Researchers at Washington University in St. Louis discovered the 55,000-foot double Everests and some secrets to how they were made while mapping the moon-sized Io's surface using stereo-images taken by the Voyager and Galileo spacecrafts.

They applied the same technique to find what appear to be ancient ice channels filled by ice volcanoes on Io's sister moon, Ganymede. The findings have been released in two papers in the current issues of Geology and Nature.

"What we think is going on is that instead of (crustal) plates moving horizontally, they are moving vertically," says Jovian moon mapper Paul Shenk.

Like Earth, Io has rift zones, where lava wells up from below as plates spread apart. That's where the similarity ends, however. Io's volcanoes are low-lying and set apart from its mountains, which has puzzled scientists for some time.

Now it appears that two volcanic forces have created Io's mountains. First, when too much lava piles up, it depresses the crust until there's no room for more in the depths of Io, says Shenk.

When that happens, the crust "pops" and huge tilted blocks of crust rise vertically out of the moon. The second mountain-building force is heat that can get trapped up under the crust. The heat causes rocks to expand and crowd upwards for space.

"It seems like quite a reasonable explanation," says NASA volcanologist Rosaly Lopes. "What seems to be happening is the stress is breaking up the crust."

Theoretically, Io's mountains could grow even higher since there is less gravity pulling on them then Earth's mountains, says William McKinnon of Washington University

Cooler volcanoes were at work on Ganymede, says Shenk. Bizarre dark- and light-banded terrain made of water ice on the cold moon appears to have been caused by ice volcanoes during a turbulent period in Ganymede's past. "It's very distinct and beautiful," says McKinnon.

As on Io, the maps of Ganymede show that melted crust apparently spewed out of the ground and filled in low-lying areas. The main differences are that Ganymede's crust is made mostly of water ice and is no longer subject to the tidal forces from Jupiter that keep Io so hot and violent.