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

Abstract

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 $J_2$ and $J_4$ 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 H$_2$O-H/He or H$_2$O-CH$_4$-NH$_3$ mixtures are consistent with the Voyager 2 measurements if the heavy element mass fraction, $Z$, in the mantle $\lesssim85\%$, or if atmospheric $Z$ $\gtrsim25\%$. Our gravity harmonics model shows that UOP $J_2$ and $J_4$ measurements can distinguish between high ($Z\geq25\%$) and low ($Z=12.5\%$) atmospheric metallicity scenarios. The UOP can robustly constrain $J_6$ and potentially $J_8$ given polar orbits within rings. An ice-rich composition can naturally explain the source of Uranus' 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.