Localized ejection of dust and chunks on comet 67P/Churyumov–Gerasimenko: testing how comets work.

We extend an existing thermophysical activity model of comet 67P/Churyumov–Gerasimenko to include pressure build-up inside the pebbles making up the nucleus. We test various quantities of H |$_{2}$| O and CO |$_{2}$|⁠ , in order to simulate the material inside and outside of proposed water enriched bodies (WEBs). We find that WEBs can reproduce the peak water flux observed by Rosetta , but that the addition of a time-resolved heat-flow reduces the water fluxes away from perihelion as compared to the previously assumed equilibrium model. Our modelled WEBs eject dust continuously but with a rate that is much higher than the observed erosion and mass-loss, thus requiring an active area smaller than the total comet surface area or very large quantities of dust fallback. When simulating the CO |$_{2}$| -rich non-WEB material, we only find the ejection of large chunks under specific conditions (e.g. low diffusivities between the pebbles or intense insolation at southern summer), while we also find CO |$_{2}$| outgassing rates that are much greater than observed. This is a general problem in models where CO |$_{2}$| drives erosion, alongside difficulties in simultaneously ejecting chunks from deep while eroding the surface layer. We therefore conclude that ejection of chunks by CO |$_{2}$| must be a localized phenomenon, occurring separately in space or time from surface erosion and water emission. Simulating the global production rates of gas, dust, and chunks from a comet thus remains challenging, while the activity mechanism is shown to be very sensitive to the material structure (i.e. porosity and diffusivity) at various scales. [ABSTRACT FROM AUTHOR]