Uranus' Complex Internal Structure

TL;DR

This paper introduces a new method to interpret Uranus' internal structure models, revealing non-adiabatic regions that imply higher internal temperatures and different composition estimates, emphasizing the importance of accurate rotation and gravitational data.

Contribution

The paper presents a novel approach to link empirical structure models with composition, accounting for non-adiabatic regions and the effects of rotation period and winds on Uranus' internal structure.

Findings

Uranus' interior likely contains non-adiabatic regions.

Internal temperatures can reach several thousand Kelvin.

Maximum water-to-rock ratio ranges between 2.6 and 21.

Abstract

Uranus' bulk composition remains unknown. Although there are clear indications that Uranus' interior is not fully convective, and therefore has a non-adiabatic temperature profile, many interior models continue to assume an adiabatic interior. In this paper we present a new method to interpret empirical structure models in terms of composition and for identifying non-convective regions. We also explore how the uncertainty in Uranus' rotation period and winds affect the inferred composition and temperature profile. We use Uranus' density profiles from previous work where the density is represented by up to three polytropes. Using our new method, we find that these empirical models imply that Uranus' interior includes non-adiabatic regions. This leads to significantly hotter internal temperatures that can reach a few 10$^3$ K and higher bulk heavy-element abundances (up to 1 M$_\oplus$) compared to standard adiabatic models. We find that the assumed rotation period strongly affects the inferred composition while the winds have a negligible effect. Although solutions with only H-He and rock are possible, we find that the maximum water-to-rock ratio in Uranus for our models ranges between 2.6 and 21. This is significantly lower compared to standard adiabatic models. We conclude that it is important to include non-adiabatic regions in Uranus structure models as they significantly affect the inferred temperature profile, and therefore the inferred bulk heavy-element abundance. In addition, we suggest that it is of great value to measure Uranus' gravitational field and determine its rotation period in order to decrease the uncertainty in Uranus' bulk composition.