The volcanoes are not the same as the typical terrestrial variety, but they operate similarly. "Volcanism involves the melting of solid crust or mantle," Schenk told SPACE.com. "In this case, the crust happens to be ice."

Though the ice volcanoes spew melted water, they are unlike Yellowstone's Old Faithful or an undersea hydrothermal vent, both of which recycle surface water that is heated by hot rocks. Water magma on Ganymede originates deep within the moon, in the same way that molten volcanic rock, called magma, does on Earth.

This image, showing an area roughly 44 miles (70 kilometers) wide, shows how flowing material from an ice volcano seems to have butted up against higher, rugged terrain.

Schenk described the resulting smooth structures as looking something like the lunar mare. A visitor standing on Ganymede's surface would see vast flats that would appear virtually unblemished. One trough extends an estimated 600 miles (900 kilometers), roughly the distance between St. Louis and New Orleans. The features, though younger than much of the surface, are estimated to be a billion years old.

"This is the most persuasive evidence yet that there is volcanism on Ganymede," said Louise Prockter, a Johns Hopkins University researcher who was not involved in the study. "I think [the researchers] are definitely onto something."

But Prockter said in a telephone interview that the smooth areas appear very well confined -- uncharacteristic for lava flows. And, she said, she would have expected to see more evidence of volcanism in more places than the researchers uncovered.

The Nature paper discusses two areas that appear to have been resurfaced by volcanism, and Schenk said another extensive region was spotted at Ganymede's south pole. Several other areas suggest similar topography, he said. Volcanism typifies the surface of another Jovian moon -- Io.