On Steep Cliffs and Fountains of Dust: The Rosetta Comet 67P/Churyumov-Gerasimenko

Comet 67P/Churyumov-Gerasimenko shows remarkable surface features as well as an unusual bulk shape and high porosity, with early activity seen at 4.2 AU heliocentric distance in approach of the Rosetta spacecraft. Observations from the OSIRIS scientific imaging system onboard Rosetta show that the nucleus of 67P consists of two lobes which are connected by a short neck. The nucleus has a bulk density less than half that of water. The shape raises the question of whether the two lobes represent a contact binary formed 4.5 Gyr ago or rather a single body in which a gap has evolved via erosion. The surface morphology suggests the loss of larger volumes of material by mass-loss events, sublimation forming overhangs, or cliff collapses with subsequent release of volatiles.