Observing and modeling water activity and surface changes over dust cover on comet 67P/Churyumov-Gerasimenko

The excitement of the Rosetta Mission culminated in 2014 when the spacecraft arrived at comet 67P/Churyumov-Gerasimenko (67P) after a ten-year chase to escort the comet through its following perihelion passage. Characterizing the distribution of activity and surface changes over the nucleus was among the main scientific objectives of OSIRIS, the camera system onboard Rosetta, that would shed light on the physical and compositional properties of the nucleus and its evolution. Some fundamental geometric and photometric methods of image analysis are reviewed and refurbished in this work that are instrumental to the determination of sources of dust activity and quantification of surface changes on 67P observed by OSIRIS. Deriving nucleus properties from the observed dust activity and surface changes relies on modeling of thermo-physical conditions of the nucleus at the epochs of observations. The general formulation and numerical recipes of two cometary thermo-physical models, as well as strategies of model parameterization, are explicated in this work in order to facilitate the determination of nucleus subsurface properties. Dust jets from the night side had been recurrently observed on 67P, in often cases near the dusk terminators. The thermo-physical models are parameterized and applied to simulate the thermal and mechanical conditions of the nucleus subsurface over the source areas of jets under which the observed dust activity could have been sustained after sunset. These jets are found to have probably originated from the depth of a few millimeters below the surface where water ice was present and where the residual warmth could sustain strong water outgassing even one hour after dark. The source areas of the sunset jets had undergone significant changes when 67P reached 2 AU inbound from the Sun. It will be shown that these changes, as well as numerous others found at roughly the same latitudes, were erosive in nature and induced by sublimation of water ice accumulated over months. The quantification of the changes based on OSIRIS observations in comparison with the estimation of accumulated water ice loss via thermo-physical modeling revealed a low ice abundance on the order of 1% in the dust cover on average. These results allude to a fundamental but unresolved question regarding physics of cometary activity and evolution, namely, the detailed mechanism of ejection of dust induced by sublimation of lesser amounts of water ice underneath.