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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Rickman, H. et al. Comet 67P/Churyumov-Gerasimenko: constraints on its origin from OSIRIS observations. Astron. Astrophys. 583, A44 (2015)
Guilbert-Lepoutre, A. et al. On the evolution of comets. Space Sci. Rev. 197, 271–296 (2015)
Chen, J. & Jewitt, D. On the rate at which comets split. Icarus 108, 265–271 (1994)
Boehnhardt, H. in Comets II (eds Festou, M., Keller, H. U. & Weaver, H. A. ) 301–316 (Univ. Arizona Press, 2004)
Keller, H. U. et al. Isolation, erosion, and morphology of comet 67P/Churyumov-Gerasimenko. Astron. Astrophys. 583, A34 (2015)
Massironi, M. et al. Two independent and primitive envelopes of the bilobate nucleus of comet 67P. Nature 526, 402–405 (2015)
Kring, D. A. & Cohen, B. A. Cataclysmic bombardment throughout the inner solar system 3.9–4.0 Ga. J. Geophys. Res. 107, 4 (2002)
Strom, R. G., Malhotra, R., Ito, T., Yoshida, F. & Kring, D. A. The origin of planetary impactors in the inner solar system. Science 309, 1847–1850 (2005)
Sierks, H. et al. On the nucleus structure and activity of comet 67P/Churyumov-Gerasimenko. Science 347, http://dx.doi.org/10.1126/science.aaa1044 (2015)
El-Maarry, M. R. et al. Fractures on comet 67P/Churyumov-Gerasimenko observed by Rosetta/OSIRIS. Geophys. Res. Lett. 42, 5170–5178 (2015)
Mottola, S. et al. The rotation state of 67P/Churyumov-Gerasimenko from approach observations with the OSIRIS cameras on Rosetta. Astron. Astrophys. 569, L2 (2014)
Barnes, T. & Farnham, T. Shape models of 67P/Churyumov-Gerasimenko v1.0, RO-C-multi-5-67P-shape-v1.0. NASA Planetary Data System and ESA Planetary Science Archive http://pdssbn.astro.umd.edu/holdings/ro-c-multi-5-67p-shape-v1.0/dataset.html 2015
Preusker, F. et al. Shape model, reference system definition, and cartographic mapping standards for comet 67P/Churyumov-Gerasimenko—stereo-photogrammetric analysis of Rosetta/OSIRIS image data. Astron. Astrophys. 583, A33 (2015)
Pollard, D. D. & Fletcher, R. C. Fundamentals of Structural Geology 1st edn (Cambridge Univ. Press, 2005)
Biele, J. et al. The landing(s) of Philae and inferences about comet surface mechanical properties. Science 349, aaa9816–aaa9816-6 (2015)
Groussin, O. et al. Gravitational slopes, geomorphology, and material strengths of the nucleus of comet 67P/Churyumov-Gerasimenko from OSIRIS observations. Astron. Astrophys. 583, A32 (2015)
Steckloff, J. K. et al. Dynamic sublimation pressure and the catastrophic breakup of comet ISON. Icarus 258, 430–437 (2015)
Scheeres, D. J. Rotational fission of contact binary asteroids. Icarus 189, 370–385 (2007)
Jutzi, M. & Asphaug, E. The shape and structure of cometary nuclei as a result of low-velocity accretion. Science 348, 1355–1358 (2015)
Keller, H. U., Mottola, S., Skorov, Y. & Jorda, L. The changing rotation period of comet 67P/Churyumov-Gerasimenko controlled by its activity. Astron. Astrophys. 579, L5 (2015)
Scheeres, D. J. The dynamical evolution of uniformly rotating asteroids subject to YORP. Icarus 188, 430–450 (2007)
Levison, H. F. & Duncan, M. J. The long-term dynamical behavior of short-period comets. Icarus 108, 18–36 (1994)
Thomas, N. et al. Morphological diversity of comet 67P/Churymov-Gerasimenko. Science 347, aaa0440-1–aaa0440-6 (2015)
Pravec, P. et al. Formation of asteroid pairs by rotational fission. Nature 466, 1085–1088 (2010)
Jacobson, S. A. & Scheeres, D. J. Dynamics of rotationally fissioned asteroids: source of observed small asteroid systems. Icarus 214, 161–178 (2011)
Steckloff, J. K. & Jacobson, S. A. The formation of striae with cometary dust tails by a sublimation-driven YORP-like effect. Icarus 264, 160–171 (2016)
Hirabayashi, M., Sánchez, D. P. & Scheeres, D. J. Internal structure of asteroids having surface shedding due to rotational instability. Astrophys. J. 808, 63 (2015)
Marchi, S. et al. The onset of the lunar cataclysm as recorded in its ancient crater populations. Earth Planet. Sci. Lett. 325–326, 27–38 (2012)
Kohnke, P. 2009, Theory Reference for the Mechanical APDL and Mechanical Applications, 12th edn (ANSYS, Inc.)
Marsden, B. G., Sekanina, Z. & Yeomans, D. K. Comets and nongravitational forces. Astron. J. 78, 211–225 (1973)
Si, H. TetGen, a Delaunay-based quality tetrahedral mesh generator. ACM Trans. Math. Software 41, 11 (2015)
Hirabayashi, M. & Scheeres, D.J. Stress and failure analysis of rapidly rotating asteroid (29075) 1950 DA. Astrophys. J. Lett. 798, L8 (2015)
Hirabayashi, M. & Scheeres, D. J. Analysis of asteroid (216) Kleopatra using dynamical and structural constraints. Astrophys. J. 780, 160 (2014)
Chen, W. F. & Han, D. J. Plasticity for Structural Engineers Ch. 2 (Springer, 1988)
Byerlee, J. Friction of rocks. Pure Appl. Geophys. 116, 615–626 (1978)
Willam, K. J. & Warnke, E. P. Constitutive model for the triaxial behaviour of concrete. In IABSE Reports of the Working Commissions 19, III-1–III-30 (1974)
El-Maarry, M. R. et al. Regional surface morphology of comet 67P/Churyumov-Gerasimenko from Rosetta/OSIRIS images. Astron. Astrophys. 583, A26 (2015)
Lambe, T. W. & Whitman, R. V. Soil Mechanics Ch. 11, 12 (John Wiley, 1969)
Jaeger, J. C., Cook, N. G. W. & Zimmerman, R. Fundamentals of Rock Mechanics Ch. 6 (John Wiley, 2009)
Hirabayashi, M. Failure modes and conditions of a cohesive, spherical body due to YORP spin-up. Mon. Not. R. Astron. Soc. 454, 2249–2257 (2015)
Unger, D. J. Analytical Fracture Mechanics 25–28 (Dover, 1995)
Gross, D. & Seelig, T. Fracture Mechanics Ch. 2, 4 (Springer, 2011)
Wiederhorn, S. M. & Bolz, L. H. Stress corrosion and static fatigue of glass. J. Am. Ceram. Soc. 53, 543–548 (1970)
Goodman, D. J. Critical stress intensity factor (KIc) measurements at high loading rates for polycrystalline ice in Physics and Mechanics of Ice 129–146 (Springer, 1980)
Atkinson, B. K. Subcritical crack growth in geological materials. J. Geophys. Rev. 89, 4077–4114 (1984)
Scheeres, D. J. Stability in the full two body problem. Celestial Mech. Dyn. Astron. 83, 155–169 (2002)
McMahon, J. W. & Scheeres, D. J. Improving space object catalog maintenance through advances in solar radiation pressure modeling. J. Guid. Control Dyn. 38, 1366–1381 (2015)
Farnham, T. L. & Thomas, P. C. Plate shape model of comet 103P/Hartley 2 V1.0, DIF-C-HRIV/MRI-5-HARTLEY2-SHAPE-V1.0. NASA Planetary Data System https://pds.nasa.gov (2013)
Sagdeev, R. Z. et al. Television observations of comet Halley from Vega spacecraft. Nature 321, 262–266 (1986)
Keller, H. U., Britt, D., Buratti, B. J. & Thomas, N. In situ observations of cometary nuclei in Comet II (eds Festou, M. C., Keller, H. U. & Weaver, H. A. ) 211–222 (Univ. Arizona Press, 2004)
Harmon, J. K., Nolan, M. C., Giorgini, J. D. & Howell, E. S. Radar observations of 8P/Tuttle: a contact-binary comet. Icarus 207, 499–502 (2010)
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.
The authors declare no competing financial interests.
Extended data figures and tables
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.
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.
a, The bulk density is 500 kg m−3. b, The bulk density is 570 kg m−3.
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.
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.
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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