Constraints on the ejecting-crust activity model on comet 67P/Churyumov-Gerasimenko

Abstract: Reproducing the observed activity of comets with thermophysical models remains a primary challenge of cometary science. We use a pebble-based thermophysical model of gas-pressure build-up in the subsurface to reproduce the global emission rates of dust, water, CO${2}$, and CO observed by Rosetta at comet 67P/Churyumov-Gerasimenko (hereafter 67P). For sufficiently low diffusivities, the low tensile strength is overcome, leading to the ejection of $\sim$ millimetre- to decimetre-sized dust-particles as well as roughly the correct outgassing rates. All the ejections, and thus the bulk of the outgassing, come from the southern hemisphere during the time that it is strongly illuminated at perihelion. This leads to a 'blow-off' of the dust-crust that otherwise forms: volatiles are much closer to the surface in the south (within the top centimetre) than in the north (10-or-more cm deep), naturally explaining the strong southern water-outgassing expected from 67P's non-gravitational accelerations and torques. We find that low gas-diffusivity, as well as large heat-capacity and steeply decreasing tensile strength with depth or ice-content, are in best agreement with the outgassing data. However, even in these cases, we struggle not to exceed the observed emission rates of dust, CO${2}$, and CO. In the south, it is difficult for models to achieve a balance between triggering activity and generating too much of it (with CO$_{2}$ the critical driving-species here); while in the north, it remains challenging to generate activity at all. Strong constraints are placed on the nature of the activity mechanism by the location of dust-ejection and erosion.