Motions in Saturn's atmosphere: Observations before Voyager encounters

Abstract

A summary is presented of all the telescopic observations of the clouds in Saturn's atmosphere made prior to the Voyager encounters with the planet. Saturn displays a pattern of belts and zones like Jupiter, although they are practically constant in latitude and without significant changes over the last century. Some dark and white spots with diameters of about 6000 to 10,000 km have been observed, allowing us to ascertain the atmospheric rotational period. Only a strong equatorial jet can be inferred from these measurements. The most obvious features of Saturn are the great white spots (GWS) with diameters of 20,000 km or more, but only five outbursts have been detected since 1876. Corrected measurements have been obtained of the zonal velocities at different latitudes, u(φ), with variations between 430 and −14 msec−1 with respect to the magnetic rotation of the planet [M. L. Kaiser, M. D. Desch, J. W. Warwick, and J. B. Pierce, Science 209, 1238–1240 (1980)]. The expansion velocities of some GWS have been measured to be u′ ∼ 30 to 100 msec−1 and their lifetimes, τL ∼ from 30 to 150 days. A possible subatmospheric origin for the GWS is proposed, in which it is due to a temporary and local anisotropy in the internal heat source (a “hot spot” in the deeper atmosphere). All the multiple features seen in both 1903 and 1933 appear to be linked to a unique system of longitudes rotating with the magnetic period.