The Nucleus of Comet 67P/Churyumov-Gerasimenko : a New Case of Contact Binary ?

Abstract:

The nucleus of comet 67P/Churyumov-Gerasimenko appears to be formed of two components as revealed by images obtained with the OSIRIS narrow angle camera and the derived 3-D shape model (Jorda et al. 2014). This shape raises questions far beyond the obvious differences with the nuclei visited so far. We will explore the possible scenarios that may explain it. Whereas contact- binaries have been advocated for elongated nuclei (e.g., 19P/Borrelly and 103P/Hartley 2), the only unquestionable case remains that of 8P/Tuttle based on radar imaging (Harmon at al. 2010) and indirectly confirmed by HST observations (Lamy et al. 2008). However 8P originates from the Oort cloud where the collisional activity is essentially non-existent. A contact-binary among the Jupiter family comets (JFC) such as 67P would have profound implications since it must be primordial and the comet must have survived a possible history of collisions in the Kuiper belt. The present cumulative distribution function of size of nuclei of JFC comets indeed suggests a collisionally-relaxed population. Asymmetric erosion of an initially larger more regular body or even of a pre-existing contact binary will be studied in the framework of the past dynamical evolution of 67P (e.g., Groussin et al. 2007). Although unlikely, the scenario of a re-accumulated body following a catastrophic collision will be considered. Forthcoming images at higher spatial resolution with OSIRIS on the orbiter and CIVA and the lander will hopefully help deciphering the origin of the nucleus of 67P and restricting the number of possible scenarios.