Setting the Stage for Uranian Seismology from Rings and Radial Velocities

Abstract

A Uranus orbiter would be well positioned to detect the planet’s free oscillation modes, whose frequencies can resolve 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 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 resonate with Uranus’s narrow rings. We show that even a single ℓ = 2 f- or g-mode detected in ring imaging or occultations can constrain Uranus’s core extent and density. Fully fluid models typically have ℓ = 2-7 f-mode frequencies slightly too high to resonate among the narrow rings. If Uranus has a solid core that f-modes cannot penetrate, their frequencies are reduced, rendering them more likely to be observed. A single ℓ ≳ 7 f-mode detection would constrain Uranus’s unknown rotation period. Meanwhile, the different technique of Doppler-imaging seismology requires specialized instrumentation but could deliver many detections, with best sensitivity to acoustic (p) modes at mHz frequencies. Deviations from uniform frequency spacing can be used to locate density interfaces in Uranus’s interior, such as a sharp core boundary. Shallower nonadiabaticity and condensation layers complicate this approach, but higher-order frequency differences can be analyzed to disentangle deep and near-surface effects. The detection of normal modes by a Uranus orbiter would help to discern among the degenerate solutions permitted by conventional measurements of the planet’s static gravity field.