The release of micron-sized dust grains from evaporating micro-granular dust-ice mixtures Applications to cometary activity and protoplanetary disks

Context. When comets approach the Sun, they release micrometer-sized solid particles, which thereafter form the dust tail. The cohesion among grains with these sizes is typically on the order of 1 kPa. Typical gas pressures of the sublimating volatiles are <1 Pa so that a model describing the release of the grains has to overcome this cohesion bottleneck. The cohesion bottleneck does, however, not exist for centimeter-sized dust aggregates ("pebbles"), which might be the building blocks of comets and whose cohesion typically is <1 Pa, due to their reduced contact area. Aims. We propose a new, purely geometrical model that reduces the cohesion between micron-sized grains to zero by evaporating the volatile ice that either covers the surfaces of all grains or exists as solid ice particles between refractory grains. Methods. We used computer simulations of micro-granular particle clusters with power-law size-frequency distributions of the monomer grains and determined how the evaporation affects the formation of isolated particles or particle clusters. Results. Micro-granular assemblages of core-mantle particles can emit single dust grains or small clusters of grains for virtually all dust-to-ice volume ratios. In contrast, intimate mixtures of dust and ice grains of similar size can only become dust-active if the dust-to-ice volume ratio does not exceed 60:40. Conclusions. Our model differs from previous ones that rely on tensile strength and gas pressure mechanisms insofar as it provides cohesion-free dust emission caused by ice evaporation and new insights into cometary dust emission. It predicts that the bulk of the dust grains in comets cannot be of core-mantle type and that the Water-ice-Enriched Blocks observed on comet 67P/Churyumov-Gerasimenko, with their dust-to-ice mass ratio of similar to 2, should be the primary source of small particles emitted from the nucleus. Additionally, our model has potential applications in studying the evolution of icy pebbles as they cross the snowline in protoplanetary disks.