Ejection and dynamics of aggregates in the coma of comet 67P/Churyumov-Gerasimenko

Context . The process of gas-driven ejection of refractory materials from cometary surfaces continues to pose a challenging question in cometary science. The activity modeling of comet 67P/Churyumov-Gerasimenko, based on data from the Rosetta mission, has significantly enhanced our comprehension of cometary activity. But thermophysical models have difficulties in simultaneously explaining the production rates of various gas species and dust. It has been suggested that different gas species might be responsible for the ejection of refractory material in distinct size ranges. Aims . This work focuses on investigating the abundance and the ejection mechanisms of large aggregates (≳1 cm) from the comet nucleus. We aim to determine their properties and map the distribution of their source regions across the comet surface. This can place constraints on activity models for comets. Methods . We examined 189 images acquired at five epochs by the OSIRIS/NAC instrument on board the Rosetta spacecraft. Our goal was to identify bright tracks produced by individual aggregates as they traversed the camera field of view. In parallel, we generated synthetic images based on the output of dynamical simulations involving various types of aggregates. By comparing these synthetic images with the observations, we determined the characteristics of the simulated aggregates that most closely resemble the observations. Results . We have identified over 30 000 tracks present in the OSIRIS images, derived constraints on the characteristics of the aggregates, and mapped their origins on the nucleus surface. The aggregates have an average radius of ≃5 cm and a bulk density consistent with that of the comet’s nucleus. Due to their size, gas drag exerts only a minor influence on their dynamical behavior, so an initial velocity is needed to bring them into the camera field of view. The source regions of these aggregates are predominantly located near the boundaries of distinct terrains on the surface.