The prospects for gravitational seismology at Uranus with an orbiting spacecraft

We study the feasibility of conducting gravitational seismology at Uranus, that is, of detecting and characterizing Uranian normal-mode oscillations through their perturbations on the trajectory of an orbiting spacecraft. Characterizing such oscillations has the potential to provide important constraints on models of the interior structure of the planet. We use an orbit determination model to create a realistic, synthetic radio science data set that includes the effect of gravitational perturbations on the orbiter exerted by normal modes. The accelerations produced by these perturbations are then used to compute the torque that the modes exert on the spacecraft along its simulated trajectory. We compare the component of this torque parallel to the planetary spin axis to an analytical model and examine the accuracy and precision to which the mode parameters can be recovered. Our approach is novel in that it allows the normal modes’ properties to be retrieved without any need to tie them to a prespecified spectrum based on a hypothesized interior model. Assuming an orbit with eccentricity of 0.98 and inclination of 45 passing between the planet and the inner edge of the ζring, a noise level similar to that associated with the Juno radio science experiment at Jupiter, and mode energies comparable to that of the modes that excite density waves in Saturn’s C ring, we demonstrate that we are able to recover mode frequencies to within 1% and amplitudes and phases to within 20% and 10%, respectively, of their true values when one or two normal modes dominate the spectrum. We conclude that gravitational seismology can add valuable information concerning Uranus’s normal modes, complementing any that might be obtained through ring seismology or Doppler imaging, when a small number of modes dominate the spectrum. We recommend exploring its potential for revealing normal mode properties, when the spectrum contains many modes of similar amplitude, as a subject for future investigation.