It’s the signature accessory of the largest planet in our solar system: Jupiter’s Great Red Spot, an enormous anticyclone over twice the width of our entire planet. Visible in even modest backyard telescopes, the GRS has been churning away for at least several hundred years. But, based on recent analysis of data gathered by the Cassini spacecraft during its pass by Jupiter in December 2000, the Great Red Spot’s rusty coloration may actually only be skin-deep – a “sunburn” created by interaction between Jupiter’s upper atmosphere and solar radiation.

Edited from a JPL news release on the Cassini mission site:

In the lab, JPL researchers Kevin Baines, Bob Carlson, and Tom Momary 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. 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).

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.

FACT: the winds in Jupiter’s Great Red Spot reach speeds of over 270 mph (435 km/h)!

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

These findings are 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.

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 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 researchers 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 ultraviolet 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.

NASA’s Juno mission will explore the GRS further when it arrives at Jupiter in 2016.

The results were presented at the American Astronomical Society’s Division for Planetary Science Meeting in Tucson, Arizona.

Source: NASA JPL