The formation environment of the Galilean Moons

Abstract

The regular satellites of the gas giant planets were formed in circumplanetary disks of gas and dust, whose evolution was governed by mass and angular momentum transport of uncertain origins. Turbulence driven by the magneto-rotational instability can provide the transport if the gas is sufficiently ionised to couple to the embedded magnetic fields. We compute the ionisation states of the minimum-mass and gas-starved models of the Jovian sub-nebula, including the key effects known from Solar nebula studies: ionisation by cosmic rays and stellar X-rays, charge transfer to metal atoms, and recombination on grains. The results show that magneto-rotational turbulence develops in a region whose size depends on the absorbing mass column, the grain surface area, the gas-phase metal abundance and the strength of the turbulent mixing. The turbulence is almost entirely absent from the minimum-mass sub-nebula model over a wide range in these parameters. In contrast, turbulence occurs in the gas-starved model in the surface layers and throughout the parts furthest from the planet, provided sub-micron grains are underabundant. This work was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology with support from the NASA Outer Planets Research program, and at the University of Hong Kong under RGC grant HKU 7024/08P.