A cyclone the size of Europe lashed Saturn for five years, becoming the longest-lasting storm ever seen on the solar system's great planets, scientists say.
The cyclone, with a vortex 4000 kilometres wide, is being tracked by Spanish scientists with images from the Cassini spacecraft.
"Our observations make this cyclone the longest-lasting one ever seen on the giant planets of the solar system, Jupiter and Saturn," says the report's lead author, Teresa del Rio-Gaztelurrutia.
The discovery is surprising, for cyclones - where the wind spins in the same direction as the planet - usually do not last long, said the researcher, who led a team from the University of the Basque Country.
"We still know very little about these kinds of structures," she says.
Scientists began to track the cyclone in 2004 when Cassini beamed back first images of the planet.
They managed to analyse the horizontal and vertical structure of the cyclone, its circulation and its interaction with winds, using mathematical simulations.
Not very intense
Despite its colossal size, the scientists detected "not very intense" winds.
The whirlwind was moving at 245 kilometres per hour, dragged by a strong jet stream, while the maximum speed of the winds around its edge was a leisurely 72 kilometres per hour.
Anticyclones, which form the Great Red Spot on Jupiter and the Great Dark Spot on Neptune, are much more stable than cyclones and last for much longer periods.
NASA releases images from Cassini with a one-year delay, so the scientists have only seen evidence for 2009 and are waiting to see if the mammoth cyclone has survived into this year.
Saturn, Jupiter, Neptune and Uranus are so-called gas giants that orbit the Sun beyond the asteroid belt.
They are massive planets that are believed to have a small solid core, enveloped by a thick atmosphere of suffocating gases such as helium, hydrogen, methane and ammonia.