Thermal imaging of Uranus: Upper-tropospheric temperatures one season after Voyager

We report on 18-25 μm thermal imaging of Uranus that took place between 2003 and 2011, a time span roughly one season after the thermal maps made by the Voyager-2 IRIS experiment in 1986. We re-derived meridional variations of temperature and para-H₂ fraction from the Voyager experiment and compared these with the thermal images, which are sensitive to temperatures in the upper troposphere of Uranus around the 70-400 mbar atmospheric pressure range. The thermal images display a maximum of 3 K of equivalent temperature changes across the disk, and they are consistent with the temperature distribution measured by the Voyager IRIS experiment. This implies that there has been no detectable change of the meridional distribution of upper-tropospheric/lower-stratospheric temperatures over a season. This is inconsistent with seasonally dependent radiative-convective-dynamical models and full global climate models that predict some variability with season if the effective temperature is meridionally constant. We posit that the effective temperature of Uranus could be meridionally variable, with the additional possibility that even the small temperature variations predicted by the GCMs are overestimated.