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 mission2,3. 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 layers4,5, may not be primordial.
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S.R.S. and P.M. acknowledge support from the Centre National d’Études Spatiales, as well as the Academies of Excellence on Complex Systems and Space, Environment, Risk and Resilience of the Initiative d’EXcellence ‘Joint, Excellent, and Dynamic Initiative’ (IDEX JEDI) of the Université Côte d’Azur. M.J. acknowledges support from the Swiss National Centre of Competence in Research PlanetS, and S.M. acknowledges support from the Jet Propulsion Laboratory. Computation was performed using the YORP computing cluster run by the Center for Theory and Computation at the University of Maryland’s Department of Astronomy and the Mésocentre de Calcul Intensif ‘Simulations Intensives en Géophysique, Astronomie, Mécanique, et Mathématiques’ hosted by the Côte d’Azur Observatory in Nice. For data visualization, the authors made use of the freeware, multi-platform, ray-tracing package, Persistence of Vision Raytracer.
Supplementary Figures 1–4, captions for Supplementary Videos 1–4.
Animation showing the formation of the largest remnant after a 150 m s–1 catastrophic impact (18° friction angle).
Animation showing the formation of the largest remnant after a 150 m s–1 catastrophic impact (29° friction angle).
One example of gravitational reaccumulation of marginally bound impact fragments.
One example of the formation of a bilobate body from a catastrophic disruption simulation.