Io's Tidal Dissipation and Longitudinal Drift

Previously, Io's tidal heating has been calculated assuming synchronous rotation and a Keplerian orbit. However, Io's orbit is significantly non-Keplerian (due to Jupiter's oblateness), and Io's rotation may include significant departures (librations) from the synchronous state, which introduce additional terms to the tidal potential. To quantify the effect of these additional tidal modes on the dissipation within Io, the dynamical evolution of Io has been studied by coupling the orbital dynamics to rotation and tidal deformation. Io's heat flow is computed utilizing both a simplified (Q parameterization) model of dissipation, and a complete solution of the tidal deformation in a layered, viscoelastic body. We will compare the Keplerian, synchronous case with the unrestricted case to determine the sensitivity of the inferred internal structure of Io to the dynamical assumptions. We will also study the phenomenon of longitudinal drift or asynchronous rotation of Io. Longitudinal drift can occur in a deformable body because a degree-two deformation combined with a rotation can leave the shape of the body unchanged but result in a slow drift of surface features relative to perfect synchronous rotation. Orbital eccentricity causes longitudinal drift only in one direction, inclination can induce drift in either direction. We have determined that the drift rate scales with the square of the orbital eccentricity and is controlled by the relaxation time (or viscosity) of the body. We carry out a systematic numerical and analytical study to quantify the rate of longitudinal drift as a function of orbital parameters and internal structure. We will also look for the signature of longitudinal drift by analysis of the distribution of mountains on Io as a function of longitude. If volcanic activity destroys mountains (as suggested by the anti-correlation of volcanoes and mountains on Io), then we should observe progressive degradation of the mountain population as the longitudinal drift carries them across the sub- and anti-jovian volcanic regions (where tidal heating is a maximum).