Hubble telescope image of Ganymede auroral belts (mapped with blue-white splotches) are imposed over a grayish Galileo orbiter image of Ganymede. Pink dotted lines indicate the paths of the belts across the moon's surface.

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NASA Hubble Space Telescope images of Ganymede's auroral belts (colored blue in this illustration) are overlaid on a Galileo orbiter image of the moon. The amount of rocking of the moon's magnetic field suggests that the moon has a subsurface saltwater ocean.

Credits: NASA/ESA