On the nucleus structure and activity of comet 67P/Churyumov-Gerasimenko

Images from the OSIRISscientific imaging system onboard Rosetta show that the nucleus of 67P/Churyumov-Gerasimenko consists of two lobes connected by a short neck. The nucleus has a bulk density less than half that of water. Activityat a distance from the Sun of >3 astronomical unitsis predominantly from the neck, where jets have been seen consistently. The nucleus rotates about the principal axis of momentum. The surface morphology suggests that the removal of larger volumes of material, possibly via explosive release of subsurface pressure or via creation of overhangs by sublimation, may be a major mass loss process. The shape raises the question of whether the two lobes represent a contact binaryformed 4.5 billion yearsago, or a single body where a gap has evolved via mass loss.