The Hubble Space Telescope has recently strengthened the case for a subsurface ocean on Ganymede by observing

Question

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The Hubble Space Telescope has observed a thin atmosphere of water vapor above the surface of Ganymede, which strengthens the case for a subsurface ocean on the moon. Salts on its surface and jets of water erupting from the surface are not mentioned as evidence for a subsurface ocean. Changes in time of auroras surrounding the moon may be studied for other purposes but are not directly related to the presence of a subsurface ocean.

The Hubble Space Telescope Has Recently Strengthened the Case for a Subsurface