Infrared Observations of the Uranian System

The infrared interferometer spectrometer (IRIS) on Voyager 2 recorded thermal emission spectra of Uranus between 200 and 400 cm^-1 and of Miranda and Ariel between 200 and 500 cm^-1 with a spectral resolution of 4.3 cm^-1. Reflected solar radiation was also measured with a single-channel radiometer sensitive in the visible and near infrared. By combining IRIS spectra with radio science results, a mole fraction for atmospheric helium of 0.15 ± 0.05 (mass fraction, 0.26 ± 0.08) is found. Vertical temperature profiles between 60 and 900 millibars were derived from average polar and equatorial spectra. Temperatures averaged over a layer between 400 to 900 millibars show nearly identical values at the poles and near the equator but are 1 or 2 degrees lower at mid-latitudes in both hemispheres. The cooler zone in the southern hemisphere appears darker in reflected sunlight than the adjacent areas. An upper limit for the effective temperature of Uranus is 59.4 kelvins. Temperatures of Miranda and Ariel at the subsolar point are 86 ± 1 and 84 ± 1 kelvins, respectively, implying Bond albedos of 0.24 ± 0.06 and 0.31 ± 0.06, respectively. Estimates of phase integrals suggest that these satellites have unusual surface microstructure.