The missing clouds are one of two primary bands of clouds circling the giant planet's equatorial region. Twice the width of Earth and 20 times longer, the brown clouds are known as the South Equatorial Belt (SEB).
The SEB has gone missing before, most recently in 2007. Some astronomers suspect the brown clouds may be temporarily blocked from view due to another, higher layer of clouds.
"It's possible some ‘ammonia cirrus' has formed on top of the SEB," Orton said in a NASA website. "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.
He suspects that a change in the planet's winds may have carried ammonia-rich material into the clear, cold zone above the SEB, setting the stage for icy, high-altitude clouds to form.
According to John Rogers, director of the British Astronomical Association's Jupiter Section, when SEB returns, the event can be dramatic.
"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," said Rogers. "However we can't predict when or where it will start."
Image credit: NASA