Nicholas Attree
Cometary outgassing produces a back-reaction force on a nucleus that can alter its trajectory and rotation state. Understanding this activity is key to exploring the physics of the upper layers of cometary surfaces (with implications for their formation and subsequent evolutionary history) and can be constrained by observing the orbit and rotation changes. For comet 67P/Churyumov-Gerasimenko, detailed measurements have been made by the Rosetta spacecraft and various attempts have been made to model the activity.
Here we present updated work using a previously published activity model to fit to Rosetta outgassing, trajectory, and rotation data. We test a number of different activity distributions over the surface of the comet by varying the Effective Active Fraction (EAF), relative to pure water ice, of facets on a shape model. The previous work has shown that, in order to fit the fast ramp-up and fall-off in outgassing either side of perihelion, 67P’s EAF must vary with time. We therefore investigate a number of different EAF curves to see if different parametric models can be ruled out. The objective here is to constraint the shape of the activity curve that a more advanced thermo-physical model must produce in order to fit the data. We also investigate different spatial patterns in EAF, and attempt to correlate them to physical features on the cometary surface. Here we are able, for the first time, to achieve a good fit to the Rosetta data by parameterising EAF in terms of the different geological unit types on 67P. This may have important implications for understanding how activity works on the different types of surface observed on cometary nuclei, including ‘smooth’, ‘dusty’ and ‘rocky’ surface morphologies.
Slide two shows an outline of the method, showing on the left two views of a 3D model of the comet. On one surface temperature has been computed from a thermal model and is shown mapped on the surface. On the other a map of Effective Active Fraction, i.e. how active the surface is relative to pure water ice is shown. From these two the outgassing, force, and torque can be calculated. On the right the outputs of the model are shown, compared to the Rosetta measurements in three plots. These show: water outgassing rate, Earth to comet range, and comet torque, between 300 days before and 300 days after the comet's closest approach to the sun (perihelion). The model curves fit the data relatively well. At the bottom of the slide is shown how Effective Active Fraction varies over this time period in a preliminary model.
Slide 3 shows the main results. The figure shows the Effective Active Fraction mapped over the comet that best reproduces the Rosetta data for a model divided up by geological regions on the surface (the text and references describe this in more detail). The plot on the right shows how the Effective Active Fraction varies with time in this model. This model agrees with the data better than the preliminary result in the previous slide, and with the results of the previously published paper (Attree et al. 2019).
The conclusions are that the Rosetta data can be fitted with a model where activity is dependent on geological terrain type: e.g. different types of terrain have different amounts of outgassing relative to pure water ice, and also vary in different ways with time. This is probably due to different amounts of dust present on the surface, which can reduce activity.