Catastrophic disruptions as the origin of bilobate comets

Several comets observed at close range have bilobate shapes¹, including comet 67P/Churyumov-Gerasimenko (67P/C-G), which was imaged by the European Space Agency's Rosetta mission^2,3. Bilobate comets are thought to be primordial because they are rich in supervolatiles (for example, N₂ and CO) and have a low bulk density, which implies that their formation requires a very low-speed accretion of two bodies. However, slow accretion does not only occur during the primordial phase of the Solar System; it can also occur at later epochs as part of the reaccumulation process resulting from the collisional disruption of a larger body⁴, so this cannot directly constrain the age of bilobate comets. Here, we show by numerical simulation that 67P/C-G and other elongated or bilobate comets can be formed in the wake of catastrophic collisional disruptions of larger bodies while maintaining their volatiles and low density throughout the process. Since this process can occur at any epoch of our Solar System's history, from early on through to the present day⁵, there is no need for these objects to be formed primordially. These findings indicate that observed prominent geological features, such as pits and stratified surface layers^4,5, may not be primordial.