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 $A^{\ast }=0.338\pm 0.011$, with a phase integral of $q^{\ast }=1.36\pm 0.03$. 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 $\overline{A^{\ast }}=0.349\pm 0.016$ 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.

11 pages, 5 figures