According to a new study led by Dr Kevin Baines of NASA’s Jet Propulsion Laboratory in Pasadena, the reddish color of the Great Red Spot on Jupiter is likely a product of chemicals being broken apart by solar ultraviolet (UV) light in the planet’s upper atmosphere.

“Our models suggest most of the Great Red Spot is actually pretty bland in color, beneath the upper cloud layer of reddish material,” Dr Baines said.

“Under the reddish ‘sunburn,’ the clouds are probably whitish or grayish.”

“A coloring agent confined to the top of the clouds would be inconsistent with the competing theory, which posits that the spot’s red color is due to upwelling chemicals formed deep beneath the visible cloud layers. If red material were being transported from below, it should be present at other altitudes as well, which would make the red spot redder still.”

In the lab, Dr Baines and his colleagues blasted ammonia and acetylene gases – chemicals known to exist on Jupiter – with UV 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 scientists compared to the Great Red Spot as observed by the Visual Infrared Mapping Spectrometer (VIMS) on NASA’s Cassini spacecraft.

They found that the light-scattering properties of their red concoction nicely matched a model of the Great Red Spot in which the red-colored material is confined to the uppermost reaches of the giant cyclone-like feature.

The team initially set out to determine if the Great Red Spot’s color might derive from Sun-induced breakdown of a more complex molecule, ammonium hydrosulfide, which makes up one of Jupiter’s main cloud layers.

They quickly found that instead of a red color, the products their experiment produced were a brilliant shade of green.

This surprising negative result prompted them to try simple combinations of ammonia with hydrocarbons that are common at Jupiter’s high altitudes.

Breaking down ammonia and acetylene with UV light turned out to best fit the data collected by Cassini.

As for why the intense red color is seen only in the Great Red Spot and a few much smaller spots on the planet, the scientists think altitude plays a key role.

“The Great Red Spot is extremely tall. It reaches much higher altitudes than clouds elsewhere on Jupiter,” Dr Baines said.

He and his colleagues think the spot’s great heights both enable and enhance 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 UV light.”

“In addition, the vortex nature of the spot confines particles, preventing them from escaping. This causes the redness of the spot’s cloud tops to increase beyond what might otherwise be expected.”

Other areas of Jupiter display a mixed palette of oranges, browns and even shades of red.

“These are places where high, bright clouds are known to be much thinner, allowing views to depths in the atmosphere where more colorful substances exist,” Dr Baines said.

The results were presented November 14, 2014 at the 46th Annual Meeting of the American Astronomical Society’s Division of Planetary Sciences in Tucson, Arizona.

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Baines K.H. et al. 2014. Why is the Great Red Spot Red? The Exogenic, Photolytic Origin of the UV/Blue-Absorbing Chromophores of Jupiter’s Great Red Spot as Determined by Spectral Analysis of Cassini/VIMS Observations using New Laboratory Optical Coefficients. American Astronomical Society, DPS meeting #46, #511.05