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The low luminosity of Uranus is a long-standing challenge in planetaryscience. Simple adiabatic models are inconsistent with the measured luminosity,which indicates that Uranus is non-adiabatic because it has thermal boundarylayers and/or conductive regions. A gradual composition distribution acts as athermal boundary to suppress convection and slow down the internal cooling.Here we investigate whether composition gradients in the deep interior ofUranus can explain its low luminosity, the required composition gradient, andwhether it is stable for convective mixing on a timescale of some billionyears. We varied the primordial composition distribution and the initial energybudget of the planet, and chose the models that fit the currently measuredproperties (radius, luminosity, and moment of inertia) of Uranus. We presentseveral alternative non-adiabatic internal structures that fit the Uranusmeasurements. We found that convective mixing is limited to the interior ofUranus, and a composition gradient is stable and sufficient to explain itscurrent luminosity. As a result, the interior of Uranus might still be veryhot, in spite of its low luminosity. The stable composition gradient alsoindicates that the current internal structure of Uranus is similar to itsprimordial structure. Moreover, we suggest that the initial energy content ofUranus cannot be greater than 20% of its formation (accretion) energy. We alsofind that an interior with a mixture of ice and rock, rather than separated iceand rock shells, is consistent with measurements, suggesting that Uranus mightnot be "differentiated". Our models can explain the luminosity of Uranus, andthey are also consistent with its metal-rich atmosphere and with thepredictions for the location where its magnetic field is generated.

Explaining the low luminosity of Uranus: A self-consistent thermal and structural evolution