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The Great Red Spot is a persistent anticyclonic storm, 22° south of Jupiter's equator. Why is it reddish?

From Wikipedia:

It is not known exactly what causes the Great Red Spot's reddish color.

Are there updated data?

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    $\begingroup$ It should be noted that its color and size change in time. Right now the GRS is at its smallest in a very long time, and it's at its least reddish. Basically, it's the mild color of a coffee shop latte, more or less, difficult to see against the similar-colored background of the equatorial belts nearby. Observers using relatively small amateur telescopes have a bit of trouble seeing it even in good conditions - at an aperture of 150...200mm it's more visible at first as a dent in the nearby belts. This is in stark contrast with the deep red and large size of the GRS in previous decades. $\endgroup$ Dec 17 '14 at 23:44
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It's red because it's a 'sunburn'. The clouds in the red spot reach to higher altitudes than the surrounding ones and are more exposed to solar UV radiation, which in turn changes the structure of some of the organic molecules etc. This is at least the explanation suggested by recent data from NASA's Cassini mission, see this 5-day-old press release.

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    $\begingroup$ When I read "sunburn" I immediately thought you were crazy and needed a downvote. But it's serious. Neat. +1 $\endgroup$ Nov 16 '14 at 18:36
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    $\begingroup$ wow, totally unexpected. great job! $\endgroup$ Nov 16 '14 at 20:48
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    $\begingroup$ @Deuterium I find it amazing that you ask this question just 3 days after this press release! What triggered the question? $\endgroup$
    – Walter
    Nov 16 '14 at 21:20
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    $\begingroup$ @Walter I heard that years ago on a video, two days ago I was reading wikipedia and did not believe that is not known yet. I just hope that my chance on lottery was not spent here. $\endgroup$ Nov 16 '14 at 21:55
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    $\begingroup$ @HDE226868 I accidentally copied (instead of edited) it. I then had to delete one of them. $\endgroup$
    – Walter
    Nov 17 '14 at 16:12
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@Walter's answer still stands correct.

TL;DR: UV light by the Sun shone on ammonium hydrosulfide produced reddish compounds, which may be the cause of the reddish hue of the Great Red Spot.

Source (from 2018, abstract below):

Mark J. Loeffler, Reggie L. Hudson, Coloring Jupiter's clouds: Radiolysis of ammonium hydrosulfide (NH4SH), Icarus, Volume 302, 2018, Pages 418-425, ISSN 0019-1035, https://doi.org/10.1016/j.icarus.2017.10.041.

Abstract: Here we present our recent studies on the color and spectral reflectance changes induced by ∼0.9 MeV proton irradiation of ammonium hydrosulfide, NH4SH, a compound predicted to be an important tropospheric cloud component of Jupiter and other giant planets. Ultraviolet-visible spectroscopy was used to observe and identify reaction products in the ice sample and digital photography was used to document the corresponding color changes at 10–160 K. Our experiments clearly show that the resulting color of the sample depends not only on the irradiation dose but also the irradiation temperature. Furthermore, unlike in our most recent studies of irradiation of NH4SH at 120 K, which showed that higher irradiation doses caused the sample to appear green, the lower temperature studies now show that the sample becomes red after irradiation. However, comparison of these lower temperature spectra over the entire spectral range observed by HST shows that even though the color and spectrum resemble the color and spectrum of the GRS, there is still enough difference to suggest that another component may be needed to adequately fit spectra of the GRS and other red regions of Jupiter's clouds. Regardless, the presence of NH4SH in the atmosphere of Jupiter and other gas giants, combined with this compound's clear alteration via radiolysis, suggests that its contribution to the ultraviolet-visible spectra of any of these object's clouds is significant.

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