The case of the missing Ceres family

Abstract

Ceres is unusual among large (>250km) asteroids in lacking a dynamical family. We explore possible explanations, noting that its particularly large size and the ubiquity of families associated with other large asteroids makes avoidance of a sufficiently-sized collision by chance exceedingly unlikely. Current models of Ceres’ thermal history and interior structure favor a differentiated object with an icy near-surface covered by an ∼0.1–1km lag deposit, which could result in a collisional family of diverse, predominately icy bodies. We predict that sublimation of an icy Ceres family would occur on timescales of hundreds of millions of years, much shorter than the history of the Solar System. Sublimation on a Ceres family body would be aided by a low non-ice fraction and a high average temperature, both of which would inhibit lag deposit development. Because there seems to be no likely mechanism for removing a rocky Ceres family, and because the formation of a Ceres family of some kind seems nearly statistically inevitable, the lack of a Ceres family is indirect but independent evidence for Ceres’ differentiation.All of the other large asteroids lacking dynamical families (704 Interamnia, 52 Europa, and 65 Cybele) have spectral properties similar to Ceres, or otherwise suggesting ice at their surfaces. While other large asteroids with similar spectral properties do have families (24 Themis, 10 Hygiea, 31 Euphrosyne), their families are not well understood, particularly Hygiea.