Is near-surface ice the driver of dust activity on 67P/Churyumov-Gerasimenko

Context. Mainly for historical reasons, nearly all of the current thermophysical models of dust activity rely on the poorly justified assumption of cohesionless dust lifted by a gas drag force against the weak nucleus gravity. The interpretation of Rosetta data and our understanding of comet activity is particularly sensitive to this assumption. Aims. We investigate the role that cohesion forces among the dust grains play in the evolution of temperature and pressure at the ice-dust interface and the resulting dust activity (lifting). Methods. We used a 1D thermophysical numerical model that provides a realistic description of cohesion forces among dust aggregates. Several conditions of solar illumination on the nucleus are investigated for the H2O, CO, and CO2 ices below the dust layer. We examine a wide range of dust grain sizes. Results. The simulations confirm an increase in temperature and pressure at the ice boundary between the two model layers with respect to exposed pure ice. Furthermore, we show that a non-monotonic behavior of temperature and pressure versus layer thickness is expected at the ice-dust interface for fine aggregates (of sizes <= 30 mu m), but not for the larger grains. The ratio of vapor pressure to the physically determined tensile strength for various agglomerate sizes and layer thicknesses provides further evidence that the gas drag is not sufficient to remove dust grains of sizes < 1 mm, which is a result of taking cohesion forces among the particles into account. Conclusions. In the framework of the presented model, which can be considered common in terms of assumptions and physical parameters in the cometary community, the dust removal by a gas drag force is not a plausible physical mechanism. The sublimation of not only water ice, but also of super-volatile ice (i.e., CO) is unable to remove dust grains for illumination conditions corresponding to 1.3 AU. A way out of this impasse requires revision of the most common model assumption employed by the cometary community.