Setting the Stage for Uranian Seismology from Rings and Radial Velocities

A Uranus orbiter would be well positioned to detect the planet's free oscillation modes, whose frequencies can resolve open questions about Uranus's weakly constrained interior. We calculate the spectra that may manifest in resonances with ring orbits or in Doppler imaging of Uranus's visible surface, using a wide range of new interior models that satisfy the present constraints. Recent work has shown that Uranus's fundamental (f) and {internal} gravity (g) modes have appropriate frequencies to resonantly confine Uranus's narrow rings, which in most cases have no clear confinement mechanism. We show that even a single low spherical harmonic degree f or g mode detected in imaging or occultations can constrain Uranus's core structure, compared to the degenerate solution space permitted by current or more precise orbiter-derived zonal gravity constraints. Degeneracies persist due to possible confusion between f, g, and interface modes; in this respect a second mode detection would clarify the interpretation dramatically. If Uranus has a frozen core that low degree f modes cannot penetrate, their frequencies are reduced and they are more likely to resonate among the narrow rings. A single high degree f mode resonating in the rings would constrain Uranus's unknown interior rotation period. The different technique of Doppler imaging seismology requires specialized instrumentation but could deliver many more detections, with best sensitivity to acoustic (p) mode oscillations at mHz frequencies. Their deviations from uniform spacing can be used to locate density interfaces in Uranus's interior, such as that of a sharp core boundary. Shallower nonadiabaticity and condensation layers complicate this approach, but we show how higher order frequency differences based on the same spectra can be analyzed to disentangle deep and near-surface effects.