Uranus’s complex internal structure

Abstract

Context . Uranus’s bulk composition remains unknown. Although there are clear indications that Uranus’s interior is not fully convec-tive, and therefore has a non-adiabatic temperature profile, many interior models continue to assume an adiabatic interior. Aims . In this paper we present a new method for interpreting empirical structure models in terms of composition and for identifying non-convective regions. We also explore how the uncertainty in Uranus’s rotation period and winds affects the inferred composition and temperature profile. Methods . We used Uranus’s density profiles from previous work in which the density is represented by up to three polytropes. Results . Using our new method, we find that these empirical models imply that Uranus’s interior includes non-adiabatic regions. This leads to significantly hotter internal temperatures, which can reach several tens of thousands of kelvins, and higher bulk heavy-element abundances (up to 1 M ⊕ ) 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. Conclusions . 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 to decrease the uncertainty in Uranus’s bulk composition, it is of great value to measure Uranus’s gravitational field and determine its rotation period.

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