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What evidence suggests that Ganymede has an underground ocean?
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Jupiter
Photograph of Jupiter taken by Voyager 1 on February 1, 1979, at a range of 32.7 million km (20.3 million miles). Prominent are the planet's pastel-shaded cloud bands and Great Red Spot (lower centre).
Evidence for a subsurface ocean on Ganymede is primarily derived from observations of the moon's magnetic environment.
As the largest of Jupiter's satellites, Ganymede is unique in that it possesses its own permanent magnetic field, which is strong enough to generate a magnetosphere and auroras. Scientists utilized the Hubble Space Telescope to monitor how these auroras shift in response to interactions with Jupiter’s own powerful magnetic field. The specific behavior of these auroras provides strong evidence for the existence of a liquid, electrically conducting layer—likely a subsurface ocean—approximately 100 km (60 miles) thick beneath the moon's icy crust.12