Explaining saturn's great white spots.

• Position: Planetary Thunderstorms

Every 20 to 30 years, Saturn's atmosphere roils with giant, planet-encircling thunderstorms that produce intense lightning and enormous cloud disturbances. The head of one of these storms--popularly called "great white spots," in analogy to the Great Red Spot of Jupiter--can be as large as Earth. Unlike Jupiter's spot, which is calm at the center and has no lightning, the Saturn spots are active in the center and have long tails that eventually wrap around the planet.

Six such storms have been observed on Saturn over the past 140 years, alternating between the equator and midlatitudes, with the most recent emerging in December 2010 and encircling the planet within six months. The storms usually occur when Saturn's northern hemisphere is most tilted toward the sun. Just what triggers them and why they occur so infrequently, however, has been unclear.

Now, a study by two planetary scientists from the California Institute of Technology, Pasadena, suggests a possible cause. Using numerical modeling, Andrew Ingersoll and graduate student Cheng Li simulated the formation of the storms and found that they may be caused by the weight of the water molecules in the planet's atmosphere.

Because these water molecules are heavy compared to the hydrogen and helium that comprise most of the gas-giant planet's atmosphere, they make the upper atmosphere lighter when they rain out, and that suppresses convection. Over time, this leads to a (cooling of the upper atmosphere, but that cooling eventually overrides the suppressed convection, and warm, moist air rises rapidly and triggers a thunderstorm. 'The upper atmosphere is so cold and so massive that it takes 20 to 30 years for this cooling to trigger...