  The intrinsic luminosity of Uranus is a factor of 10 less than that of
Neptune, an observation that standard giant planetary evolution models, which
assume negligible viscosity, fail to capture. Here we show that more than half
of the interior of Uranus is likely to be in a solid state, and that thermal
evolution models that account for this high viscosity region satisfy the
observed faintness of Uranus by storing accretional heat deep in the interior.
A frozen interior also explains the quality factor of Uranus required by the
evolution of the orbits of its satellites.
