The Great Red Spot on Jupiter -- the persistent high-pressure 'anticyclone' that was first observed by Gian Domenico Cassini in 1665 -- is likely only red because of what scientists at the Jet Propulsion Laboratory are calling a "sunburn."
There have been competing theories as to why the Great Red Spot has its color. NASA scientist Kevin Baines said that one theory is that "the spot’s red color is due to upwelling chemicals formed deep beneath the visible cloud layers."
But, he added, "[i]f 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 models that Baines and his team constructed at the JPL, however, "suggest most of the Great Red Spot is actually pretty bland in color, beneath the upper cloud layer of reddish material."
His team believes that the storm's altitude makes the ammonia and acetylene present throughout the atmosphere more vulnerable to UV light.
"The Great Red Spot is extremely tall. It reaches much higher altitudes than clouds elsewhere on Jupiter," he said. "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."
Moreover, he added, "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."