The effect of thermal conductivity on the outgassing and local gas dynamics from cometary nuclei

Abstract

Aims. The aim of this work is to investigate the parameters influencing the generation of the inner comae of a comet with a spher- ical nucleus and to model the gas activity distribution around its nuclei. Here, we investigate the influence of thermal conductivity combined with sub-surface H2O and CO2-ice sources on insolation-driven sublimation and the resulting gas flow field. In the process, we adopted some of the rotational and surface properties of the target of the Rosetta mission, comet 67P/Churyumov-Gerasimenko (67P/CG). Methods. We used a simplified model of heat transport through the surface layer to establish sublimation rates from a H2O- and CO2-ice sub-surface into a vacuum. We then applied the 3D Direct Simulation Monte Carlo method to model the coma as a sublimation- driven flow. The free parameters of the model were used to test the range of effects arising from thermal inertia and the depth of the source on the gas outflow. Results. Thermal inertia and the depth of the sublimation front can have a strong effect on the emission distribution of the flow at the surface. In models with a thermal inertia up to 80 TIU (thermal inertia units: J m−2 K−1 s−1/2), the H2O distribution can be rotated about the rotation axis by about 20◦ relative to models with no thermal lag. For CO2, the maximum activity can be shifted towards the sunset terminator with activity going far into the nightside for cases with low thermal diffusivity. The presence of a small amount of CO2 can reduce the presence of H2O by at least an order of magnitude on the nightside by blocking H2O flow. In addition, CO2 can also decrease the speed of the mixed flow in the same region up to 200 m s−1, compared to cases with no CO2 activity. Conclusions. Even low values of the thermal inertia can substantially modify the gas flow field. Including CO2 leads to strong varia- tions in the local CO2/H2O density ratio between the dayside and nightside. CO2 can dominate the gas composition above the nightside and can also act to modify the H2O flow field close to the terminator.