Abstract

Comets are thought to preserve almost pristine dust particles, thus providing a unique sample of the properties of the early solar nebula. The microscopic properties of this dust played a key part in particle aggregation during the formation of the Solar System1,2. Cometary dust was previously considered to comprise irregular, fluffy agglomerates on the basis of interpretations of remote observations in the visible and infrared3,4,5,6 and the study of chondritic porous interplanetary dust particles7 that were thought, but not proved, to originate in comets. Although the dust returned by an earlier mission8 has provided detailed mineralogy of particles from comet 81P/Wild, the fine-grained aggregate component was strongly modified during collection9. Here we report in situ measurements of dust particles at comet 67P/Churyumov–Gerasimenko. The particles are aggregates of smaller, elongated grains, with structures at distinct sizes indicating hierarchical aggregation. Topographic images of selected dust particles with sizes of one micrometre to a few tens of micrometres show a variety of morphologies, including compact single grains and large porous aggregate particles, similar to chondritic porous interplanetary dust particles. The measured grain elongations are similar to the value inferred for interstellar dust and support the idea that such grains could represent a fraction of the building blocks of comets. In the subsequent growth phase, hierarchical agglomeration could be a dominant process10 and would produce aggregates that stick more easily at higher masses and velocities than homogeneous dust particles11. The presence of hierarchical dust aggregates in the near-surface of the nucleus of comet 67P also provides a mechanism for lowering the tensile strength of the dust layer and aiding dust release12.

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Acknowledgements

Rosetta is an ESA mission with contributions from its member states and NASA. We also thank the Rosetta Science Ground Segment and Mission Operations Centre for their support in acquiring the presented data. MIDAS became possible through support from funding agencies including the European Space Agency’s PRODEX programme, the Austrian Space Agency, the Austrian Academy of Sciences and the German funding agency DARA (later DLR). A.-C.L.-R. acknowledges support from the French Space Agency, CNES. M.S.B. and T.M. acknowledge funding from the Austrian Science Fund (FWF): P 28100-N36. T.M. also acknowledges the Steiermärkische Sparkasse and the Karl-Franzens Universität Graz for their financial support. P.E. acknowledges support from the NASA Astrobiology Institute. R.S. thanks F. Hofer and H. Plank for discussions and the Austrian Research Promotion Agency (FFG) for financial support. All data presented here will be made available in the ESA Planetary Science Archive (http://www.cosmos.esa.int/web/psa/rosetta).

Author information

Author notes

    • Mark S. Bentley
    • , Roland Schmied
    •  & Thurid Mannel

    These authors contributed equally to this work.

Affiliations

  1. Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria

    • Mark S. Bentley
    • , Roland Schmied
    • , Thurid Mannel
    • , Klaus Torkar
    •  & Harald Jeszenszky
  2. Physics Institute, University of Graz, Universitätsplatz 5, 8010 Graz, Austria

    • Thurid Mannel
  3. European Space Research and Technology Centre, Future Missions Office (SREF), Noordwijk, The Netherlands

    • Jens Romstedt
  4. UPMC (Sorbonne Université); CNRS/INSU; LATMOS-IPSL, BC 102, 4 place Jussieu, 75005 Paris, France

    • Anny-Chantal Levasseur-Regourd
  5. Institut für Planetologie, Universität Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany

    • Iris Weber
    •  & Elmar K. Jessberger
  6. Leiden Observatory, Postbus 9513, 2300 RA Leiden, The Netherlands

    • Pascale Ehrenfreund
  7. Space Policy Institute, George Washington University, 20052 Washington DC, USA

    • Pascale Ehrenfreund
  8. Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria

    • Christian Koeberl
  9. Natural History Museum, Burgring 7, 1010 Vienna, Austria

    • Christian Koeberl
  10. Department of Physics and Technology, UiT The Arctic University of Norway, N-9037 Tromsø, Norway

    • Ove Havnes

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Contributions

R.S., T.M. and M.S.B. planned the experiments on MIDAS, analysed and interpreted the data and wrote the manuscript. M.S.B. developed the planning and data processing pipelines. R.S. and T.M. implemented the elongation calculations. R.S. performed the post-processing and calibration as well as the particle/grain measurement and is responsible for the graphical data presentation. T.M. considered the uncertainties for all data. A.-C.L.-R. provided information on cometary dust derived from polarimetric observations and its interpretation. H.J. supported the experiments with software updates. All authors discussed the results and commented on the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Mark S. Bentley.

Reviewer Information Nature thanks M. Fulle and L. Kolokolova for their contribution to the peer review of this work.

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DOI

https://doi.org/10.1038/nature19091

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