The hexagon is about the size of Jupiter's Great Red Spot, a feature that would cover two to three Earths side by side, Dr. Ingersoll explained in an interview.
Saturn's polar storm is small compared with Jupiter's Great Red Spot, the current record-holder for largest storm on any planet in the solar system. That storm sits in Jupiter's southern hemisphere at a latitude comparable to the central coast of Queensland in Australia, where tropical cyclones frequently make landfall. Winds speeds for the Great Red Spot hover at around 400 miles an hour.
Astronomers have observed Jupiter's ruddy blemish regularly since 1831, although some records hint at an initial observation in the 1600s. Indeed, it may be a permanent feature in the giant planet's atmosphere.
At Saturn, NASA's Voyager missions identified the hexagon feature surrounding Saturn's north polar region some 30 years ago. It was up to Cassini to spot the storm. The craft recorded initial evidence for the hurricane shortly after the craft arrived at Saturn in 2004. Saturn's north pole was cloaked in the darkness of a northern-hemisphere winter that lasts seven years, so the first evidence emerged from infrared measurements. The Cassini team had to wait until spring arrived to gather stunning visual images of the storm.
Studies of Jupiter's Great Red Spot indicate that the energy driving the storm comes from water vapor, as it does with Earth's tropical cyclones. On Saturn, the energy source for the polar storm is a mystery. Seventy-five percent of the planet's atmosphere is hydrogen, roughly 25 percent is helium, along with tiny quantities of other gases and water.
Two energy sources come to mind, Ingersoll says. One is sunlight, which at Saturn's distance is relatively weak compared with the amount of the sun's energy reaching the five planets closer in to the sun. The other energy source is the planet's internal heat, left over from the planet's formation some 4.6 billion years ago.