Infrared Observations of the Neptunian System

Abstract

The infrared interferometer spectrometer on Voyager 2 obtained thermal emission spectra of Neptune with a spectral resolution of 4.3 cm^-1. Measurements of reflected solar radiation were also obtained with a broadband radiometer sensitive in the visible and near infrared. Analysis of the strong C₂H₂ emission feature at 729 cm^-1 suggests an acetylene mole fraction in the range between 9 x 10^-8 and 9 x 10^-7. Vertical temperature profiles were derived between 30 and 1000 millibars at 70° and 42°S and 30°N. Temperature maps of the planet between 80°S and 30°N were obtained for two atmospheric layers, one in the lower stratosphere between 30 and 120 millibars and the other in the troposphere between 300 and 1000 millibars. Zonal mean temperatures obtained from these maps and from latitude scans indicate a relatively warm pole and equator with cooler mid-latitudes. This is qualitatively similar to the behavior found on Uranus even though the obliquities and internal heat fluxes of the two planets are markedly different. Comparison of winds derived from images with the vertical wind shear calculated from the temperature field indicates a general decay of wind speed with height, a phenomenon also observed on the other three giant planets. Strong, wavelike longitudinal thermal structure is found, some of which appears to be associated with the Great Dark Spot. An intense, localizd cold region is seen in the lower stratosphere, which does not appear to be correlated with any visible feature. A preliminary estimate of the effective temperature of the planet yields a value of 59.3 ± 1.0 kelvins. Measurements of Triton provide an estimate of the daytime surface temperature of 38⁺³_-4 kelvins.