There is a basic understanding of the way gases are released from cometary nuclei in order to form the gas and dust comae as they approach the Sun. We know that the production of these gases is driven by the incident solar radiation on the nucleus, and this leads to the sublimation of cometary ices.
The composition of the coma depends on the composition and thermal properties of its nucleus. In general, comets are often assumed to be a mixture of ice and dust (Whipple 1950, Mendis 1977, Fanale et al. 1984). There is however a lack of knowledge on the actual internal structure of comets both at macroscopic and microscopic levels. For comet 67P/Churyumov-Gerasimenko (67P/CG), the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) determined H2O, CO2, CO and O2 to be the most abundant gases in the coma (Hässig et al. 2015, Le Roy et al. 2015, Bieler et al. 2015b), which is a clear indication of the composition of dominant ices within the nucleus. How these different ices are physically related to each other and distributed within the nucleus is a more intricate issue for which we have no clear answer at this time.
The aim of our work is to investigate the surface parameters influencing the generation of the inner comae of comets and model the gas activity distribution around their nuclei. Herein the influence of thermal inertia combined with sub-surface sources on insolation-driven sublimation and the resulting gas flow field is investigated using both H2O and CO2 as driving volatiles. We apply this study to a spherical nucleus comet and for the complex shape of comet 67P/CG (Preusker et al. 2017).
We use a simplified model of heat transport through the surface layer to establish sublimation rates from a H2O- and CO2-ice subsurface into vacuum. The 3D Direct Simulation Monte Carlo method is then used to model the coma as a sublimation-driven flow. The free parameters of the model are used to test the range of effects arising from thermal inertia and the depth of the source on the gas outflow.
Our results suggest that 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 greater than zero, the H2O distribution can be shifted in rotation by about 20 degrees relative to models with no thermal lag. For CO2, the maximum activity can be shifted towards the terminator with activity going far into the nightside. The presence of a small amount of CO2 can also reduce the presence of H2O by at least an order of magnitude on the nightside. However, it can increase the speed of the mixed flow on the dayside by about 100m/s.
H2O is the dominant species on the dayside in all tested cases. While, CO2 is clearly the main driver for activity on the nightside in several of them. This would be consistent with observations of gas density and dust column density above the nightside hemisphere of the nucleus of 67P/CG.