Imagine a thunderstorm so massive that its dark outline wrapped around the entire planet.
Such terrifying "megastorms" are common on Saturn. Also called "Great White Spots," they erupt once every 20 or 30 years in the planet's northern hemisphere and rage nonstop for months. Astronomers have spotted six of these planet-wide storms whipping around on Saturn since 1876. The most recent storm struck in December 2010, when NASA's Cassini spacecraft happened to be orbiting the planet, snagging a front-row view of the megastorm's entire 200-day life span.
