what makes the Great Red Spot red? And why has it lasted so long? These are some of the questions scientists who have spent years studying Jupiter have not found an answer to. Now Kevin Baines, a planetary scientist at National Aeronautics and Space Research's ( nasa ) Jet Propulsion Laboratory in California, usa , and his colleagues may have forged a "piece of the puzzle'. And even though there are too many such pieces in Jupiter, this could mark the beginning of solving the puzzle ( New Scientist , Vol 163, No 2201).

Infrared spectra from the Galileo spacecraft allowed the team of scientists to peer into the giant planet's complex atmospheric stew. They found that the Great Red Spot, a hurricane-like storm system twice the diameter of Earth and at least 300 years old, is capped by a tilted tower of ammonia clouds about 20 kilometres tall. These clouds form a raised oval about two-thirds the diameter of the base. Says Baines, "It looks like a wedding cake.' Spirals of clouds extend out across the top of the upper tier and deeper layers can be glimpsed through clearings.

"It would have a relatively small region below it of rapidly rising gas rich in water, ammonia and other quite exotic things,' says Fred Taylor, a physicist at Oxford University. "At a certain height, they spread like the cap of a terrestrial hurricane,' he says. Baines thinks the upwelling acts like a rock in the middle of a stream. "You get a bow wave where the east side gets lifted. Then the clouds descend as the core rotates,' he says.

Scientists have put together tentative models of the upper layers of Jupiter's atmosphere