layer.

This study was carried out with the financial support of the Russian Science Foundation (grant No. 24-12-00299, https://rscf.ru/en/project/24-12-00299/).