Sunlight Helps Give Jupiter’s Great Red Spot Its Color

Chuck Bednar for redOrbit.com – Your Universe Online
By analyzing data from the Cassini mission, NASA researchers have found that the color of Jupiter’s Great Red Spot is most likely the result of simple chemicals being broken apart by sunlight in the planet’s upper atmosphere.
Those findings, which are being presented later this week by Cassini team scientist Kevin Baines at the American Astronomical Society’s Division for Planetary Science Meeting in Arizona, contradict the commonly-held theory that the hue was the result of reddish chemicals originating from beneath Jupiter’s clouds, the US space agency announced on Tuesday.
Baines, who is based at NASA’s Jet Propulsion Laboratory (JPL), along with Bob Carlson and Tom Momary – both of whom are colleagues from the Pasadena, California-based facility, reached their conclusions based on a combination of laboratory experiments and data captured by Cassini during its December 2000 flyby of Jupiter.
During their experiments, they exposed chemicals known to exist on Jupiter – ammonia and acetylene gases – with ultraviolet light in order to simulate the effect the sun’s light would have on the substances at the extreme heights of the clouds found in the Great Red Spot. The resulting reddish material was then compared to the cyclone-like feature as observed by Cassini’s Visible and Infrared Mapping Spectrometer (VIMS) instrument.
Baines and his colleagues discovered that the light-scattering properties of the red chemical concoction was a close match to a model of the Great Red Spot in which the red-colored material is confined to its uppermost reaches. In a statement, he added that, underneath the upper-most cloud layer, most of the phenomenon is “actually pretty bland in color… the clouds are probably whitish or grayish.”
The presence of some type of coloring agent would be inconsistent with the other primary theory, which claims that the red color of the Spot is the result of “upwelling chemicals formed deep beneath the visible cloud layers,” according to NASA. If the red material was actually being transported upwards from beneath, it should also be present at lower altitudes – making the spot appear to be even redder than it actually is.
The overwhelming majority of Jupiter is made of hydrogen and helium, with only a dash of elements. Scientists are attempting to understand what elemental combinations are responsible for causing the colors observed in the giant planet’s clouds, as it could provide new insight into Jupiter’s composition, the agency explained.
Baines and his fellow investigators initially tried to determine if the color of the Spot could be linked to a sun-induced breakdown of ammonium hydrosulfide, a more complex molecule than makes up one of the planet’s main cloud layers. However, experiments focusing on this molecule wound up producing “a brilliant shade of green,” not red.
Those results prompted the research team to experiment instead with simple combinations of ammonia with hydrocarbons – elements commonly found at Jupiter’s higher altitudes. They discovered that breaking down ammonia acetylene with ultraviolet light appeared to be the best match with the data collected by Cassini.
“The team thinks the spot’s great heights both enable and enhance the reddening,” NASA explained. “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.”

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