Be prepared for a surprise the next time you look at Jupiter through a telescope. One of the Jovian giant's two main dark cloud belts has completely disappeared. This surprising development has changed the appearance of the solar system's largest planet.

Jupiter's signature appearance through any size telescope usually includes two broad reddish-brown equatorial belts and lighter, more bluish bands called zones.

Jupiter's belts and zones are created by chemicals that have formed at different heights. Planetary scientists believe that the higher zones are composed of crystals of frozen ammonia, while the lower belts are created from chemicals including sulphur and phosphorus.

Recent images now reveal that Jupiter's dark lower stripe, known as the South Equatorial Belt, has now disappeared. This belt is twice as wide as Earth and more than 20 times as long.

Astronomers think the belt is not actually gone but may just be hidden underneath some higher clouds. They believe that an ammonia cirrus cloud has formed on top of the South Equatorial Belt, hiding the dark belt 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 instead of water.

Scientists think that changes in Jupiter's global wind patterns may have brought ammonia-rich material into the clear, cold zone above the South Equatorial Belt, setting the stage for formation of the high-altitude, icy clouds.

This is not the first time that Jupiter's South Equatorial Belt has disappeared. The belt fades at irregular intervals, most recently in 1989-90, 1993 and 2007.

According to planetary scientists, the return of the South Equatorial Belt can be dramatic. We can expect a spectacular outburst of storms and vortices when the belt's revival begins. Scientists, however, can't predict when or where it will start. It could be any time in the next two years.

-- By Dan Malerbo, Buhl Planetarium and Observatory

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