Despin Mechanism for Protogiant Planets and Ionization State of Protogiant Planetary Disks

Abstract

The giant planets Jupiter and Saturn probably experienced despinning from a rapid rotation rate near rotational break-up to their present rotational states. This is presumed to occur when these planets possessed gaseous disks, during and immediately after planetary accretion. We modeled the despinning mechanism arising from hydromagnetic torque due to the interaction of the planetary magnetic field with the partially conducting gaseous disk, assuming parameters typical of the late stages of the planet's formation. The main idea of the model is that the angular momentum is transferred from the proto_planet to the disk at all radii beyond the corotation point because of the coupling of planetary dipole field lines to the disk fluid and the resulting current and Lorentz force thereby created. The ionization degree of the gas of the protojovian disk was estimated to be about 10^(−15)at one scale height, but increases rapidly with height. The calculations suggest that under the condition of a giant planetary surface magnetic field of 100 G, a mean magnetic diffusivity of the disk of 10^(16)–10^(17) cm^2s^(−1)and the minimum mass of the disk sufficient to create the Galilean satellites, the planetary angular momentum can be reduced to its current value from an initial critical rotational period in about 10^6–10^7years. If the dissipation time scale of the disk is of this order, then this model is capable in principal of explaining the present spin states of Jupiter and Saturn, provided that the protoplanet had a strong magnetic dipole.