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GANYMEDE AURORAE

NASA's Hubble Space Telescope observed a pair of auroral belts encircling the Jovian moon Ganymede. The belts were observed in ultraviolet light by the Space Telescope Imaging Spectrograph and are colored blue in this illustration.enter image description here

Why is Ganymede's aurora only visible in ultraviolet while Earth's is in the visible part of the spectrum?

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    $\begingroup$ The fact that it was observed in UV does not mean that it is "only visible in ultraviolet". Likewise, do our aurora's not have UV emissions? In other words: without additional information, this may not be special at all (not a 'valid' question). $\endgroup$
    – user1569
    Jan 14, 2020 at 15:49
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    $\begingroup$ Did you do any prior research on what the composition of the UV-emitting particles is? $\endgroup$
    – AtmosphericPrisonEscape
    Jan 14, 2020 at 18:16
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    $\begingroup$ @AtmosphericPrisonEscape This is what I came across during my research, "Follow-up observations will be difficult because the auroras were only visible in ultraviolet light," astrobio.net/history/… $\endgroup$
    – Bob516
    Jan 15, 2020 at 3:08
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    $\begingroup$ I suppose in visible light it's much harder to see Ganimede's aurora in telescope, because it's swamped in reflected light from the surface. $\endgroup$
    – Heopps
    Jan 15, 2020 at 8:14
  • $\begingroup$ @JanDoggen. As cited in my comment to AtmosphericPrisonEscape I had reason to believe the Ganymede aurora could not be detected in visible light. Also, I did not say Earth's aurora did not emit in the UV part of the spectrum, I only pointed out that it emits in the visible part of the spectrum, though I may have worded it imperfectly. $\endgroup$
    – Bob516
    Jan 15, 2020 at 12:36

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Quoting from Nat.Geo. article (which has that same UV image),

Then, in 2010 and 2011, Hubble took a close look at the moon. More specifically, it looked at the auroral bands ringing Ganymede’s poles. Hubble studied the auroras in the ultraviolet, but Saur said the shimmering lights would be visible to human eyes.

“If somebody could be standing on Ganymede looking up into the night sky, it would appear as red aurora, to you,” he described.

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    $\begingroup$ Yes. Keep in mind that while Ganymede's atmosphere is very thin, Earth's aurorae are also generated in an extremely thin part of its atmosphere (the thermosphere), and are of course entirely visible. It's just that in the case of Ganymede, the "suitably thin" layer starts much closer to, if not at, the surface. $\endgroup$
    – The_Sympathizer
    Jan 15, 2020 at 5:28

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