The Global Atmospheric Circulation of Saturn

Abstract

Over the past decade, the Cassini spacecraft has provided an unprecedented observational record of the atmosphere of Saturn, which in many ways now surpasses Jupiter as the best-observed giant planet. These observations, along with data from the Voyager spacecraft and Earth-based telescopes, demonstrate that Saturn, like Jupiter, has an atmospheric circulation dominated by zonal (east-west) jet streams, including a broad, fast eastward equatorial jet and numerous weaker jets at higher latitudes. Imaging from Voyager, Cassini, and groundbased telescopes also document a wide range of tropospheric features, including vortices, waves, turbulence, and moist convective storms. At large scales, the clouds, ammonia gas, and other chemical tracers exhibit a zonally banded pattern whose relationships to the zonal jets remains poorly understood. Infrared observations constrain the stratospheric thermal structure and allow the derivation of stratospheric temperatures; these exhibit not only the expected seasonal changes but a wealth of variations that are likely dynamical in origin and highlight dynamical coupling between the stratosphere and the underlying troposphere. In parallel to these observational developments, significant advances in theory and modeling have occurred over the past decade, especially regarding the dynamics of zonal jets, and we survey these new developments in the context of both Jupiter and Saturn. Highly idealized two-dimensional models illuminate the dynamics that give rise to zonal jets in rapidly rotating atmospheres stirred by convection or other processes, while more realistic three-dimensional models of the atmosphere and interior are starting to identify the particular conditions under which Jupiterand Saturn-like flows—including the fast equatorial superrotation, multiple jets at higher latitudes, storms, and vortices—can occur. Future data analysis and models have the potential to greatly increase our understanding over the next decade.