Tides of io

Abstract

Jupiter's satellite Io is the most active earth-like planetary body in the solar system with a surface heat flow of, at least, 2.5 W m−2, a resurfacing rate of 1.3 cm a−1, and, possibly, a self-sustained magnetic field. It is universally accepted that the activity is driven by tidal energy dissipated in Io's mantle. Tides with amplitudes two orders of magnitude larger than the lunar tides on Earth are raised on Io by Jupiter. Since Io rotates synchronously with its orbital revolution, substantial tidal deformation requires an eccentric orbit. The orbital eccentricity is maintained by the Laplace resonance between the inner Jovian satellites against the damping induced by tidal dissipation in Io's interior. Models of tidal dissipation assume a visco-elastic mantle rheology and require a fluid (outer) core to allow sufficiently strong tidal deformation. The mantle most likely is partially molten and there may be an asthenosphere or magmasphere underneath the lithosphere. The energy that is dissipated in Io is drawn from Jupiter's rotational energy and is transferred to Io's orbital energy before part of it is dissipated in the satellite. Tidal dissipation thus is a sink in the orbital energy balance and a source in the energy balance of the interior. The energy balances are coupled through the temperature dependent rheology parameters. Models of the thermal-orbital evolution indicate that a quasi-stationary high dissipation state is possible as well as oscillations of the thermal and orbital parameters. A magnetic field is unlikely in a quasi-stationary state. The time rate of change of orbit parameters such as the mean motion are constrained by astrometrical observation over the past 300 years. These data can be used to constrain the present tidal dissipation rate. These constraints indicate that the present heat flow is an order of magnitude larger than the present dissipation rate. A model of time dependent heat transfer with local hot spots in the mantle where melt is generated by viscous dissipation is proposed. This model may explain the gap between the present heat flow and the tidal dissipation rate.