Skygazers here are watching one of Jupiter's dark brown stripes that faded out last spring slowly regain its color, tipped off by an amateur astronomer from the central Philippines.
In early November, amateur astronomer Christopher Go of Cebu City observed a prominent bright spot in the unusually whitened belt, piquing the interest of professional and amateur astronomers around the world. Amateur astronomers had earlier noticed that the long-standing stripe, known as the South Equatorial Belt (SEB) just south of Jupiter's equator, had turned white.
Now astronomers can observe a rare and mysterious phenomenon caused by the planet's winds and cloud chemistry, thanks to Go, of the Physics Department of the University of San Carlos.
After follow-up observations with NASA's Infrared Telescope Facility, the 10-meter Keck telescope and the 8-meter Gemini telescope, all atop Mauna Kea in Hawaii, scientists at the University of California, Berkeley, and elsewhere now believe the stripe is making a comeback.
Astronomers announced first-glimpse images of the reappearing stripe Nov. 24.
"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," said Glenn Orton, a research scientist at NASA's Jet Propulsion Laboratory (JPL) 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 South Equatorial Belt 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 South Equatorial Belt, making it unique to Jupiter and to the entire solar system.
The bright storm that Go observed in the faded belt was quite unusual, said 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 said. "The storms are surrounded by darker areas, bluish-grey in the visible, indicative of `clearings' in the cloud deck."
"I was fortunate to catch the outburst," Go said. "I had a meeting that evening, and it went late. I caught the outburst just in time as it was rising. Had I imaged earlier, I would not have caught it."
Go witnessed the disappearance of the stripe earlier this year, and in 2007 he was the first to catch the stripe's return. "I was able to catch it early this time around because I knew exactly what to look for," he said.
To confirm the presence of such clearings, the team obtained data at longer wavelengths (5 micron) sensitive to thermal emission from Jupiter's deep atmosphere. These data confirm that the visibly dark material indeed is being seen through holes in the cloud deck, "perhaps signaling the start of the SEB revival," added Orton.
The white band wasn't the only change on the big, gaseous planet. At the same time, Jupiter's Great Red Spot became a darker red color. Orton said the color of the spot – a giant storm on Jupiter that is three times the size of Earth and a century or more older – will likely brighten a bit again as the SEB makes its comeback.
The SEB underwent a slight brightening, known as a "fade," just as NASA's New Horizons spacecraft was flying by on its way to Pluto in 2007. Then there was a rapid "revival" of its usual dark color three to four months later. The last full fade and revival was a double-header event, starting with a fade in 1989, revival in 1990, then another fade and revival in 1993. Similar events have been captured visually and photographically back to the early 20th century, and they are likely to be a long-term phenomenon in Jupiter's atmosphere.
Scientists are particularly interested in this event because it's the first time they've been able to use modern instruments to determine the details of the chemical and dynamical changes of this phenomenon.
"These observations may help to unravel the mystery of why this transition occurs, and may allow us to understand the longevity of Jupiter's belt/zone structure," added Leigh Fletcher, a scientist at Oxford University in England.
The event also signifies another close collaboration between professional and amateur astronomers. The amateurs, located worldwide, are often well equipped with instrumentation and are able to track the rapid developments of planets in the solar system. These amateurs are collaborating with professionals to further study the changes that are of great value to scientists and researchers everywhere.
Since the discovery of the first spot, there have been several more outbreaks of varying strengths. The SEB revival is happening fast, with violent eruptions, de Pater said.
Observing this event carefully may help to refine the scientific questions that will be posed by NASA's Juno spacecraft, due to arrive at Jupiter in 2016, and a larger mission to orbit Jupiter and explore its satellite Europa after 2020.
The observations were conducted by a large team of observers around the world. At UC Berkeley, the following researchers are involved: Imke de Pater, Michael Wong, James Graham, Shelley Wright and Franck Marchis. More observations at near- and mid-infrared wavelengths.