Modelling comet 67P/Churyumov-Gerasimenko's global production rates with an ejecting dust-crust

Simulating cometary activity by overcoming material cohesion in order to eject dust particles is currently challenging for numerical models. Here we will present the results of a pebble-based 1D thermophsical model running across a number of points on the surface of comet 67P/Churyumov-Gerasimenko (hereafter 67P). We find that, for sufficiently low diffusivities,  the cohesion can be overcome by sublimating gas pressure build-up in the subsurface, leading to the ejection of the dust crust and resulting in global emission rates of dust, water, CO₂, and CO that roughly match those observed by the Rosetta mission. Ejected particles are of millimetre- to decimetre-scale, and all come from the southern hemisphere during the time that it is strongly illuminated at perihelion, leading to the 'blow-off' of the dust-crust here. Volatiles are then 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 water outgassing expected here from modelling of 67P's non-gravitational accelerations and torques. We find that a low gas-diffusivity, as well as a large heat-capacity and a steeply decreasing tensile strength with depth, are in best agreement with the outgassing data. However, the model struggles not to exceed the observed emission rates of dust, CO₂, and CO. It is difficult to achieve a balance between triggering activity and generating too much of it in the south (with CO₂ the critical driving-species here); while in the north, it remains challenging to generate activity at all. The local and global locations of dust ejection and erosion place strong constraints on the nature of the cometary activity mechanism.