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 HST/STIS, Gemini/NIFS and IRTF/SpeX from 2000 – 2009, we make a new estimate 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 – 2016, we model how Uranus’s reflectivity and heat budget vary during its orbit and determine new orbital-mean average value for the bolometric Bond albedo of 𝐴∗ = 0.349 ± 0.016 and for the absorbed solar flux of 𝑃in = 0.604 ± 0.027Wm−2. Assuming the outgoing thermal flux to be 𝑃out = 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.