Estimates of the bolometric albedos and radiation balance of Uranus and Neptune

Abstract

We have attempted to bound the wavelength-averaged phase integrals and bolometric albedos of Uranus and Neptune by fitting a wide range of aerosol model atmospheres to their observed geometric albedo spectra. These models are characterized by an upper haze layer of finite optical depth and a lower cloud layer of infinite optical depth at discrete altitudes. Alternative models differ in the assumed value of the particles' single scattering phase function and the wavelength dependence of the haze optical depth. Phase functions ranging from isotropic to those characteristic of particles in the atmospheres of Titan, Jupiter, and Saturn are considered. We have partially tested the models of Uranus by comparing the dependence of their disk-integrated brightness on phase angle with that derived from a combination of ground-based and Voyager 1 data that span phase angles from 0 to 85° and by comparing their predicted shapes of several H 2 quadrupole lines with observed shapes. Predictions of the Neptune models were compared with determinations of the planet's disk-integrated brightness from 0 to 48° phase angle. The derived model parameters lie within useful bounds. In the case of Uranus, the cloud pressure for all seven models considered falls between 2.2 and 2.5 bars, implying methane mixing ratios in the deeper portion of the atmosphere that are at least 30 times higher than expected from solar elemental abundances if the cloud is interpreted as being a methane condensation cloud. The range of haze pressure (≤0.5 bars) and optical depth (0.06 to 0.6 at a wavelength of 0.6435 μm) imply that haze aerosols are a significant absorber of sunlight and hence constitute a significant heating source in Uranus's upper troposphere and stratosphere. The haze aerosols absorb strongly at both short and long visible wavelenths, unlike the aerosols in Titan's atmosphere. Qualitatively similar conclusions apply to our model atmospheres of Neptune, with the cloud pressure being somewhat higher than for Uranus (2.7–3.2 bars) and the methane abundance being at least 60 times higher than expected from solar elemental abundances. At the current epoch, the wavelength-averaged phase integrals of Uranus and Neptune equal 1.26 ± 0.11 and 1.25 ± 0.1, respectively. The corresponding bolometric albedos are 0.343 ± 0.055 and 0.282 ± 0.044, respectively. When averaged over an orbital period, these albedos may be 7% lower for Uranus and altered little for Neptune, based on measurements of their secular brightness variability. Comparison of these results with thermal observations implies that the internal heat source for Uranus is less than 0.27 times the solar input (specific luminosity ≤1.6 × 10 −7 erg/g/sec), while this value for Neptune is (1.85 ± 0.56) times its solar input (specific luminosity = 3.4 ± 1.1) × 10 −7 erg/ g/ sec). These results imply that the meteorological regimes in the observable atmospheres of Uranus and Neptune may be very different, with internal heat flux playing a much more important role for Neptune than for Uranus.