Isolating the Gravitational Influence of Uranus’s Winds Requires Close Passages Inward of the Rings

Abstract Close orbits by the Uranus Orbiter and Probe (UOP) could be used to deduce Uranus’s multipolar gravity field to a higher precision and angular degree than the J 2 and J 4 currently measured from ground-based ring occultations and the Voyager 2 flyby. We examine J n sensitivity limits obtained from simulations of candidate UOP trajectories, pairing these with Uranus interior and wind models to perform retrievals from the gravity moments. We consider zonal wind profiles derived from recent feature-tracking data, assuming that zonal winds extend into the planet along cylinders, with a radial decay function similar to those that explain Jupiter and Saturn gravity. Present knowledge of J 2 and J 4 permits a fairly wide range of possible wind depths in Uranus, up to 1800 km or 7% of the planet’s radius. Measuring additional gravity moments is essential to separate this unknown wind depth from other interior properties of interest, but J 6 is found to be too dominated by bulk rotation to be a useful probe of the wind depth. Odd moments arising from Uranus’s observed north–south asymmetric flow are strong functions of the wind depth, but the usefulness of J 3 is hindered by its sensitivity to present uncertainties in the wind profile. The even moment J 8 or the odd moments J 5 and J 7 are the best probes of the depth of Uranus’s winds. J 8 and, most likely, J 5 and J 7 are measurable in a highly inclined orbit making ≳10 pericenter passages inward of the ζ ring, approximately 1000–2500 km above Uranus’s cloud tops.