The effect of thermal inertia on the outgassing and gas dynamics in the inner-coma of cometary nuclei

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 [1, 2, 3]. 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), for example, 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[4, 5, 6], 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.

Previous studies have produced mathematical models of the surface layer of cometary nuclei based on heat and gas diffusion through pores [7, 8, 9, 10, 11]. These studies use a one-dimensional geometry. Our aim is to use a similar approach to model the gas activity distribution around three-dimensional nuclei. This is known in the literature as "standard thermal model" for slow-rotators [12, 13, 14] and it calculates the insolation condition at a certain heliocentric distance for one point on the surface after Nrot nucleus rotations on its axis. Here in the influence of thermal inertia combined with sub-surface sublimation sources at different depths and the resulting gas flow field is investigated using both H2O and CO2 as driving volatiles. The additional complexity in geometry prevents more sophisticated numerical solutions. We have therefore ignored the influence of heating of gas through collision with a hotter dust mantle in the present model, considering that these additional effects probably only result in relatively small changes in the position of the sublimation front due to the exponential change in sublimation rate with temperature.

We apply this study to a spherical nucleus comet, but we adopt some of the rotational and surface properties of the target of the Rosetta mission, comet 67P/CG. In order to model gas activity in the inner coma, we have chosen a nucleus with a 2 km radius and an outer limit of our simulation domain to be at 8km from the surface of the comet. The 3D Direct Simulation Monte Carlo method is then used to model the coma as a sublimation-driven flow.

Simulation results displayed in figure 1 show 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. We determine that for cases with a thermal inertia larger than zero, the H2O distribution can be shifted in rotation by about 20º relative to models with no thermal lag. For CO2 cases with different thermal inertia values and sublimation fronts, the activity distribution can be shifted towards the terminator making CO2-ice the main source of nightside activity. This would be consistent with observations of gas density and dust column density above the nightside hemisphere of the nucleus of 67P/CG.

There is also a strong effect of CO2 activity on the distribution of the H2O flow field in the nightside of the comet, which can decrease the amount of H2O by up to 98% compared to a pure H2O case. CO2 gas also decreases the temperature of the flow, as well as the flow velocities on the nightside. In the cases we studied, temperatures were reduced by a factor of 2 and velocities were up to 150m/s slower than the cases without CO2 activity.

Figure 1: Slice of the 3D simulation domain on the xy-plane, with information of number density, temperature and velocity within the flow for a case without thermal inertia (upper row), a pure H2O case with thermal inertia of 40 J/(m2K√s) (second row) and a mixture case with 40 J/(m2K√s) (third and fourth row). The arrows on the left side indicate the position of the sub-solar point.

 

Acknowledgements

This work has been carried out within the frame- work of the National Centre of Competence in Re- search PlanetS supported by the Swiss National Sci- ence Foundation. The authors acknowledge the finan- cial support of the SNSF.

Raphael Marschall acknowledges the support from the Swiss National Science Foundation grant 184482.

 

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