The interior of Uranus: Thermal profile, bulk composition and the distribution of rock, water and hydrogen and helium

Abstract

We present improved empirical density profiles of Uranus and interpret them in terms of their temperature and composition using a new random algorithm. The algorithm to determine the temperature and composition is agnostic with respect to the temperature gradient in non-isentropic regions and chooses randomly amongst all possible gradients that are stable against convection and correspond to an Equation of State compatible composition. Our empirical models are based on an efficient implementation of the Theory of Figures up to 10th order including a proper treatment of the atmosphere. The accuracy of 10th order ToF enables us to present accurate calculations of the gravitational moments of Uranus up to $J_{14}$: $J_6=(5.3078\pm 0.3312)\cdot 10^{-7}$, $J_8=(-1.1114\pm 0.1391)\cdot 10^{-8}$, $J_{10}=(2.8616\pm 0.5466)\cdot 10^{-10}$, $J_{12}=(-8.4684\pm 2.0889)\cdot 10^{-12}$ and $J_{14}=(2.7508\pm 0.7944)\cdot 10^{-13}$. We consider two interior models of Uranus that differ with respect to the maximal number of materials allowed per layer of Uranus (three vs. four composition components). The case with three materials does not allow Hydrogen and Helium in deeper parts of Uranus and results in a higher water abundance which leads to lower central temperatures. On the other hand, the models with four materials allow H-He to be mixed into the deeper interior and lead to rock-dominated solutions. We find that these four composition components models are less reliable due to the underlying empirical models incompatibility with realistic Brunt frequencies. Most of our models are found to be either purely convective with the exception of boundary layers, or only convective in the outermost region. Almost all of our models possess a region that is convective and consists of ionic H${}_2$O which could explain the generation of Uranus' magnetic field.

Accepted for publication in A&A

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• The Evolution and Internal Structure of Neptunes and Sub-Neptunes: The importance of thermal conductivity in non-convective regions
• Towards an agnostic algorithm for sampling empirical structure models: The case of Uranus and Neptune
• Reassessing planetary composition: Evidence of rock-dominated envelopes in Uranus and Neptune