Letter | Published:

Catastrophic disruptions as the origin of bilobate comets

Nature Astronomyvolume 2pages379382 (2018) | Download Citation


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.

Author information


  1. Université Côte d’Azur, Observatoire de la Côte d’Azur, Centre National de la Recherche Scientifique, Laboratoire Lagrange, Nice, France

    • Stephen R. Schwartz
    •  & Patrick Michel
  2. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

    • Stephen R. Schwartz
  3. Physics Institute, University of Bern, National Centre of Competence in Research PlanetS, Bern, Switzerland

    • Martin Jutzi
  4. Southwest Research Institute, Boulder, CO, USA

    • Simone Marchi
  5. School of Aerospace Engineering, Tsinghua University, Beijing, China

    • Yun Zhang
  6. Department of Astronomy, University of Maryland, College Park, MD, USA

    • Yun Zhang
    •  & Derek C. Richardson


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S.R.S. performed the soft-sphere/N-body numerical simulations, analysed the numerical results and led the research. M.J. performed the SPH numerical simulations and contributed to the analyses. P.M. initiated the collaboration between the institutions to investigate catastrophic impacts as an origin story, and helped steer the study to address questions in the context of Solar System origins. S.M. provided essential and detailed context for the Rosetta findings and outstanding questions regarding comet 67P/G–C. Y.Z. analysed the friction angles and implemented the rolling friction subroutine used. D.C.R. guided the development of all N-body-related code. All authors contributed to interpretation of the results and preparation of the manuscript.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to Stephen R. Schwartz.

Supplementary information

  1. Supplementary Information

    Supplementary Figures 1–4, captions for Supplementary Videos 1–4.


  1. Supplementary Video 1

    Animation showing the formation of the largest remnant after a 150 m s–1 catastrophic impact (18° friction angle).

  2. Supplementary Video 2

    Animation showing the formation of the largest remnant after a 150 m s–1 catastrophic impact (29° friction angle).

  3. Supplementary Video 3

    One example of gravitational reaccumulation of marginally bound impact fragments.

  4. Supplementary Video 4

    One example of the formation of a bilobate body from a catastrophic disruption simulation.

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