The diverse shapes of binary asteroid satellites born from sub-escape-velocity moonlet mergers

Context. Recent direct observations of atypically shaped rubble-pile satellites of sub-kilometre asteroids in the form of the spherically oblate Dimorphos and bilobate Selam challenge classical binary asteroid formation theories, which only explain the predominantly elongated population. Aims. This study further explores a rubble-pile satellite formation scenario for binary asteroid systems involving debris discs by investigating how mergers between moonlets with impact velocities below the mutual escape speed (sub-escape-velocity mergers) and tidal disruptions can create atypically shaped moons. Methods. We simulated sub-escape-velocity mergers between moonlets and studied the resulting structural evolution of the formed moon in a tidal environment using the polyhedral discrete elements method N-body code GRAINS. Results. Firstly, we find that the shapes of rubble-pile moons formed by mergers in this regime are highly dependent on the shape and initial orientation of the involved moonlets. This can be explained by the moonlets largely retaining their individual structures during the impact. Secondly, we observed that mass loss via tidal disruption for a bilobate object occurs in discrete regimes of distance to the primary. Closer to the primary, the innermost lobe is completely stripped off, while only a small piece of it is lost when further out. Conclusions. Due to moonlets largely retaining their shape after undergoing a sub-escape-velocity merger, it is necessary to account for their non-sphericity to accurately model satellite formation in circumasteroidal debris discs. Moreover, the reshaping of merged objects via tidal disruption and distortion can produce oblate spheroid moons such as Dimorphos and highly elongated bilobate satellites with distinct necks such as Selam.