Rotation of cometary muclei

Abstract

Asymmetric comas and repetitive appearances of structures in the heads and tails of comets are used to infer nuclei rotation periods. However, periodic behaviour of optically-thick expanding comas or of ion plasma production may contribute spurious results. The spin periods of comets are longer than those of asteroids, ranging generally over 10-100 h and above probable limits for gravitational escape. The periods show a flatter distribution, which may reflect an accretional rather than collisional fragmentation history. Arguments for spin-up with age due to sublimating gases are weak; the converse is possible and spin-down due to preferential escape of particles from equatorial regions appears likely.