Infrared Observations of the Saturnian System from Voyager 1

Abstract

During the passage of Voyager 1 through the Saturn system, the infrared instrument acquired spectral and radiometric data on Saturn, the rings, and Titan and other satellites. Infrared spectra of Saturn indicate the presence of H 2 , CH 4 , NH 3 , PH 3 , C 2 H 2 , C 2 H 6 , and possibly C 3 H 4 and C 3 H 8 . A hydrogen mole fraction of 0.94 is inferred with an uncertainty of a few percent, implying a depletion of helium in the atmosphere of Saturn relative to that of Jupiter. The atmospheric thermal structure of Saturn shows hemisphere asymmetries that are consistent with a response to the seasonally varying insolation. Extensive small-scale latitudinal structure is also observed. On Titan, positive identifications of infrared spectral features are made for CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , and HCN; tentative identifications are made for C 3 H 4 and C 3 H 8 . The infrared continuum opacity on Titan appears to be quite small between 500 and 600 cm –1 , implying that the solid surface is a major contributor to the observed emission over this spectral range; between 500 and 200 cm –1 the opacity increases with decreasing wave number, attaining an optical thickness in excess of 2 at 200 cm –1 . Temperatures near the 1-millibar level are independent of longitude and local time but show a decrease of ∼ 20 K between the equator and north pole, which suggests a seasonally dependent cyclostrophic zonal flow in the stratosphere of ∼ 100 meters per second. Measurements of the C ring of Saturn yield a temperature of 85 ± 1 K and an infrared optical depth of 0.09 ± 0.01. Radiometer observations of sunlight transmitted through the ring system indicate an optical depth of 10 –1.3 ±0.3 for the Cassini division. A phase integral of 1.02 ± 0.06 is inferred for Rhea, which agrees with values for other icy bodies in the solar system. Rhea eclipse observations indicate the presence of surface materials with both high and low thermal inertias, the former most likely a blocky component and the latter a frost.