The primordial nucleus of Comet 67P/Churyumov-Gerasimenko

Bjorn Davidsson 1 Holger Sierks 2 Carsten Guettler 2 Francesco Marzari 3 Maurizio Pajola 4 Hans Rickman 5 Michael A'Hearn 6 Anne-Therese Auger 7 Mohamed El-Maarry 8 Sonia Fornasier 9 Pedro Gutierrez 10 Horst U. Keller 11 Matteo Massironi 12 Colin Snodgrass 13 Jean-Baptiste Vincent 2 Cesare Barbieri 3 Philippe Lamy 7 Rafael Rodrigo 14 Detlef Koschny 15 Antonella Barucci 9 Jean-Loup Bertaux 16 Ivano Bertini 4 Gabriele Cremonese 17 Vania da Deppo 18 Stefano Debei 19 Mariolino de Cecco 20 Clement Feller 21 Marco Fulle 22 Olivier Groussin 7 Stubbe Hviid 23 Sebastian Hoefner 2 Wing-Huen Ip 24 Laurent Jorda 7 Joerg Knollenberg 23 Gabor Kovacs 2 Joerg-Rainer Kramm 2 Ekkehard Kuehrt 23 Michael Kueppers 25 Fiorangela La Forgia 3 Luisa Lara 10 Monica Lazzarin 3 Jose Lopez-Moreno 10 Richard Moissl-Fraund 25 Stefano Mottola 23 Giampiero Naletto 26 Nilda Oklay 2 Nicolas Thomas 8 Cecilia Tubiana 2
Abstract : Observations of Comet 67P/Churyumov-Gerasimenko by Rosetta show that the nucleus is bi-lobed, extensively layered, has a low bulk density, a high dust-to-ice mass ratio (implying high porosity), and weak strength except for a thin sintered surface layer. The comet is rich in supervolatiles (CO, CO2, N2), may contain amorphous water ice, and displays little to no signs of aqueous alteration. Lack of phyllosilicates in Stardust samples from Comet 81P/Wild 2 provides further support that comet nuclei did not contain liquid water.These properties differ from those expected for 50-200 km diameter bodies in the primordial disk. We find that thermal processing due to Al-26, combined with collisional compaction, creates a population of medium-sized bodies that are comparably dense, compacted, strong, heavily depleted in supervolatiles, containing little to no amorphous water ice, and that have experienced extensive aqueous alteration. Irregular satellites Phoebe and Himalia are potential representatives of this population. Collisional rubble piles inherit these properties from their parents. We therefore conclude that observed comet nuclei are primordial rubble piles, and not collisional rubble piles.We propose a concurrent comet and TNO formation scenario that is consistent with these observations. We argue that TNOs form due to streaming instabilities at sizes of about 50-400 km and that about 350 of these grow slowly in a low-mass primordial disk to the size of Triton, causing little viscous stirring during growth. We propose a dynamically cold primordial disk, that prevents medium-sized TNOs from breaking into collisional rubble piles, and allows for the survival of primordial rubble-pile comets. We argue that comets form by hierarchical agglomeration out of material that remains after TNO formation. This slow growth is necessary to avoid thermal processing by Al-26, and to allow comet nuclei to incorporate 3 Myr old material from the inner Solar System, found in Stardust samples. Growth in the Solar Nebula creates porous single-lobe nuclei, while continued growth in a mildly viscously stirred primordial disk creates denser outer layers, and allow bi-lobe nucleus formation through mergers.
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Submitted on : Tuesday, November 17, 2015 - 5:50:48 PM
Last modification on : Friday, August 2, 2019 - 2:30:11 PM

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  • HAL Id : insu-01230109, version 1

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Bjorn Davidsson, Holger Sierks, Carsten Guettler, Francesco Marzari, Maurizio Pajola, et al.. The primordial nucleus of Comet 67P/Churyumov-Gerasimenko. 47th DPS Annual Meeting, Nov 2015, Washington, United States. pp.413.15. ⟨insu-01230109⟩

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