The bolometric Bond albedo and energy balance of Uranus

Abstract

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 $A^* = 0.338 \pm 0.011$, with a phase integral of $q^* = 1.36 \pm 0.03$. 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 $\overline{A^*} = 0.349 \pm 0.016$ and for the absorbed solar flux of $\overline{P_\mathrm{in}}=0.604 \pm 0.027$ W m$^{-2}$. Assuming the outgoing thermal flux to be $\overline{P_\mathrm{out}}=0.693 \pm 0.013$ W m$^{-2}$, as previously determined from Voyager 2 observations, we arrive at a new estimate of Uranus's average heat flux budget of $P_\mathrm{out}/P_\mathrm{in} = 1.15 \pm 0.06$, finding considerable variation with time due to Uranus's significant orbital eccentricity of 0.046. This leads the flux budget to vary from $P_\mathrm{out}/P_\mathrm{in} = 1.03$ near perihelion, to 1.24 near aphelion. We conclude that although $P_\mathrm{out}/P_\mathrm{in}$ is considerably smaller than for the other giant planets, Uranus is not in thermal equilibrium with the Sun.