One of Jupiter's dark brown stripes that faded out last spring is regaining its color, providing an unprecedented opportunity for astronomers to observe a rare and mysterious phenomenon caused by the planet's winds and cloud chemistry.
In early 2010, amateur astronomers noticed that the long-standing stripe, known as the South Equatorial Belt (SEB), just south of Jupiter's equator, had turned white. In early November 2010, amateur astronomer Christopher Go of Cebu City in the Philippines observed a prominent bright spot in the unusually whitened belt, piquing the interest of professional and amateur astronomers around the world.
After follow-up observations with NASA's Infrared Telescope Facility (IRTF), the 10 m Keck telescope, and the 8 m Gemini telescope--all atop Mauna Kea in Hawaii--scientists at the University of California (UC), Berkeley and elsewhere now believe the stripe is making a comeback.
"The reason Jupiter seemed to 'lose' this band--camouflaging itself among the surrounding white bands--is that the usual downwelling winds that are dry and keep the region clear of clouds died down," says Glenn Orton, a research scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. "One of the things we were looking for in the infrared was evidence that the darker material appearing in visible light was actually the start of clearing in the cloud deck, and that is precisely what we saw."
This white cloud deck is made up of white ammonia ice. When the white clouds float at a higher altitude, they obscure the view of the lower brown clouds. Every few decades or so, the SEB turns completely white for perhaps one to three years--an event that has puzzled scientists for decades. This extreme change in appearance has only been seen with the SEB, making it unique to Jupiter and to the entire solar system.
The bright storm that Go observed in the faded belt was quite unusual, says Imke de Pater, UC Berkeley professor of astronomy.
"At infrared wavelengths, images in reflected sunlight show that the spot is a tremendously energetic 'outburst,' a vigorous storm that reaches extreme high altitudes," de Pater says. …