High-Phase-Angle Observations of Uranus at 2650 Å: Haze Structure and Particle Properties

Abstract

Spatially resolved photometric observations of Uranus at 2650 Å obtained with the Voyager 2 Photopolarimeter Subsystem (PPS) are presented and analyzed with model atmospheres. A vertically homogenous atmospheric model with Rayleigh scattering and absorption fits Uranus well at 2650 Å at phase angles from 16° to 157°; no strong forward scattering typical of abundant large particles is observed. A microphysical model developed for Uranus to fit Voyager imaging system data (K. Rages, J. B. Pollack, M. G. Tomasko, and L. R. Doose, 1991,Icarus89, 359–376) predicts the vertical distribution and properties of stratospheric haze aerosols at 22.5° and 65°S. This microphysical model also fits the 2650-Å PPS data at low latitudes. Increasing the aerosol sizes in the Rageset al.size distribution by a factor of 1.2 creates an unobserved forward scattering peak and can be excluded. The low-latitude upper stratospheric aerosols on Uranus are smaller (<0.1 μm radius) and/or much less abundant than the 0.2 to 0.25-μm stratospheric aerosol population found by PPS on Neptune; however, to fit the Uranus 2650-Å data at all latitudes requires darker and/or more abundant (1.5–3 times more) aerosols near the sunlit pole.