Why is Ganymede's aurora only visible in ultraviolet while Earth's is bright green and red? 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.

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