Bilobate comet morphology and internal structure controlled by shear deformation

Abstract

Bilobate comets—small icy bodies with two distinct lobes—are a common configuration among comets, but the factors shaping these bodies are largely unknown. Cometary nuclei, the solid centres of comets, erode by ice sublimation when they are sufficiently close to the Sun, but the importance of a comet’s internal structure on its erosion is unclear. Here we present three-dimensional analyses of images from the Rosetta mission to illuminate the process that shaped the Jupiter-family bilobate comet 67P/Churyumov–Gerasimenko over billions of years. We show that the comet’s surface and interior exhibit shear-fracture and fault networks, on spatial scales of tens to hundreds of metres. Fractures propagate up to 500 m below the surface through a mechanically homogeneous material. Through fracture network analysis and stress modelling, we show that shear deformation generates fracture networks that control mechanical surface erosion, particularly in the strongly marked neck trough of 67P/Churyumov–Gerasimenko, exposing its interior. We conclude that shear deformation shapes and structures the surface and interior of bilobate comets, particularly in the outer Solar System where water ice sublimation is negligible.

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Fig. 1: Fracture pattern on 67P’s southern hemisphere showing fracture interpretations and comparison with typical Earth analogues or equivalents.

(a, b(i), b(ii), c, d(i), d(ii) and e(i)): ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Fig. 2: Fracture length distribution and direction statistics.
Fig. 3: Block diagram revealing 67P’s fractured internal structure and its evolution through increasing mechanical erosion.

(b): ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Fig. 4: Chronology of the evolution of the shape of 67P (from a primordial or collisional later event), showing the effects of the two complementary erosion processes (mechanical erosion and sublimation erosion).

Data availability

All the images analysed during the current study are available in the ESA-PSA repository (https://archives.esac.esa.int/psa). The data that support the findings of this study are in the Supplementary Information and available from the corresponding author upon reasonable request (matonti@cerege.fr).

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Acknowledgements

OSIRIS was built by a consortium of the Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany; the CISAS University of Padova, Italy; the Laboratoire d’Astrophysique de Marseille, France; the Instituto de Astrofisica de Andalucia, CSIC, Granada, Spain; the Research and Scientific Support Department of the ESA, Noordwijk, Netherlands; the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain; the Universidad Politéchnica 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 Rosetta Science Operations Centre and the Rosetta Mission Operations Centre for the successful rendezvous with comet 67 P/Churyumov–Gerasimenko. We also thank Emerson E&P Software, Emerson Automation Solutions, for providing SKUA-GOCAD licenses in the scope of the Emerson Grant Program, V. A. La Bruna for the interesting Structural Geology discussions related to this study, and Y. Guglielmi for his advices on the submission.

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C.M. led this study, mapped the lineaments, performed geological interpretation and wrote most of the manuscript. N.A. performed the 3D projection of the lineaments as well as the statistical calculations and interpretations, and participated in the manuscript writing. O.G. contributed significantly to the interpretations and to the manuscript writing. L.J. provided local and global 3D models and developed tool for images selection and data projection. S.V. contributed to the 3D statistical analysis and data importing to the Gocad software. S.F.H. provided the 3D stress model for 67P. S.B. contributed to improved design of the study, interpretations and manuscript. D.N. contributed to the local and global 3D shape model creation. A.-T.A. contributed to the image selection and geological interpretation. P.L. provided the stereo anaglyph images used for interpretation. H.S., G.N., R.R., D.K. and B.D. 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.

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Correspondence to C. Matonti.

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Matonti, C., Attree, N., Groussin, O. et al. Bilobate comet morphology and internal structure controlled by shear deformation. Nat. Geosci. 12, 157–162 (2019). https://doi.org/10.1038/s41561-019-0307-9

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