A quantitative description of comet 67P’s dust and gas production remains enigmatic

Abstract

ABSTRACT The mechanism of dust emission from a cometary nucleus is still an open question and thermophysical models have problems reproducing outgassing and dust productions rates simultaneously. In this study, we investigate the capabilities of a rather simple thermophysical model to match observations from Rosetta instruments at comet 67P/Churyumov–Gerasimenko and the influence of model variations. We assume a macroporous surface structure composed of pebbles and investigate the influence of different model assumptions. Besides the scenario in which dust layers are ejected when the vapour pressure overcomes the tensile strength, we use artificial ejection mechanisms, depending on ice depletion of layers. We find that dust activity following the pressure criterion is only possible for reduced tensile strength values or reduced gas diffusivity and is inconsistent with observed outgassing rates, because activity is driven by CO2. Only when we assume that dust activity is triggered when the layer is completely depleted in H2O, the ratio of CO2 to H2O outgassing rates is in the expected order of magnitude. However, the dust-to-H2O ratio is never reproduced. Only with decreased gas diffusivity, the slope of the H2O outgassing rate is matched, however absolute values are too low. To investigate maximum reachable pressures, we adapted our model equivalent to a gas-impermeable dust structure. Here, pressures exceeding the tensile strength by orders of magnitude are possible. Maximum activity distances of $3.1$, $8.2$, and $74 \, \mathrm{au}$ were estimated for H2O-, CO2-, and CO-driven activity of $1\hbox{-}\mathrm{cm}$-sized dust, respectively. In conclusion, the mechanism behind dust emission remains unclear.