Nebular drag and capture into spin-orbit resonance

Abstract

The majority of planetary satellites whose spin period is known are observed to be in synchronous spin-orbit resonance. The commonly accepted explanation for this observation is that it is due to the effects of tidal evolution. However, cosmogonic theories state that the formation of planetary and satellite systems occurs within a primordial solar nebula and circumplanetary nebulae, respectively. In this paper the influence of nebular drag on the capture into spin-orbit resonance is analysed. The results show that the torques generated are important for these resonances in a wide range of cases. Using the protojovian nebula model by Lunine and Stevenson (1982), conservative estimates of the despinning time scales for the Galilean satellites are computed. In comparison the despinning time scale from tidal effects are several orders of magnitude larger.