Whether called “dirty snowballs” or “icy dirtballs,” comets are rich in water ice. However, until today, little is known about the nature of cometary water ice, such as its origin, formation, and evolution process. For example, sufficient water gas can be found in the coma, yet all observed comet nuclei appear dark and have low reflectivity.
Only a limited amount of water ice has been detected on some cometary surfaces. Why is that? Why are we able to detect water but can hardly see it on the nucleus surface? Where is the ice hiding?
The orbiter Rosetta may provide us with clues to clear up this mystery. With the in-situ, long-term observation and measurement, water ice has been identified in small patches or bright spots on the surface of Comet 67P/Churyumov-Gerasimenko. Furthermore, thin frosts sublimating close to receding shadows have also been observed. In order to explain the observed ice on the cometary surface, possible theories of the water ice cycle on the comet have been proposed:
- Ice sublimation from the subsurface layer condenses in the uppermost layer after sunset;
- The backflux from the inner coma deposits on the cold area.
The former — subsurface ice sublimation mechanism — has been widely accepted and adopted by many recent works about Comet 67P/Churyumov-Gerasimenko, while the latter has not been extensively explored. Most studies consider coma deposition mainly for thermo-physical models of the nucleus because the condensing gas molecules may heat the surface by releasing the latent energy. Some simplified models have been used to examine the mass transport around the nucleus by considering the condensation on the shadowy parts of the nucleus surface, yet more precise and modern models for simulation and comparison with observational results are still needed.
A recent study (1) has approached the condensation process of 67P’s inner gas coma numerically by introducing rarified gas dynamics and data from Rosetta. The authors designed several cases to see how the water deposition distributes with the varying surface conditions such as different illuminated regions and terrains. The simulation results show that, firstly, it is more likely to have water vapor condensing in shadowy regions on the dayside rather than the nightside of the nucleus surface. That is because the deposition comes from the backflux, which can only be gas molecules reaching the surface from the vicinity due to intermolecular collisions. The nightside hardly has outgassing activities thus fails to provide a certain amount of backflux. Secondly, the neck region of the comet is another preferable place for water deposition because of the concave terrain.
Lastly, the deposition acquired from coma condensation in the near-perihelion environment shows comparable to the ice accumulation from the condensation of subsurface sublimation, which suggests that the coma condensation mechanism may also play an important role in the water cycle of 67P during its perihelion passage.
These findings are described in the article entitled Water vapor deposition from the inner gas coma onto the nucleus of Comet 67P/Churyumov-Gerasimenko, recently published in the journal Planetary and Space Science. This work was conducted by Ying Liao from Macau University of Science and Technology, I. L. Lai from the National Central University (Taiwan), and her former colleagues from the University of Bern, Switzerland.