By combining observations from NASA’s Cassini spacecraft with lab experiments, astronomers are saying that Jupiter’s Great Red Spot is basically a sunburn: That ruddy color is a product of simple chemicals being broken apart by sunlight in the planet’s upper atmosphere.
As wide as two Earths, the famous spot is a long-lived, cyclone-like feature of Jupiter's atmosphere. Three main cloud layers occupy specific altitudes in Jupiter’s skies: from highest to lowest, they’re ammonia, ammonium hydrosulfide, and water clouds. One of the leading theories for the spot’s striking color argues that reddish chemicals are coming from beneath all of those clouds. According to that theory, the Great Red Spot is more of a blush, rather than a sunburn.
A team led by Kevin Baines of NASA's Jet Propulsion Laboratory set out to determine if the spot’s color might derive from the sun-induced breakdown of ammonium hydrosulfide (the middle cloud layer), but they quickly discovered that instead of a red color, the products of their lab experiment had a brilliant shade of green.
So, the team decided to blast ammonia (the top cloud layer) and acetylene gases—common hydrocarbons of Jupiter’s high altitudes—with ultraviolet light. This simulates the sun's effects at extreme cloud heights in the Great Red Spot. The result was a reddish material, which the team then compared to observations by Cassini's Visible and Infrared Mapping Spectrometer (VIMS) during its December 2000 Jupiter flyby. The light-scattering properties of their red lab concoction fit nicely with a model of the spot where the red-colored material is confined to the uppermost reaches.
"Our models suggest most of the Great Red Spot is actually pretty bland in color, beneath the upper cloud layer of reddish material," Baines says in a news release. "Under the reddish 'sunburn' the clouds are probably whitish or grayish."
Having the coloring agent confined to the top of the clouds contradicts the leading theory, which says the 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, they say, which would make the red spot redder still.
According to the team, altitude explains why intense red is only seen in the vibrant spot (and maybe a few other smaller spots). "The Great Red Spot is extremely tall," Baines says. "It reaches much higher altitudes than clouds elsewhere on Jupiter." The spot's great height enables (and enhances) the reddening: Its winds transport ammonia ice particles high into the atmosphere where they’re exposed to much more of the sun's ultraviolet light. And the spot’s vortex nature prevents particles from escaping its cloud tops. The oranges and browns are thanks to thin, high clouds where we get to see into the depths of the atmosphere where more colorful substances exist.
The work was presented at the American Astronomical Society's Division for Planetary Science meeting in Tucson this week.