Sublimative torques as the origin of bilobate comets

Abstract

About 70 percent of observed cometary nuclei are bilobate (made up of two masses of material connected by a narrow neck). In contrast, only 10-20 percent of similarly-sized asteroids are contact binaries or bilobate, suggesting that some process unique to comets is responsible for the formation of bilobate shapes. We examine a new mechanism for creating bilobate nuclei in Jupiter-family comets (JFCs), in which sublimative torques acting on a comet during its migration through the Centaur region spin the nucleus up to disruption, after which it may reform in a bilobate shape. We find that JFCs smaller than approximately 100 kilometers in radius should experience enough torque from carbon monoxide and carbon dioxide sublimation over their dynamical evolution through the Centaur region to be restructured into bilobate shapes. This suggests that the observed bilobate distribution of comet shapes could be the result of cometary evolution, rather than a feature of primordial cometary reservoirs.