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.
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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.
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Additional information
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 figures and tables
Extended Data Figure 1 Seth region.
a, b, General view (a) and interpreted image (b) with the main terraces (pale green), and strata margins (red dashed lines) of the Seth region. c, Stereographic projection (north-polar aspect) of the main lobe, displaying heights derived from a stereo-photogrammetric shape model23. Colour scale indicates height above a sphere of radius 1.5 km centred in the main lobe. Terraces are aligned along different levels of the same elevation. d, NAC view of landing site candidate A (see location in panels a–c). Terraces in the upper wall of site A (small white arrows) together with parallel lineaments (large white arrows), define a continuous stratification. Yellow arrows indicate other terraces of the Seth region.
Extended Data Figure 2 Imhotep regions and surroundings.
a, NAC overview of the Ash, Apis, Khepry and Imhotep regions; in the background are regions of the head. The red dashed line separates the head from the main body. The white square is the location of c. The eye symbol shows the view of d. b, Map of the terrace, cuesta and mesa margins (dashed red lines) showing the general attitudes of strata in the main body. c, Close view of the stratification within Ash, where thinner strata are only a few metres thick. d, Imhotep terrace margins and inner stratification. Strata of site B dip underneath smooth deposits, showing their exhumed nature. e, Metre-to-centimetre scale strata heads within Imhotep.
Extended Data Figure 3 The Babi and Aten regions.
a, NAC overview of Babi region and surroundings. White squares are the locations of b and d. b, Detailed NAC view of Babi stratification: white arrows indicate small terraces and strata. c, The Babi–Aten boundary is underlined by staircase terraces, indicated by black arrows. d, Detailed view of stratification in Aker: white arrows show strata cut by fractures.
Extended Data Figure 4 The Hatmehit region.
a, NAC mosaic of the Hatmehit region and surroundings. The red dashed lines show the cuesta-like margins in Ma’at and Maftet. White arrows indicate the terrace step within Hatmehit. White squares are the locations of b and c. The apparently centripetal morphologies all around Hatmehit are due to stratified scarps, highlighted by the red dashed lines in b and c. The terrace step within Hatmehit (white arrows in d and e) and the terraces at its margin (black arrows in e) are visible in the stereo-photoclinometry Digital Terrain Model (d) and the related perspective view (e).
Extended Data Figure 5 Standard deviations of best-fitting planes.
Distribution of the ratios between the standard deviations of the 3D coordinates in the local reference system (where the z axis is normal to the best-fitting plane). N is the number of best fitting planes; σz is the standard deviation along the z axis of the points used to retrieve a given plane; σx and σy are the standard deviations along the other two perpendicular axes. The mean ratio is 0.0574, which corresponds to a maximum error on the dip angle and dip direction of the best-fitting planes lower than 2°.
Extended Data Figure 6 3D views of the nucleus and strata of comet 67P/Churyumov–Gerasimenko.
On the left side are four different 3D views of the nucleus of comet 67P/Churyumov–Gerasimenko with best-fitting planes derived from the stratification. On the right side are the best-fitting planes alone. Each plane indicates the orientation of strata at that specific location (corresponding to the centre of the drawn plane) on the comet nucleus. Note how the two lobes show independent envelopes. The colour scale indicates the angular deviation between the plane vector and the local gravity vector (calculated for the whole body, assuming uniform density).
Extended Data Figure 7 Eastern sector cross-sectional slices through comet 67P/Churyumov–Gerasimenko along its major axis (longitudinal sections).
On the left are perspective views of the comet nucleus showing the different cross-sectional slices. In red are shown the best-fitting planes projected into the geological section; in blue all the other best-fitting planes. On the right are geological sections. In blue are shown the best-fitting planes; green arrows are vectors perpendicular to each best-fitting plane; yellow lines show the field of lines used for drawing strata within the comet nucleus (strata are perpendicular to the yellow lines); dashed black lines are the strata departing from measurements at the surface of each longitudinal section. The traces of transversal sections A, B, C and D and the locations of some regions are also reported.
Extended Data Figure 8 Western sector cross-sectional slices through the comet along its major axis (longitudinal sections).
On the left are perspective views of the comet nucleus showing the different cross-sectional slices. In red are shown the best-fitting planes projected into the geological section, in blue is shown all the other best-fitting planes. On the right are geological sections. In blue are shown the best-fitting planes; green arrows are vectors perpendicular to each best-fitting plane; yellow lines show the field of lines used for drawing strata within the comet nucleus (strata are perpendicular to the yellow lines); dashed black lines are strata departing from measurements at the surface of each longitudinal section; dashed grey lines are inferred strata. The traces of transversal sections A, B, C and D and the locations of some regions are also reported.
Extended Data Figure 9 Cross-sectional slices through the comet perpendicular to the major axis (transversal sections).
On the left are perspective views of the comet nucleus showing the different cross-sectional slices. In red are shown the best-fitting planes projected into the geological section; in blue are shown all the other best-fitting planes. On the right are geological sections. In blue are shown the best-fitting planes; green arrows are vectors perpendicular to each best-fitting plane; yellow lines show the field of lines used for drawing strata within the comet nucleus (strata are perpendicular to the yellow lines); dashed black lines are strata departing from measurements at the surface of each section and from the intercept of strata of the longitudinal sections; dashed grey lines are inferred strata. The traces of longitudinal sections 1, 2, 3, 4, 5 and 6 and the locations of some regions are also reported.
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Massironi, M., Simioni, E., Marzari, F. et al. Two independent and primitive envelopes of the bilobate nucleus of comet 67P. Nature 526, 402–405 (2015). https://doi.org/10.1038/nature15511
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DOI: https://doi.org/10.1038/nature15511
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