Interior and Gravity Field Models for Uranus Suggest a Mixed-composition Interior: Implications for the Uranus Orbiter and Probe

The interior composition and structure of Uranus are ambiguous. It is unclear whether Uranus is composed of fully differentiated layers dominated by an icy mantle or has smooth compositional gradients. The Uranus Orbiter and Probe (UOP), the next NASA flagship mission prioritized by the Planetary Science and Astrobiology Survey 2023-2032, will constrain the planet's interior by measuring its gravity and magnetic fields. To characterize the Uranian interior, here we present CORGI, a newly developed planetary interior and gravity model. We confirm that high degrees of mixing are required for Uranus interior models to be consistent with the J2 and J4 gravity harmonics measured by Voyager 2. Empirical models, which have smooth density profiles that require extensive mixing, can reproduce the Voyager 2 measurements. Distinct-layer models with mantles composed of H2O-H/He or H2O-CH4-NH3 mixtures are consistent with the Voyager 2 measurements if the heavy element mass fraction, Z, in the mantle less than or similar to 85%, or if atmospheric Z greater than or similar to 25%. Our gravity harmonics model shows that UOP J2 and J4 measurements can distinguish between high (Z >= 25%) and low (Z = 12.5%) atmospheric metallicity scenarios. The UOP can robustly constrain J6 and potentially J8 given polar orbits within rings. An ice-rich composition can naturally explain the source of Uranus's magnetic field. However, because the physical properties of rock-ice mixtures are poorly known, magnetic field generation by a rock-rich composition cannot be ruled out. Future experiments and simulations on realistic planetary building materials will be essential for refining Uranus interior models.