Comet 67P/Churyumov-Gerasimenko: Constraints on its origin from OSIRIS observations

¹ P.A.S. Space Research Center, Bartycka 18A, 00-716 Warszawa, Poland
² Dept. of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
³ Southwest Research Institute, 1050 Walnut St., Boulder, CO 80302, USA
e-mail: marchi@boulder.swri.edu
⁴ Department for Astronomy, University of Maryland, College Park, MD 20742-2421, USA
⁵ Department of Physics and Astronomy “G. Galilei”, University of Padova, Vic. Osservatorio 3, 35122 Padova, Italy
⁶ Physikalisches Institut, Sidlerstrasse 5, University of Bern, 3012 Bern, Switzerland
⁷ Max-Planck Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁸ Institute for Space Science, National Central University, 32054 Chung-Li, Taiwan
⁹ Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, 38106 Braunschweig, Germany
¹⁰ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
¹¹ Dipartimento di Geoscienze, University of Padova, via G. Gradenigo 6, 35131 Padova, Italy
¹² Centro di Ateneo di Studi ed Attivitá Spaziali, “Giuseppe Colombo” (CISAS), University of Padova, via Venezia 15, 35131 Padova, Italy
¹³ Department of Information Engineering, University of Padova, via G. Gradenigo 6/B, 35131 Padova, Italy
¹⁴ CNR-IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy
¹⁵ Scientific Support Office, European Space Agency, 2201 Noordwijk, The Netherlands
¹⁶ Centro de Astrobiologia (INTA-CSIC), 28691 Villanueva de la Canada, Madrid, Spain
¹⁷ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹⁸ LESIA, Obs. de Paris, CNRS, Univ. Paris 06, Univ. Paris-Diderot, 5 Pl. J. Janssen, 92195 Meudon, France
¹⁹ LATMOS, CNRS/UVSQ/IPSL, 11 bd d’Alembert, 78280 Guyancourt, France
²⁰ INAF–Osservatorio Astronomico, vicolo dell’Osservatorio 2, 35122 Padova, Italy
²¹ Department of Mechanical Engineering, University of Padova, via Venezia 1, 35131 Padova, Italy
²² UNITN, Univ. di Trento, via Mesiano 77, 38100 Trento, Italy
²³ INAF–Osservatorio Astronomico, via Tiepolo 11, 34143 Trieste, Italy
²⁴ Instituto de Astrofisica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
²⁵ Institute of Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany
²⁶ Operations Department, European Space Astronomy Centre/ESA, PO Box 78, 28691 Villanueva de la Canada, Madrid, Spain
²⁷ Institut für Datentechnik und Kommunikationsnetze der TU Braunschweig, Hans-Sommer-Str. 66, 38106 Braunschweig, Germany
²⁸ Instituto Nacional de Técnica Aeroespacial, Carretera de Ajalvir, p.k. 4, 28850 Torrejon de Ardoz, Madrid, Spain

Received: 13 March 2015
Accepted: 12 May 2015

Abstract

Context. One of the main aims of the ESA Rosetta mission is to study the origin of the solar system by exploring comet 67P/Churyumov-Gerasimenko at close range.

Aims. In this paper we discuss the origin and evolution of comet 67P/Churyumov-Gerasimenko in relation to that of comets in general and in the framework of current solar system formation models.

Methods. We use data from the OSIRIS scientific cameras as basic constraints. In particular, we discuss the overall bi-lobate shape and the presence of key geological features, such as layers and fractures. We also treat the problem of collisional evolution of comet nuclei by a particle-in-a-box calculation for an estimate of the probability of survival for 67P/Churyumov-Gerasimenko during the early epochs of the solar system.

Results. We argue that the two lobes of the 67P/Churyumov-Gerasimenko nucleus are derived from two distinct objects that have formed a contact binary via a gentle merger. The lobes are separate bodies, though sufficiently similar to have formed in the same environment. An estimate of the collisional rate in the primordial, trans-planetary disk shows that most comets of similar size to 67P/Churyumov-Gerasimenko are likely collisional fragments, although survival of primordial planetesimals cannot be excluded.

Conclusions. A collisional origin of the contact binary is suggested, and the low bulk density of the aggregate and abundance of volatile species show that a very gentle merger must have occurred. We thus consider two main scenarios: the primordial accretion of planetesimals, and the re-accretion of fragments after an energetic impact onto a larger parent body. We point to the primordial signatures exhibited by 67P/Churyumov-Gerasimenko and other comet nuclei as critical tests of the collisional evolution.