A Multiarc Approach to Detecting Uranus’s Normal Modes via Doppler Tracking of a Planetary Orbiter

Abstract Gravity science investigations as part of the Uranus Orbiter and Probe (UOP) mission concept are recommended to address key knowledge gaps in our understanding of Uranus’s internal structure. Measurements of the planet’s static gravity field and tidal parameters may be insufficient to fully constrain its mass distribution and core structure, hence complementary approaches are being considered, including the investigation of normal modes via planetary seismology. These global-scale oscillations, observed in Saturn and possibly Jupiter, are linked to interior properties and may be observable using a gravity science instrument. Yet, Uranus’s mode frequencies and amplitudes remain unknown, posing a challenge for their detection. Here, we draw comparisons with Saturn’s observed oscillations and perform numerical simulations of the UOP orbit determination process. The goal is to assess the minimum number of Uranus periapses necessary to detect the presence of normal modes as a function of orbiter trajectory. We do not attempt here to determine how precisely their amplitudes and frequencies can be estimated. We find that if Uranus’s oscillations produce gravitational potential amplitudes comparable to those inferred for Saturn from Cassini ring seismology, an orbiter is likely to detect their signal after a few low-altitude periapses, for modes with angular degree ℓ < 5. Detecting the signal of higher-degree modes (5 < ℓ < 10) is more challenging if the number of periapses is fewer than eight, as their gravitational potential rapidly weakens with distance and the limited data may be insufficient to accumulate a detectable signature.