What's a monster hurricane doing on top of Saturn? (+video)

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.

During the northern hemisphere's seven-year long winter, the planet's internal heat probably keeps the storm going. During the northern hemisphere spring and summer, the two sources can combine to spin up the storm for another Saturnian year pirouetting at the top of the planet.

The storm's winds can maintain their momentum in part because nothing lies beneath the storm to sap it of energy. On Earth, friction with land surfaces sap a hurricane's strength, in addition to a cut-off in the ocean warmth and moisture hurricanes need as fuel, Ingersoll says.

The winds maintain their momentum as well because less sunlight reaches the gas giants. This means each gas-giant's atmosphere exhibits successively less turbulence with distance from the sun. That turbulence also can act as a drag on wind flow. This explains why the fastest winds found so far are on Neptune, even farther from the sun than Saturn, Ingersoll says.

The hexagonal polar jet stream also presents a puzzle. On Earth, the polar jet in the Northern Hemisphere interacts with the continents, contributing to its north-south, river-like meanders. Saturn has no continents.

The length and positions of the hexagon's sides can vary slightly with time, the stream of fast-flowing air nevertheless maintains this general shape around the polar region – a six-sided corral for one of the solar system's most powerful storms.

"I wish I had all the answers" to the puzzles the storm and hexagon pose, Ingersoll says. "But that's life in science."

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