Lost! A giant belt of brown clouds big enough to swallow Earth twenty times over…

May 20, 2010: In a development that has transformed the appearance of the solar system's largest planet, one of Jupiter's two main cloud belts has completely disappeared.

"This is a big event," says planetary scientist Glenn Orton of NASA's Jet Propulsion Lab. "We're monitoring the situation closely and do not yet fully understand what's going on." Known as the South Equatorial Belt (SEB), the brown cloudy band is twice as wide as Earth and more than twenty times as long. The loss of such an enormous "stripe" can be seen with ease halfway across the solar system.

 In any size telescope, or even in large binoculars, Jupiter's signature appearance has always included two broad equatorial belts. But, anyone who turns their telescope on Jupiter at the moment, however, will see a planet with only one belt - a very strange sight! Many astronomers noticed the belt fading late last year, but certainly didn't expect to see it completely disappear. It is believed that the belt is not actually gone, but may be just hiding underneath some higher clouds. Without the SEB present, Jupiter's Great Red Spot is surrounded by almost uninterrupted white. 

It's possible that some 'ammonia cirrus' has formed on top of the SEB, hiding the SEB from view. On Earth, white wispy cirrus clouds are made of ice crystals. On Jupiter, the same sort of clouds can form, but the crystals are made of ammonia (NH₃) instead of water (H₂0). What would trigger such a broad outbreak of "ammonia cirrus"? Glenn Orton (NASA's Jet Propulsion Lab), suspects that changes in global wind patterns have brought ammonia-rich material into the clear, cold zone above the SEB, setting the stage for formation of the high-altitude, icy clouds. Indeed, Jupiter's atmosphere is a mysterious place which would benefit from exploration. No one knows, for instance, why the Great Red Spot is red - or what has sustained the raging storm for so many years. Neither does theory explain why the twin equatorial belts are brown, nor why one should vanish while the other remains.

 This isn't the first time the SEB has faded out. "The SEB fades at irregular intervals, most recently in 1973-75, 1989-90, 1993, 2007, 2010," says John Rogers, director of the British Astronomical Association's Jupiter Section. "The 2007 fading was terminated rather early, but in the other years the SEB was almost absent, as at present."

 The return of the SEB can be dramatic. "We can look forward to a spectacular outburst of storms and vortices when the 'SEB Revival' begins," says Rogers. "It always begins at a single point, and a disturbance spreads out rapidly around the planet from there, often becoming spectacular even for amateurs eyeballing the planet through medium-sized telescopes. However we can't predict when or where it will start. On historical precedent it could be any time in the next 2 years. We hope it will be in the next few months so that everyone can get a good view. Anyone could be the first to spot the return of the SEB.

Credits: NASA Science News

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