Formation of bi-lobed shapes by sub-catastrophic collisions - A late origin of comet 67P’s structure

Context. The origin of the particular shape of comet 67P/Churyumov-Gerasimenko (67P) is a topic of active research. How and when it acquired its peculiar characteristics has distinct implications on the origin of the solar system and its dynamics. Aims. We investigate how shapes such as that of comet 67P can result from a new type of low-energy, sub-catastrophic impact involving elongated, rotating bodies. We focus on parameters potentially leading to bi-lobed structures. We also estimate the probability of such structures surviving subsequent impacts. Methods. We used a smooth particle hydrodynamics (SPH) shock physics code to model the impacts, the subsequent re-accumulation of material and the reconfiguration into a stable final shape. The energy increase as well as the degree of compaction of the resulting bodies were tracked in the simulations. Results. Our modelling results suggest that the formation of bi-lobed structures like 67P is a natural outcome of the low-energy, sub-catastrophic collisions considered here. Conclusions. Sub-catastrophic impacts have the potential to alter the shape of a small body significantly, without leading to major heating or compaction. The currently observed shapes of cometary nuclei, such as 67P, may be a result of such a major shape forming impact.