Scientists have long known that the Great Red Spot is a huge whirling storm rising high above the surrounding cloud tops. But the reason for its reddish hue has been a mystery. One leading theory has been that red-colored compounds are swirling up from deeper down to tint the Spot, a bit like the blush from blood rushing to your cheeks. Now, new results presented at recent meeting of the American Astronomical Society are suggesting something different. [Kevin H. Baines, Robert W. Carlson and Thomas W. Momary, Why is the Great Red Spot Red? The Exogenic, Photolytic Origin of the UV/Blue-Absorbing Chromophores of Jupiter’s Great Red Spot as Determined by Spectral Analysis of Cassini/VIMS Observations using New Laboratory Optical Coefficients]
Working at the NASA-Caltech Jet Propulsion Lab in Pasadena, California, researchers exposed two chemicals known to exist in Jupiter’s atmosphere—ammonia and acetylene—to levels of ultraviolet light like those found near the top of the planet’s atmosphere. The ammonia and acetylene turned a shade of red very similar to the Great Red Spot’s, as seen up-close by passing spacecraft.
But the lab compounds only matched the Spot’s color if on Jupiter they were confined to its highest cloud tops. If they were welling up from beneath and distributed through all of the Spot’s layers, the Spot would be a much more brilliant crimson hue than its present pale red color.
So it looks like UV light’s effect means that Jupiter’s Great Red Spot is less like a blush—and more of a sunburn.