Thermophysical modelling of comets: one dimensional microphysical modelling of cometary activity

The activity evolution of comets along their orbits around the Sun has not yet been fully understood. Especially around perihelion, comets show an abnormally high dust and water activity, growing far more rapidly than the increase in energy from the incoming sunlight (e.g. Skorov et al., 2020; Combi et al., 2020). To better understand this complex phenomenon, we developed a thermophysical model that encompasses mass-transfer of volatile species and the resulting build-up of pressure. We based our model on the assumption of a comet nucleus that was formed by gravitational collapse of dust-ice pebbles (Blum et al., 2017).

With these conditions in mind, this model was designed for numerically solving the one-dimensional heat transfer equation (HTE) for porous media. It solves the HTE through an explicit finite-difference scheme to more easily incorporate the temperature-dependent thermal conductivity. The thermal conductivity for a pebble system was implemented in accordance with Gundlach and Blum, 2012. The model calculates sublimating and resublimating particles via the Hertz-Knudsen equation and uses the resulting values to calculate the latent heat transfer. Sub-surface gas pressure levels become available for analysis through this microphysical approach. For ice-dust layers, we calculate the reduced gas flux through dust layers via a reduction function, as used in Gundlach, Fulle and Blum, 2020. Furthermore, dust ejection through pressure build-up for ultra-low tensile strength layers is an included effect, which can be used for calculating the ejected dust mass of a comet.

The current model is designed for applications with dust-ice pebbles, where H₂O and CO₂ are the main volatile species, but the implementation is adaptable for different scenarios and allows applications for non-porous media and addition of multiple volatile species. 

Figure 1: An example of the resulting temperature profile of our model. It shows the temperature and the volatile species content of the layers as a function of depth for a simulation of comet 67P/Churyumov-Gerasimenko close to perihelion. The two dotted lines mark the current position of the sublimation fronts of the volatile species.

Figure 2: An example of an ejection event of the top three layers during a simulation of comet 67P. The first plot shows the temperature profile and volatile species content on the time step before the ejection occurred, the second plot shows these parameters immediately after the ejection occurred

Figure 1 shows the effect of the latent-heat transfer on the temperature profile. The profile shows a  dent at the diurnal skin depth and another dent where the sublimation front of the water ice is located. Figure 2 shows the effect of a dust-ejection event on the temperature profile. We will discuss these effects on the temperature profile as well as different applications of the model. Furthermore, we will discuss the way our model includes the microphysical properties of the pebbles and the latent-heat transfer numerically. 

 

References:

Skorov Y. et al., 2020, MNRAS 494.3, pp. 3310-3316.

Combi M. R. et al., 2020, The Planetary Science Journal 1.3, p. 72.

Blum J. et al., 2017, MNRAS 469, pp. 755-773.

Gundlach B. and Blum J., 2012, Icarus 219.2, pp. 618-629.

Gundlach B., Fulle M. and Blum J., 2020, MNRAS 493.3, pp. 3690-3715.