Jupiter gets its stripe back

Jupiter's South Equatorial Belt

This November 18 Gemini North Telescope image of Jupiter combines blue, red and yellow images into a false-colour composite that clearly shows the storm in the South Equatorial Belt. The belt is now turning dark after a brief fade to white.

One of Jupiter’s dark brown cloud bands that faded out earlier this year is regaining its colour, providing an unprecedented opportunity for astronomers to observe a rare and mysterious phenomenon caused by the planet’s winds and cloud chemistry.

Earlier this year, amateur astronomers noticed that the long-standing band, known as the South Equatorial Belt (SEB), just south of Jupiter’s equator, had turned white.

But just weeks ago, amateur astronomer Christopher Go of Cebu City in the Philippines saw 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-metre 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.

“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).

One of the things the astronomers were looking for—using telescopes that see infrared wavelengths—was evidence that the darker material appearing in visible light was actually the start of clearing in the cloud deck. And that’s what they saw.

Jupiter's South Equatorial Belt

This false-colour image, taken November 11 by the Keck telescope, shows sunlight reflected off Jupiter's upper cloud deck. The bright spot in the South Equatorial Belt is the outbreak where winds are lofting particles to high altitudes.

This white cloud deck is made up of white ammonia ice particles. 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.

The bright storm that Go saw 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.”

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 colour. Orton said the colour of the spot—a giant storm on Jupiter that is three times the size of Earth and a century or more old—will likely brighten a bit again as the South Equatorial Belt makes its comeback.

Adapted from information issued by JPL, University of Oxford, UC Berkeley, Gemini Observatory, University of San Carlos, Philippines / NASA / ESA / H. Hammel (STScI) / JIT.

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