Researchers at the University of the Basque Country say they've been monitoring a cyclone on Saturn for more than five years, making it the longest-lasting cyclone ever found on any of the giant planets.
"Cyclones - where the wind turns in the same direction as the planet - do not usually last for a long time, and so we were interested to discover one that had gone on for several years on Saturn," said researcher Teresa del Río-Gaztelurrutia.
The team started to track the cyclone in 2004, when the Cassini spacecraft started to send back images from Saturn. It's as big as the European continent, with a vortex more than 4,000 kilometres across.
It's been difficult to measure the internal winds of the vortex because of the poor contrast of the images, but the researchers have been able to detect that they're not very intense compared with the movement of the cyclone itself. This is traveling at 245 kmh, dragged by a strong jet stream, while the maximum speed of the winds around its edge is 72 kmh.
"Another reason why it attracted our attention was its visual appearance, which is very reminiscent of the Great Dark Spot on Neptune, which - like the Great Red Spot on Jupiter - is an anticyclone", says del Río-Gaztelurrutia.
The winds of anticyclones turn against the direction of the planet's rotation, and are much more stable, meaning that their vortices tend to last much longer than those of cyclones.
In contrast with the large and long-lasting anticyclones on Jupiter, the cyclone on Saturn has a weak circulation, with properties very similar to those in its immediate environment.
The scientists say they are now waiting impatiently for the data from 2010 so they can find out how the disturbance has evolved over the past year. NASA releases the images from the Cassini probe with a time lag of a year.