Recently completed International Team. See the summary slides here: Attree_ISSI_Summary
Comets are objects from the early Solar System made of a mixture of dust and ices. When heated by the Sun, the ices outgass, lifting dust (which forms the visible coma and tail) whilst also pushing on the comet nucleus in the opposite direction, changing its rotation and orbit. This ISSI team gathered experts in cometary observations, trajectories, and thermophysical modelling to analyse comet 67P/Churyumov-Gerasimenko’s outgassing activity in order to improve our understanding of these ancient objects.
Members of the team published a new extraction of the outgassing-induced non-gravitational acceleration (NGA) and trajectory from tracking the ESA/Rosetta spacecraft (Lasagni Manghi et al. 2024) and compared it to previous work (Attree et al. 2024a). We then looked at the other Rosetta data, namely measurements of 67P’s total water-outgassing rate as well as its rotation rate and the orientation of its spin axis, both of which changed due to the non-gravitational torque (NGT). When comparing with a simple thermal outgassing model, we found that uniform surface activity could not explain the observed spin-axis changes (Attree et al. 2024a): activity levels have to vary, relative to a pure water-ice surface, across different areas of the nucleus in order to explain all the data. The efficiency of the momentum transfer from the gas must also vary spatially.
The team also published a thorough analysis of 67P’s thermal environment, showing how its large spin-axis tilt leads to very strong seasonal heating in some areas and low solar input in others (Groussin et al., submitted). Comparing the NGA/T solution with the thermal maps shows that some of the activity variation can be explained by these differences. Areas with different appearances (e.g dusty/smooth terrain vs. rocky/rough terrain) also have different outgassing rates. Attempting to explain this with a detailed thermophysical model of cometary material (Attree et al. 2024b) has so far been challenging, however. The interaction of the different material types with the long-term effects of the seasonal changes may be required to fully explain 67P’s outgassing. Overall then, cometary activity is still not fully understood, but the work of this ISSI team has improved our knowledge of cometary material and its response to solar heating. Several telescope proposals have also been submitted to gather more data.