Cometary Science Newsletter

174P/Echeclus

This comet has gone into outburst. The initial report, by Paul Camilleri to the Yahoo Comets Mailing List, and subsequent follow-up by other observers, including Tony Angel, Alan Hale, Jean-Francois Soulier, Lucian Hudin, and Richard Miles, indicates a strength of approximately 2.5 magnitudes, occurring between 2016 Aug 27.7 and 28.8 UT. After the outburst, the comet had a stellar appearance and a total apparent brightness of ~15.2 mag. The comet has a Centaur class orbit, and is presently at 6.3 au from the Sun, and 5.5 au from the Earth.

- Mike Kelley, UMD

The primordial nucleus of comet 67P/Churyumov-Gerasimenko

• B. J. R. Davidsson ^1,2
• H. Sierks ³
• C. Güttler ³
• F. Marzari ⁴
• M. Pajola ⁵
• H. Rickman ^1,6
• M. F. A’Hearn ^7,8
• A.-T. Auger ⁹
• M. R. El-Maarry ¹⁰
• S. Fornasier ¹¹
• P. J. Gutiérrez ¹²
• H. U. Keller ¹³
• M. Massironi ^5,14
• C. Snodgrass ¹⁵
• J.-B. Vincent ³
• and 33 co-authors^null

1. Department of Physics and Astronomy, Uppsala University,, Sweden
2. Jet Propulsion Laboratory, Pasadena (CA), USA
3. Max–Planck–Institut für Sonnensystemforschung, Göttingen, Germany
4. University of Padova, Italy
5. CISAS, University of Padova, Italy
6. PAN Space Research Center, Warszawa, Poland
7. University of Maryland, College Park (MD), USA
8. Gauss Professor, Akademie der Wissenschaften zu Göttingen, Germany
9. Aix-Marseille Université, CNRS, LAM, Marseille, France
10. Physikalisches Institut der Universität Bern, Switzerland
11. LESIA–Observatoire de Paris, CNRS, Meudon, France
12. Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain
13. IGEP, Technische Universität Braunschweig, Germany
14. Dipartimento di Geoscienze, University of Padova, Italy
15. Planetary and Space Sciences, The Open University, Milton Keynes, UK

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.

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.

We used observations of comet 67P by the ESA Rosetta spacecraft combined with data from the NASA Stardust sample-return mission to comet 81P and from meteoritics; we also used existing observations from ground or from spacecraft of irregular satellites of the giant planets, Centaurs, and TNOs. 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, and 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 and 9P, 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.

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.

Editor's note: This abstract has been edited for length.

Astronomy & Astrophysics (Published)

DOI: 10.1051/0004-6361/201526968