Washington: The ruddy colour of Jupiter’s famous Great Red Spot is likely a product of simple chemicals being broken apart by sunlight in the planet’s upper atmosphere, NASA researchers have said.

The results, based on a combination of data from the December 2000 Jupiter flyby of NASA’s Cassini spacecraft and laboratory experiments, contradicted the other leading theory for the origin of the spot’s striking colour that the reddish chemicals come from beneath Jupiter’s clouds.

In the lab, researchers blasted ammonia and acetylene gases, chemicals 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.

The simulation produced a reddish material that “nicely matched” a model of the Great Red Spot, in which the red-coloured material was confined to the uppermost reaches of the giant cyclone-like feature, as per reported citing a statement issued by the NASA researchers Tuesday.

“Our models suggest most of the Great Red Spot is actually pretty bland in colour, beneath the upper cloud layer of reddish material,” said Kevin Baines, a Cassini team scientist based at NASA’s Jet Propulsion Laboratory. “Under the reddish ‘sunburn’ the clouds are probably whitish or greyish.”

A colouring agent confined to the top of the clouds would be inconsistent with the competing theory, which posits that the spot’s red colour is due to upwelling chemicals formed deep beneath the visible cloud layers, he said.

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

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

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

According to the researchers, the spot’s winds transport ammonia ice particles higher into the atmosphere than usual, where they are exposed to much more of the sun’s ultraviolet light.

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

The results were presented this week by Baines at the ‘s Division for Planetary Science Meeting in Tucson, Arizona.