The pristine interior of comet 67P revealed by the combined Aswan outburst and cliff collapse

Abstract

Outbursts occur commonly on comets1 with different frequencies and scales2,3. Despite multiple observations suggesting various triggering processes4,5, the driving mechanism of such outbursts is still poorly understood. Landslides have been invoked6 to explain some outbursts on comet 103P/Hartley 2, although the process required a pre-existing dust layer on the verge of failure. The Rosetta mission observed several outbursts from its target comet 67P/Churyumov–Gerasimenko, which were attributed to dust generated by the crumbling of materials from collapsing cliffs7,8. However, none of the aforementioned works included definitive evidence that landslides occur on comets. Amongst the many features observed by Rosetta on the nucleus of the comet, one peculiar fracture, 70 m long and 1 m wide, was identified on images obtained in September 2014 at the edge of a cliff named Aswan9. On 10 July 2015, the Rosetta Navigation Camera captured a large plume of dust that could be traced back to an area encompassing the Aswan escarpment7. Five days later, the OSIRIS camera observed a fresh, sharp and bright edge on the Aswan cliff. Here we report the first unambiguous link between an outburst and a cliff collapse on a comet. We establish a new dust-plume formation mechanism that does not necessarily require the breakup of pressurized crust or the presence of supervolatile material, as suggested by previous studies7. Moreover, the collapse revealed the fresh icy interior of the comet, which is characterized by an albedo >0.4, and provided the opportunity to study how the crumbling wall settled down to form a new talus.

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Figure 1: The Aswan cliff outburst.
Figure 2: The Aswan cliff pre- and post-collapse.
Figure 3: The spectrophotometric analysis after the cliff-collapse.
Figure 4: The thermophysical analysis on the Aswan cliff and plateau.
Figure 5: The talus boulder analysis during pre- and post-collapse.

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Acknowledgements

We thank M. Delbo for comments that led to substantial improvement of the paper. 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 Tecnica Aeroespacial, Madrid, Spain, the Universidad Politechnica 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), Italy (ASI), France (CNES), 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. M.P. was supported for this research by an appointment to the NASA Postdoctoral Program at the Ames Research Center administered by Universities Space Research Association (USRA) through a contract with NASA. M.F.A. acknowledges NASA funding through Jet Propulsion Laboratory contract no. 1267923 and from the Akademie der Wissenschaften zu Göttingen. W.-H.I acknowledges the Ministry of Science and Technology, Taiwan (grant no. NSC 102-2112-M-008) and Macau University of Science and Technology (grant no. FDCT 017/2014/A1).

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M.P. conceived, led and designed the study, analysed the cliff setting before and after the collapse, contributed to the spectrophotometric study, made the overall boulder size–frequency analysis and wrote the main text and Methods; S.H. carried out the thermophysical analysis and wrote part of the main text and Methods; J.B.V. performed the outburst analysis and wrote part of the main text; N.O. carried out the 6 August 2016 post-collapse spectrophotometric analysis, wrote part of the main text and Methods; F.S. and F.P. were responsible for the stereo-photoclinometric model and the 3D reconstruction of the pre- and post-collapse cases, and wrote part of the main text and Methods; S.M. contributed to the thermophysical analysis and made the illumination conditions video of 10 July 2015; G.N. contributed to designing the study and data interpretation; S.F carried out the 19 July 2015 spectrophotometric analysis, wrote part of the main text and Methods; S.L. contributed to the thermophysical analysis and wrote part of the main text; C.F. and P.H.H. contributed to the spectrophotometric study; C.G. and C.T. contributed to the data interpretation and made the Aswan observations possible; 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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Correspondence to M. Pajola.

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Supplementary information

Supplementary Information

Supplementary Figures 1–7, Supplementary Tables 1–2 and Supplementary Video 1 caption. (PDF 2107 kb)

Supplementary Video 1

Video representation of the illumination conditions at the Aswan cliff and plateau on 10 July 2015. (MP4 10311 kb)

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Pajola, M., Höfner, S., Vincent, J. et al. The pristine interior of comet 67P revealed by the combined Aswan outburst and cliff collapse. Nat Astron 1, 0092 (2017). https://doi.org/10.1038/s41550-017-0092

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