Abstract

The factors shaping cometary nuclei are still largely unknown, but could be the result of concurrent effects of evolutionary1,2 and primordial processes3,4. The peculiar bilobed shape of comet 67P/Churyumov–Gerasimenko may be the result of the fusion of two objects that were once separate or the result of a localized excavation by outgassing at the interface between the two lobes5. Here we report that the comet’s major lobe is enveloped by a nearly continuous set of strata, up to 650 metres thick, which are independent of an analogous stratified envelope on the minor lobe. Gravity vectors computed for the two lobes separately are closer to perpendicular to the strata than those calculated for the entire nucleus and adjacent to the neck separating the two lobes. Therefore comet 67P/Churyumov–Gerasimenko is an accreted body of two distinct objects with ‘onion-like’ stratification, which formed before they merged. We conclude that gentle, low-velocity collisions occurred between two fully formed kilometre-sized cometesimals in the early stages of the Solar System. The notable structural similarities between the two lobes of comet 67P/Churyumov–Gerasimenko indicate that the early-forming cometesimals experienced similar primordial stratified accretion, even though they formed independently.

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Acknowledgements

OSIRIS was built by a consortium of the Max-Planck-Institut für Sonnensystemforschung (in Göttingen, Germany), CISAS-University of Padova (Italy), the Laboratoire d’Astrophysique de Marseille (France), the Instituto de Astrofísica de Andalucia, CSIC (Granada, Spain), the Research and Scientific Support Department of the European Space Agency (Noordwijk, The Netherlands), the Instituto Nacional de Técnica Aeroespacial (Madrid, Spain), the Universidad Politćhnica de Madrid (Spain), the Department of Physics and Astronomy of Uppsala University (Sweden), and the Institut für Datentechnik und Kommunikationsnetze der Technischen Universität Braunschweig (Germany). The support of the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical Directorate is gratefully acknowledged. We thank the ESA teams at ESAC, ESOC and ESTEC for their work in support of the Rosetta mission.

Author information

Affiliations

  1. Dipartimento di Geoscienze, University of Padova, via G. Gradenigo 6, 35131 Padova, Italy

    • Matteo Massironi
    •  & Lorenza Giacomini
  2. Centro di Ateneo di Studi ed Attività Spaziali “Giuseppe Colombo” (CISAS), University of Padova, via Venezia 15, 35131 Padova, Italy

    • Matteo Massironi
    • , Maurizio Pajola
    • , Giampiero Naletto
    • , Ivano Bertini
    •  & Francesca Ferri
  3. CNR-IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy

    • Emanuele Simioni
    • , Giampiero Naletto
    •  & Vania Da Deppo
  4. University of Padova, Department of Physics and Astronomy, Vicolo dell’Osservatorio 3, 35122 Padova, Italy

    • Francesco Marzari
    • , Cesare Barbieri
    • , Fiorangela La Forgia
    • , Monica Lazzarin
    •  & Sara Magrin
  5. INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy

    • Gabriele Cremonese
  6. Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 38 rue Frédéric Joliot-Curie, 13388 Marseille, France

    • Laurent Jorda
    • , Philippe Lamy
    • , Anne-Thérèse Auger
    • , Claire Capanna
    •  & Olivier Groussin
  7. Department of Information Engineering, University of Padova, via Gradenigo 6/B, 35131 Padova, Italy

    • Giampiero Naletto
  8. The University of Kent, School of Physical Sciences, Canterbury, Kent CT2 7NZ, UK

    • Stephen Lowry
  9. Physikalisches Institut der Universität Bern, Sidlerstraße 5, 3012 Bern, Switzerland

    • Mohamed Ramy El-Maarry
    • , Antoine Pommerol
    •  & Nicolas Thomas
  10. Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Planetenforschung, Rutherfordstraße 2, 12489 Berlin, Germany

    • Frank Preusker
    • , Frank Scholten
    • , Stubbe F. Hviid
    • , Jörg Knollenberg
    • , Ekkehard Kührt
    •  & Stefano Mottola
  11. Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig Weg 3, 37077 Göttingen, Germany

    • Holger Sierks
    • , Jessica Agarwal
    • , Carsten Güttler
    • , Stubbe F. Hviid
    • , Gabor Kovacs
    • , Rainer Kramm
    • , Nilda Oklay
    • , Cecilia Tubiana
    •  & Jean-Baptiste Vincent
  12. Centro de Astrobiologia, CSIC-INTA, 28850 Torrejon de Ardoz, Madrid, Spain

    • Rafael Rodrigo
  13. International Space Science Institute, Hallerstraße 6, 3012 Bern, Switzerland

    • Rafael Rodrigo
  14. Scientific Support Office, European Space Research and Technology Centre/ESA, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk ZH, The Netherlands

    • Detlef Koschny
    •  & Sebastien Besse
  15. Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden

    • Hans Rickman
  16. PAS Space Research Center, Bartycka 18A, 00716 Warszawa, Poland

    • Hans Rickman
  17. Institut für Geophysik und extraterrestrische Physik (IGEP), Technische Universität Braunschweig, Mendelssohnstraße 3, 38106 Braunschweig, Germany

    • Horst Uwe Keller
  18. University of Maryland, Department of Astronomy, College Park, Maryland 20742-2421, USA

    • Michael F. A’Hearn
    •  & Dennis Bodewits
  19. Akademie der Wissenschaften zu Göttingen and Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig Weg 3, 37077 Göttingen, Germany

    • Michael F. A’Hearn
  20. LESIA-Observatoire de Paris, CNRS, Université Pierre et Marie Curie, Universite Paris Diderot, 5 place J. Janssen, 92195 Meudon, France

    • M. Antonella Barucci
    •  & Sonia Fornasier
  21. LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France

    • Jean-Loup Bertaux
  22. Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden

    • Björn Davidsson
  23. Department of Industrial Engineering, University of Padova, via Venezia 1, 35131 Padova, Italy

    • Stefano Debei
  24. University of Trento, Via Mesiano 77, 38100 Trento, Italy

    • Mariolino De Cecco
  25. INAF—Osservatorio Astronomico, Via Tiepolo 11, 34014 Trieste, Italy

    • Marco Fulle
  26. Planetary Science Institute, Tucson, Arizona 85719, USA

    • Robert Gaskell
  27. Instituto de Astrofisica de Andalucia (CSIC), Glorieta de la Astronomìa s/n, 18008 Granada, Spain

    • Pedro J. Gutiérrez
    • , Luisa M. Lara
    •  & Josè J. Lopez Moreno
  28. National Central University, Graduate Institute of Astronomy, 300 Chung-Da Road, Chung-Li 32054 Taiwan

    • Wing-Huen Ip
    •  & Zhong-Yi Lin
  29. Operations Department, European Space Astronomy Centre/ESA, PO Box 78, 28691 Villanueva de la Canada, Madrid, Spain

    • Michael Küppers
  30. Institut für Datentechnik und Kommunikationsnetze der TU Braunschweig, Hans-Sommer Straße 66, 38106 Braunschweig, Germany

    • Harald Michalik

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Contributions

M.M. led and designed the study, identified and mapped most of the strata, selected the areas for retrieving 3D best-fitting planes, performed the geological sections, made the overall geological interpretation and wrote most of the text; E.S. carried out the 3D reconstruction of strata attitudes and wrote part of the main text and Methods; F.M. obtained the gravity-field vectors, wrote part of the Methods and contributed to data interpretation; G.C. contributed to designing the study and data interpretation, L.G. performed the detailed geomorphological analysis of the Hatmehit region; M.P. contributed to the geomorphological analysis of the Seth region and the landing-site candidate A; L.J. was responsible for the stereo-photoclinometric model and the 3D reconstruction of the two lobes as independent objects; G.N. and S.L. substantially contributed to data interpretation and defining the related implications; F.P. and F.S. were responsible for the stereo-photogrammetric shape model; M.R.E.-M. defined the physiographic regions of the comet. H.S., C.B., P.L., R.R., D.K. and H.R. are the lead scientists of the OSIRIS project. The other authors are all co-investigators who built and ran this instrument and made the observations possible, and associates and assistants who participated in the study.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Matteo Massironi.

All data presented in this paper will be delivered to ESA’s Planetary Science Archive (http://www.rssd.esa.int/index.php?project=PSA&page=rosetta) and NASA’s Planetary Data System (https://pds.nasa.gov/) in accordance with the schedule established by the Rosetta project.

Extended data

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    Supplementary Information

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