The primordial nucleus of comet 67P/Churyumov-Gerasimenko
1 Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
2 Jet Propulsion Laboratory, M/S 183–301, 4800 Oak Grove Drive, Pasadena CA 91109, USA
3 Max–Planck–Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3, 37077 Göttingen, Germany
4 University of Padova, Department of Physics and Astronomy, via Marzolo 8, 35131 Padova, Italy
5 Centro di Ateneo di Studi ed Attivitá Spaziali “Giuseppe Colombo” (CISAS), University of Padova, via Venezia 15, 35131 Padova, Italy
6 PAN Space Research Center, Bartycka 18A, 00716 Warszawa, Poland
7 University of Maryland, Department of Astronomy, College Park, MD 20742-2421, USA
8 Gauss Professor, Akademie der Wissenschaften zu Göttingen and Max–Planck–Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
9 Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
10 Physikalisches Institut der Universität Bern, Sidlerstr. 5, 3012 Bern, Switzerland
11 LESIA–Observatoire de Paris, CNRS, Université Pierre et Marie Curie, Université Paris Diderot, 5 place J. Janssen, 92195 Meudon, France
12 Instituto de Astrofísica de Andalucía (CSIC), c/ Glorieta de la Astronomía s/n, 18008 Granada, Spain
13 Institut für Geophysik und extraterrestrische Physik (IGEP), Technische Universität Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
14 Dipartimento di Geoscienze, University of Padova, 35131 Padova, Italy
15 Planetary and Space Sciences, Department of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
16 University of Padova, Department of Physics and Astronomy, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
17 Laboratoire d’Astrophysique de Marseille, UMR 7326, CNRS & Aix-Marseille Université, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
18 Centro de Astrobiologia, CSIC–INTA, 28850 Torrejon de Ardoz, Madrid, Spain
19 International Space Science Institute, Hallerstraße 6, 3012 Bern, Switzerland
20 Scientific Support Office, European Space Research and Technology Centre/ESA, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk ZH, The Netherlands
21 LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France
22 INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
23 CNR–IFN UOS Padova LUXOR, via Trasea, 7, 35131 Padova, Italy
24 Department of Industrial Engineering – University of Padova, via Venezia 1, 35131 Padova, Italy
25 University of Trento, via Mesiano 77, 38100 Trento, Italy
26 Univ. Paris Diderot,Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris Cedex 13, France
27 INAF–Osservatorio Astronomico, via Tiepolo 11, 34014 Trieste, Italy
28 Deutsches Zentrum für Luft – und Raumfahrt (DLR), Institut für Planetenforschung, Rutherfordstraße 2, 12489 Berlin, Germany
29 National Central University, Graduate Institute of Astronomy, 300 Chung-Da Rd, 32054 Chung-Li, Taiwan
30 Operations Department, European Space Astronomy Centre/ESA, PO Box 78, 28691 Villanueva de la Canada, Madrid, Spain
31 University of Padova, Department of Information Engineering, via Gradenigo 6/B, 35131 Padova, Italy
Received: 15 July 2015
Accepted: 15 March 2016
Context. We investigate the formation and evolution of comet nuclei and other trans-Neptunian objects (TNOs) in the solar nebula and primordial disk prior to the giant planet orbit instability foreseen by the Nice model.
Aims. Our goal is to determine whether most observed comet nuclei are primordial rubble-pile survivors that formed in the solar nebula and young primordial disk or collisional rubble piles formed later in the aftermath of catastrophic disruptions of larger parent bodies. We also propose a concurrent comet and TNO formation scenario that is consistent with observations.
Methods. We used observations of comet 67P/Churyumov-Gerasimenko by the ESA Rosetta spacecraft, particularly by the OSIRIS camera system, combined with data from the NASA Stardust sample-return mission to comet 81P/Wild 2 and from meteoritics; we also used existing observations from ground or from spacecraft of irregular satellites of the giant planets, Centaurs, and TNOs. We performed modeling of thermophysics, hydrostatics, orbit evolution, and collision physics.
Results. We find that thermal processing due to short-lived radionuclides, combined with collisional processing during accretion in the primordial disk, creates a population of medium-sized bodies that are comparably dense, compacted, strong, heavily depleted in supervolatiles like CO and CO2; they contain little to no amorphous water ice, and have experienced extensive metasomatism and aqueous alteration due to liquid water. Irregular satellites Phoebe and Himalia are potential representatives of this population. Collisional rubble piles inherit these properties from their parents. Contrarily, comet nuclei have low density, high porosity, weak strength, are rich in supervolatiles, may contain amorphous water ice, and do not display convincing evidence of in situ metasomatism or aqueous alteration. We outline a comet formation scenario that starts in the solar nebula and ends in the primordial disk, that reproduces these observed properties, and additionally explains the presence of extensive layering on 67P/Churyumov-Gerasimenko (and on 9P/Tempel 1 observed by Deep Impact), its bi-lobed shape, the extremely slow growth of comet nuclei as evidenced by recent radiometric dating, and the low collision probability that allows primordial nuclei to survive the age of the solar system.
Conclusions. We conclude that observed comet nuclei are primordial rubble piles, and not collisional rubble piles. We argue that TNOs formed as a result of streaming instabilities at sizes below ~400 km and that ~350 of these grew slowly in a low-mass primordial disk to the size of Triton, Pluto, and Eris, causing little viscous stirring during growth. We thus propose a dynamically cold primordial disk, which prevented medium-sized TNOs from breaking into collisional rubble piles and allowed the survival of primordial rubble-pile comets. We argue that comets formed by hierarchical agglomeration out of material that remained after TNO formation, and that this slow growth was a necessity to avoid thermal processing by short-lived radionuclides that would lead to loss of supervolatiles, and that allowed comet nuclei to incorporate ~3 Myr old material from the inner solar system.
Key words: comets: individual: 67P/Churyumov-Gerasimenko / Kuiper belt: general / protoplanetary disks
© ESO, 2016