A Tale of Tides: Icy Satellites, Subsurface Oceans, and Tightly-Packed Planetary Systems

Abstract

Tides, a consequence of differential gravitational forces, are one of the fundamental processes that govern the evolution of planetary bodies. They lead to deformation that causes heating via friction, changes in rotation through applied torques, and alteration of planetary orbits. Oceanic tides play an important role in these and are often neglected. The dynamical response of ice-free and ice-covered oceans to tidal forcing is numerically and analytically explored in the icy satellites, with focus on Titan, Enceladus, and the Galilean moons. Numerical calculations that consider dissipation via turbulent boundary layer drag show that heat released though obliquity tides is unaffected by the thickness of a body's ocean, implying that this is a significant source of subsurface energy in moons with thick oceans, like Titan and Triton. The impact of an ice shell on oceanic tides is largest for tidal forcing from orbital eccentricity, where an ocean's gravitational wave speed is enhanced, naturally decreasing the tidal response time and resultant heating. In contrast, an ocean's fluid response to obliquity-driven tides are largely unaffected and sometimes enhanced by a global covering of ice. Tidal heating is usually driven by the central object in a system, but the TRAPPIST-1 extrasolar planetary system is so compact that significant tides will also be raised by one planet on another. The first theory of these planet-planet tides is developed here, and solid-body heating in the TRAPPIST-1 planets is explored. Calculations suggest that if the interior viscosity of these planets is low enough then planet-planet tides can generate up to 20% of all heating on TRAPPIST-1g, and Io-like volcanism can be driven on the inner two planets, even if they have circular orbits. Applying this theory to the Galilean satellites shows that subsurface oceans respond far more readily to moon-moon tides than the solid-body does. This response may lead to resonant tidal waves in these moons, in particular Europa and Io. Oceanic resonances driven by adjacent moons can release more heat than any other energy sources available to the Galilean moons, fundamentally altering their interior, orbital, and rotational evolution. If the oceans are in a non-resonant state today, then tidal heating driven by obliquity is likely the dominant source of tidal energy available to oceans in the Galilean moons.