Radio Science with Voyager 2 at Saturn: Atmosphere and Ionosphere and the Masses of Mimas, Tethys, and Iapetus

Abstract

Voyager 2 radio occultation measurements of Saturn's atmosphere probed to the 1.2-bar pressure level, where the temperature was 143 ± 6 K and the lapse rate apparently equaled the dry adiabatic value of 0.85 K per kilometer. The tropopause at both mid-latitude occultation locations (36.5°N and 31°S) was at a pressure level of about 70 millibars and a temperature of approximately 82 K. The stratospheric structures were very similar with the temperature rising to about 140 K at the 1-millibar pressure level. The peak electron concentrations sensed were 1.7 x 10⁴ and 0.64 x 10⁴ per cubic centimeter in the predawn (31°S) and late afternoon (36.5°N) locations. The topside plasma scale heights were about 1000 kilometers for the late afternoon profile, and 260 kilometers for the lower portions and 1100 kilometers for the upper portions of the topside predawn ionosphere. Radio measurements of the masses of Tethys and Iapetus yield (7.55 ± 0.90) x 10²⁰ and (18.8 ± 1.2) x 10²⁰ kilograms respectively; the Tethys-Mimas resonance theory then provides a derived mass for Afimas of (0.455 ± 0.054) x 10²⁰ kilograms. These values for Tethys and Mimas represent major increases from previously accepted ground-based values, and appear to reverse a suggested trend of increasing satellite density with orbital radius in the Saturnian system. Current results suggest the opposite trend, in which the intermediate-sized satellites of Saturn may represent several classes of objects that differ with respect to the relative amounts of water, ammonia, and methane ices incorporated at different temperatures during formation. The anomalously low density of lapetus might then be explained as resulting from a large hydrocarbon content, and its unusually dark surface markings as another manifestation of this same material.