Spectacular Storm Rages On Saturn's South Pole

A hurricane-like storm two-thirds as wide as the Earth is raging on Saturn's south pole, new images from the Cassini spacecraft reveal. Such clear hurricane-like features have never before been seen on any other planet, but scientists are not sure what is causing them.

The dark eye of the "hurricane" spans about 8000 kilometres and is surrounded by rings of clouds that tower about 30 to 75 kilometres above it.

These eye-wall clouds have never been seen anywhere other than on Earth, where they form in a process of convection when moist air flows across an ocean and rises. They drop rain in a ring around a region of falling air, which is the eye of a hurricane.

But Saturn's storm also differs from hurricanes on Earth because it is fixed in place – above the south pole – and is not powered by an ocean, since Saturn is a gaseous planet.

"It looks like a hurricane, but it doesn't behave like a hurricane," says Andrew Ingersoll, a member of Cassini's imaging team at Caltech in Pasadena, US. "Whatever it is, we're going to focus on the eye of the storm and find out why it's there."

It is unclear how long the storm has been there because Cassini has never before seen the pole at such a high resolution. And scientists are still puzzling over how it formed.

"I don't think we really have a good idea of what's sourcing it," says Cassini team member Richard Achterberg, a planetary scientist specialising in atmospheres at NASA's Goddard Space Flight Center in Greenbelt, Maryland, US.

The storm may be a seasonal phenomenon that changes over the course of Saturn's year, which lasts 29 Earth years. It is currently summertime in the planet's southern hemisphere, and both ground-based observations and higher-resolution data from Cassini's Composite Infrared Spectrometer (CIRS) show the pole is about 2° Celsius warmer than its immediate surroundings.

Mission members will try to observe the storm again with CIRS to study the chemical composition of the atmosphere at the pole. "We're going to see if there's water or something being dragged up from below," Achterberg told New Scientist. In particular, the team will look for changes in the storm over the next few years, as the southern hemisphere moves into autumn.

Achterberg says the storm appears to be unique in the solar system. Jupiter's famous storm, the Great Red Spot, for example, does not have an eye or any surrounding eye-wall clouds. It also drifts slowly around the planet with the winds and is colder than its surroundings, while Saturn's storm is warmer.

Whatever its cause, Achterberg says the most striking thing about the storm is simply its appearance. "When you look at it, the shape of those clouds surrounding the pole ... it's an amazing image."