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Jupiter's famous feature is suffering a Spot of sunburn

Jenny Winder, News Writer
Nov 15, 2014, 17:17 UTC

Sen—Giant planet Jupiter is famous for a feature called the Great Red Spot, a constantly raging storm in its upper clouds. New analysis of data from the Cassini mission suggests that this huge feature, which would dwarf our own planet, gets its colour from simple chemicals being broken apart by sunlight in the planet's upper atmosphere.

This new finding contradicts the earlier, competing theory that the reddish chemicals come from deep beneath Jupiter's cloud tops.

Jupiter is composed of hydrogen and helium, with just a sprinkling of other elements. Understanding what combinations of elements are responsible for the hues seen in Jupiter's clouds would provide insights into the giant planet's make-up. 

The Great Red Spot is a long-lived feature in Jupiter's atmosphere that is as wide as two Earths. Jupiter possesses three main cloud layers, which occupy specific altitudes in its skies From highest to lowest they are ammonia, ammonium hydrosulfphide and water clouds.

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This true colour mosaic of Jupiter was constructed from images taken by the narrow angle camera onboard NASA's Cassini spacecraft on December 29, 2000, during its closest approach to the giant planet at a distance of approximately 10 million kilometres (6.2 million miles). Image credit: NASA/JPL/Space Science Institute

Kevin Baines, a Cassini team scientist, and colleagues Bob Carlson and Tom Momary, based at NASA's Jet Propulsion Laboratory, arrived at their conclusions using a combination of data from Cassini's December 2000 Jupiter flyby and laboratory experiments.

In the lab they initially set out to determine if the Great Red Spot's colour might derive from sun-induced breakdown of a more complex molecule, ammonium hydrosulphide, which makes up one of Jupiter's main cloud layers. They quickly found that the products their experiment produced were a brilliant shade of green.

They then blasted ammonia and acetylene gases, which are known to exist on Jupiter, with ultraviolet light, to simulate the Sun's effects on these materials at the extreme heights of clouds in the Great Red Spot. This produced a reddish material, which the team compared to the Great Red Spot as observed by Cassini's Visible and Infrared Mapping Spectrometer (VIMS). 

The light-scattering properties of their concoction matched a model of the Great Red Spot with the red-coloured material confined to the uppermost reaches of the giant cyclone-like feature.

"Our models suggest most of the Great Red Spot is actually pretty bland in colour, beneath the upper cloud layer of reddish material," said Baines. "Under the reddish 'sunburn' the clouds are probably whitish or grayish." 

If that red material were being transported from below it should be present at other altitudes as well, which would make the red spot redder still.

The intense red colour is seen only in the Great Red Spot and a few much smaller spots on the planet, and the researchers think altitude plays a key role. "The Great Red Spot is extremely tall," Baines said. "It reaches much higher altitudes than clouds elsewhere on Jupiter."

The spot's height both enables and enhances the reddening. Its winds transport ammonia ice particles higher into the atmosphere than usual, where they are exposed to much more of the Sun's ultraviolet light. The vortex nature of the spot also confines particles, preventing them from escaping and causing the redness of the spot's cloud tops to increase.

Other areas of Jupiter display a mixed palette of oranges, browns and even shades of red. Baines says these are places where high, bright clouds are known to be much thinner, allowing views to depths in the atmosphere where more colourful substances exist.