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Fission and reconfiguration of bilobate comets as revealed by 67P/Churyumov–Gerasimenko

Abstract

The solid, central part of a comet—its nucleus—is subject to destructive processes1,2, which cause nuclei to split at a rate of about 0.01 per year per comet3. These destructive events are due to a range of possible thermophysical effects4; however, the geophysical expressions of these effects are unknown. Separately, over two-thirds of comet nuclei that have been imaged at high resolution show bilobate shapes5, including the nucleus of comet 67P/Churyumov–Gerasimenko (67P), visited by the Rosetta spacecraft. Analysis of the Rosetta observations suggests that 67P’s components were brought together at low speed after their separate formation6. Here, we study the structure and dynamics of 67P’s nucleus. We find that sublimation torques have caused the nucleus to spin up in the past to form the large cracks observed on its neck. However, the chaotic evolution of its spin state has so far forestalled its splitting, although it should eventually reach a rapid enough spin rate to do so. Once this occurs, the separated components will be unable to escape each other; they will orbit each other for a time, ultimately undergoing a low-speed merger that will result in a new bilobate configuration. The components of four other imaged bilobate nuclei have volume ratios that are consistent with a similar reconfiguration cycle, pointing to such cycles as a fundamental process in the evolution of short-period comet nuclei. It has been shown7,8 that comets were not strong contributors to the so-called late heavy bombardment about 4 billion years ago. The reconfiguration process suggested here would preferentially decimate comet nuclei during migration to the inner solar system, perhaps explaining this lack of a substantial cometary flux.

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Figure 1: Locations of the straight cracks and stress peaks on the surface of the Hapi region of 67P.
Figure 2: Terminal failure states for the 67P nucleus.
Figure 3: Failure types and conditions at different spin periods.
Figure 4: Dynamical variation of factors controlling spin acceleration.

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Acknowledgements

M.H. acknowledges the use of ANSYS Academic APDL, version 15.03. D.J.S. and M.H. were supported by NASA grants NNX14AL16G, NNX14AB08G and NNA14AB03A. S.R.C. carried out his work at the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. S.M. acknowledges support by the Jet Propulsion Laboratory.

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Authors

Contributions

M.H. conducted structure analysis. D.J.S., S.R.C., J.W.M. and M.H. analysed orbital and spin evolution. S.P.N. produced reduced data sets for the analysis. M.H., D.J.S., S. Ma., J.S., S. Mo. and T.B. discussed the meaning of the discovered relationships. M.H. wrote the paper with guidance from D.J.S. T.B. generated high-resolution figures. All authors commented on the manuscript.

Corresponding authors

Correspondence to Masatoshi Hirabayashi or Daniel J. Scheeres.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Different types of crack observed on the 67P nucleus.

a, Polygonal cracks on the Apis region at the edge of the large lobe; these cracks are presumably generated by thermal contraction10. b, Parallel sets of cracks on the Hathor region23. Images available at http://blogs.esa.int/rosetta/2015/08/18/do-comet-fractures-drive-surface-evolution/ (a) and at http://sci.esa.int/rosetta/55310-hapi-and-hathor/ (b). Image credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA.

Extended Data Figure 2 Tensile regions on the neck cross-section at the force balance point.

The bulk density is assumed to be 535 kg m−3. a, The maximum component of the principal stress at the force balance point, which is 7 h. The region enclosed by the yellow line indicates the neck cross-section. The red region indicates tensile regions. b, Body orientation. Green arrow, maximum moment of inertia axis; red arrow, intermediate moment of inertia axis; blue arrow, minimum moment of inertia axis.

Extended Data Figure 3 Failure types and conditions for different bulk density cases.

a, The bulk density is 500 kg m−3. b, The bulk density is 570 kg m−3.

Source data

Extended Data Figure 4 Elastic analysis for heterogeneous density cases.

The spin period was fixed at 9 h. The contour plots show the maximum component of the principal stress, with units of pascals. a, The bulk density of the neck is zero. The other regions have a bulk density of 578 kg m−3. b, The bulk density of the neck is 1,000 kg m−3, leading to a bulk density of 498 kg m−3 in the other regions. c, Schematic plot of the bulk density distribution.

Extended Data Figure 5 Plastic analysis for a neck density of 0 kg m−3.

The cross-sections displayed are the same as in Fig. 2. The averaged density is fixed at 535 kg m−3. The colours describe the stress ratio. a, Type I failure at 12.4 h. The cohesive strength used was 4 Pa. b, Type II failure at 8 h. The cohesive strength was 45 Pa. c, Type III failure at 5 h. The cohesive strength was 300 Pa. d, Body orientation.

Extended Data Figure 6 Plastic analysis for a bulk density of 1,000 kg m−3.

The cross-sections displayed are the same as in Fig. 2. The averaged density is fixed at 535 kg m−3. The colours describe the stress ratio. a, Type I failure at 12.4 h. The cohesive strength used was 5 Pa. b, Type II failure at 8 h. The cohesive strength was 25 Pa. c, Type III failure at 5 h. The cohesive strength was 200 Pa. d, Body orientation.

Extended Data Figure 7 Volume ratios of cometary nuclei imaged from spacecraft encounters or ground-based radar.

a, 67P/Churyumov–Gerasimenko. Image credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA. b, 103P/Hartley 2. Image credit: EPOXI mission MRI-VIS frame 5004057 from NASA’s Planetary Data System. c, 1P/Halley. Image credit: ESA/MPS. d, 19P/Borrelly. Image credit: PIA03500, Courtesy by NASA/JPL-Caltech. e, 8P/Tuttle. Image credit: Arecibo Observatory scans 800300017-19; resolution 1 μs × 0.5 Hz (see ref. 51 for more information).

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Hirabayashi, M., Scheeres, D., Chesley, S. et al. Fission and reconfiguration of bilobate comets as revealed by 67P/Churyumov–Gerasimenko. Nature 534, 352–355 (2016). https://doi.org/10.1038/nature17670

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