Yet there was no proof.
Then in the late 1990s, the Galileo spacecraft sent back pictures with much higher resolution. But expected evidence of volcanic activity was strangely absent.
Instead, researchers found that the smooth areas were not always so smooth, but riddled with ridges and scallops in some places. These features suggested that tectonic activity -- sudden quakes and plodding movement of surface plates -- had been at work. It was possible, they realized, that Ganymede's ancient craters and crags had been erased by the movement of the moon's crust.
The new study combined the Voyager and Galileo images to create "stereo pairs" that allow the heights of ridges and valleys to be measured with greater precision than ever before. The images revealed that the smoothest, brightest areas were also the lowest, and heavily fractured material sat up to 0.6 mile (1 kilometer) higher.
Schenk and his colleagues suggest that both ice volcanoes and tectonic activity remade Ganymede's surface.
"What happens in detail is anybody's guess," Schenk said. But liquid water, behaving like lava, "probably comes up through cracks in the crust and fills low-lying areas. This lava may come out in one massive outflow or as a series of smaller overlapping flows."
The water lava is forced to the surface either by pressure deep down, or because it is less dense and tends to float in relation to surrounding material, the researcher said. Because the water only rose to a certain level, higher terrain was not resurfaced and remains old and craggy.
Dancing with moons
In an analysis of the findings in Nature, Prockter wrote that the resurfacing of Ganymede was likely an extreme event caused by an orbital dance with sibling moons Io and Europa. The gravitational tug of the other moons may have caused ice inside Ganymede to melt.
"Studying the process that ripped apart two-thirds of Ganymede's surface, yet left the remainder virtually untouched, will help us to understand the evolution of Jupiter's moons," Prockter said.
A clearer picture is on the way. Galileo's highest-resolution images, taken in May 2000, have not yet been fully analyzed, and Prockter thinks "they should tell us more about the relative effects of volcanic and tectonic activity on this giant among moons."
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