The Formation of Bilobate Comet Shapes through Sublimative Torques

TL;DR

The paper proposes that sublimative torques during comet migration cause rotational disruption, leading to the formation of bilobate comet shapes observed today.

Contribution

It introduces a model linking sublimative activity and dynamical migration to the formation of bilobate comet nuclei, a process not previously explained.

Findings

Sublimative torques spin up comets to disruption during migration.

Disrupted rubble-pile comets reform as bilobate shapes.

Most bilobate comets likely experienced disruption 1-10 million years ago.

Abstract

Recent spacecraft and radar observations have found that ~70 percent of short-period comet nuclei, mostly Jupiter-family comets (JFCs), have bilobate shapes (two masses connected by a narrow neck). This is in stark contrast to the shapes of asteroids of similar sizes, of which ~14% are bilobate. This suggests that a process or mechanism unique to comets is producing these shapes. Here we show that the bilobate shapes of JFC nuclei are a natural byproduct of sublimative activity during their dynamical migration from their trans-Neptunian reservoir, through the Centaur population, and into the Jupiter family. We model the torques resulting from volatile sublimation during this dynamical migration and find that they tend to spin up these nuclei to disruption. Once disrupted, the rubble pile-like material properties of comet nuclei (tensile strengths of ~1-10 Pa and internal friction angles of ~35$^\circ$) cause them to reform as bilobate objects. We find that JFCs likely experienced rotational disruption events prior to entering the Jupiter family, which could explain the prevalence of bilobate shapes. These results suggest that the bilobate shapes of observed comets developed recently in their history (within the past ~1-10 Myr), rather than during solar system formation or collisions during planet migration and residency in the trans-Neptunian population.