Properties of scatterers in the troposphere and lower stratosphere of Uranus based on Voyager imaging data

Abstract

We have used photometrically and geometrically corrected Voyager images of Uranus to define spatially resolved specific intensities over a wide range of phase angles for two latitude bands and have modeled these data with scalar and vector radiative transfer and microphysical models. Our scattering model included photo-chemically produced hydrocarbon ices in the stratosphere and upper troposphere, a methane ice cloud from 1.2–1.3 bar, and an optically thick hydrogen sulfide cloud at 3 bars. We find that the methane cloud has an optical depth of 0.7 at 22.5°S and an optical depth of 2.4 at 65°S. The mean particle size in the methane cloud is ⪡10 μm for the most likely particle shapes and is probably about 1 μm. The volume absorption coefficient of the methane cloud particles is about 50% higher at 22.5°S than at 65°S, assuming the mean cloud particle size is the same at both latitudes. The mass production rate of the stratospheric hydrocarbon ice components is about 10 −16 g cm −2 sec −1, and the average particle charge in the stratosphere is around 10 electrons per micrometer diameter. The imaginary part of the stratospheric haze refractive index is ∼0.01–0.001, and there are ∼10 precipitable nanometers of non-volatile absorbing haze residue per kilometer-amagat of gas between 1.3 and 3 bar.