The Great Red Spot is the distinguishing feature that makes Jupiter one of the most easily recognizable planets in our solar system. Until recently, it was widely believed that this blemish was formed as a result of reddish-colored chemicals rising up from within the planet itself. However, using information obtained by analysis of data from the Cassini fly-by mission of Jupiter, researchers working at NASA's Jet Propulsion Laboratory have discerned that the planet's Great Red Spot may have more to do with the external action of the sun than some internal mechanism.
Basing their research on the chemical composition and quantities detected in the Jovian atmosphere by the Cassini probe, the scientists experimented in the laboratory by bombarding a mixture of acetylene and ammonia gases with ultraviolet light to replicate the emissions from the sun that reach Jupiter's upper atmosphere.
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As a result, the tests produced a reddish colored material that, when compared to data collected by Cassini's Visible and Infrared Mapping Spectrometer (VIMS), accurately represented the spectra observed in the Great Red Spot.
"Our models suggest most of the Great Red Spot is actually pretty bland in color, beneath the upper cloud layer of reddish material," said Kevin Baines, a Cassini team scientist. "Under the reddish 'sunburn' the clouds are probably whitish or grayish."
Further evidence bolstering the new research lies in the fact that, if the chemicals or material coloring the Spot are confined to the uppermost area of the clouds, then the existing theory is incorrect as chemicals bubbling up from within the planet would be seen below that area as well. Further, if the coloring material was ascending in a column below the Spot, then it would appear much more red than it is due to the greater amount of material suspended below it.
A centuries-old feature in the atmosphere of Jupiter and the size of two earths in width, the Great Red Spot is an anomaly that stretches far into the atmosphere where it reaches much higher than even the highest clouds of ammonia, ammonium hydrosulfide, and water that surround it. It is this height in the atmosphere that researchers believe makes the planet's huge blemish so visible and so prone to degradation of its constituent chemicals by ultraviolet radiation.
"The Great Red Spot is extremely tall," Baines said. "It reaches much higher altitudes than clouds elsewhere on Jupiter."
The researchers also believe that the Spot's dizzying height enables and sustains the rusty hue. They posit that winds within the Spot lift ice particles of ammonia into the atmosphere higher than it is lifted in other areas, where they are then subjected to much more of the sun's ultraviolet radiation. At the same time, the vortex action inherent in the Spot keeps the particles together, effectively corralling them in the one area. This is believed to be the reason that the ruddy color of the Spot's cloud tops form above the lower regions, and at a higher height than the researchers expected.
Known to be composed largely of hydrogen and helium (along with a modicum of other elements/compounds such as acetylene and ammonia as mentioned), Jupiter still requires greater study to determine what other chemicals exist, and how they interact. Understanding what mixtures of elements are at play in the many and varied hues observed in Jupiter's clouds and Giant Red Spot will provide valuable insights into the gas giant's chemical make-up.
The outcome of the research will be presented at NASA's Jet Propulsion Laboratory, Pasadena, California, at the American Astronomical Society's Division for Planetary Science Meeting in Tucson, Arizona.