Credit: © 2008 AAAS

Vortices are common features of planetary atmospheres, and a multitude of different variants can be found in the solar system.

On Earth, hurricanes move swiftly over the oceans, sucking up thermal energy as they go along, and decay shortly after landfall. By contrast, the polar vortices in the Arctic and Antarctic regions are stationary and persistent, sufficiently so to isolate the polar air masses and support the formation of the southern ozone hole. The polar vortices on Venus come as dipoles with a warm centre, and Jupiter's Great Red Spot and white ovals are anti-cyclones — that is, they rotate in the opposite sense from terrestrial storm systems. Images taken by the Cassini mission's imaging science subsystem on 11 October 2006 suggest that some of these characteristics are combined in a unique way in the south polar vortex on Saturn (Science 319, 1801; 2008).

The upper atmosphere above the eye of the vortex (red in the image) is free of clouds. The eye wall clouds (green) are reminiscent of terrestrial hurricanes and appear to reach up to the tropopause — the boundary between the lower and upper atmosphere of the planet. Measurements from Cassini's Composite Infrared Spectrometer show that the core of the vortex is warmer than the surrounding region, by 3–5 K depending on altitude, and that it is centred around a low-pressure system.

Saturn's polar vortex appears like a stationary, larger version of a terrestrial hurricane: an interesting, and beautiful, beast.