Jupiter's Great Red Spot has intrigued observers ever since French astronomer Gian Domenico Cassini first sighted it in 1665. The Galileo spacecraft, orbiting Jupiter, took these images at four different infrared wavelengths in September 1996; astronomers analyzed them last spring for the first time. The top image shows light reflecting off clouds high in Jupiter's atmosphere. The next image shows haze in the stratosphere over the Great Red Spot (at the lower left corner) and the North Equatorial Belt. The third shows clouds about 50 miles down into Jupiter's atmosphere. Heat emissions from still deeper in the atmosphere are visible between clouds in the bottom image. The spot resembles a huge terrestrial hurricane, says Fred Taylor, an atmospheric physicist at Oxford who studied the images. Damp air, probably containing water and ammonia, spouts upward from far below the center of the storm, spraying out over the top in a mushroom shape as it condenses into the spot's distinctive red clouds. But why is the Great Red Spot red? "It's difficult to make red clouds; it's an uncommon thing in atmospheres," says Taylor, who hopes two more years' worth of data will solve the riddle. "But it may be that what we need is another probe actually dropping into the Red Spot to make the right kinds of measurements to answer that question."