The bolometric Bond albedo and energy balance of Uranus

Using a newly developed ‘holistic’ atmospheric model of the aerosol structure in Uranus’s atmosphere, based upon observations made by Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph(STIS), Gemini/Near-Infrared Integral Field Spectrometer(NIFS), and NASA Infrared Telescope Facility(IRTF)/SpeX from 2000 to 2009, we make a new estimate of the bolometric Bond albedo of Uranus during this time of A* = 0.338 ± 0.011, with a phase integral of q* = 1.36 ± 0.03. Then, using a simple seasonal model, developed to be consistent with the disc-integrated blue and green magnitude data from the Lowell Observatory from 1950 to 2016, we model how Uranus’s reflectivity and heat budget vary during its orbit and determine new orbital-mean average values for the bolometric Bond albedo of A* = 0.349 ± 0.016 and for the absorbed solar flux of Pᵢn̄=0.604 ± 0.027 W m⁻². Assuming the outgoing thermal flux to be Pₒut̄=0.693 ± 0.013 W m⁻², as previously determined from Voyager 2 observations, we arrive at a new estimate of Uranus’s average heat flux budget of Pₒut/Pᵢn = 1.15 ± 0.06, finding considerable variation with time due to Uranus’s significant orbital eccentricity of 0.046. This leads the flux budget to vary from Pₒut/Pᵢn = 1.03 near perihelion, to 1.24 near aphelion. We conclude that although Pₒut/Pᵢn is considerably smaller than for the other giant planets, Uranus is not in thermal equilibrium with the Sun.