Inclination damping on Callisto

Callisto is thought to possess a subsurface ocean, which will dissipate energy due to obliquity tides. This dissipation should have damped any primordial inclination within 1 Gyr - and yet Callisto retains a present-day inclination. We argue that Callisto's inclination and eccentricity were both excited in the relatively recent past (∼0.3 Gyr). This excitation occurred as Callisto migrated outwards according to the 'resonance-locking' model and passed through a 2:1 mean-motion resonance with Ganymede. Ganymede's orbital elements were likewise excited by the same event. To explain the present-day orbital elements, we deduce a solid-body tidal k₂/Q ≈ 0.05 for Callisto and a significantly lower value for Ganymede.